There are two factors behind this phenomenon. First, the Moon is very bright. In fact, it is so bright that you can see it against the blue backdrop of the daytime sky. It is not as bright as the Sun, of course, because the Moon doesn’t give off any light by itself. The Moon only shines by reflecting light from the Sun. However, the Moon is one hundred thousand times brighter than the brightest nighttime star.
The Moon reflects so much light that our farming ancestors used its light to keep working to bring in the crops well into the nighttime hours. Hence, they named the full moon closest to the autumnal equinox (September 21 or 22) the harvest moon. Once every four years, the harvest moon is actually in October. The full moon after the harvest moon is referred to as the hunter’s moon.
The second idea to consider is that the Moon goes around the Earth in a little less than thirty days, or what we call roughly a month. For part of that time, around the time of the full moon, the Moon is in the sky opposite the Sun. The Moon is rising as the Sun sets. But on each successive night the Moon comes up about an hour later. The Moon gets closer and closer to the direction of the Sun in the early-morning hours, finally appearing very close to the Sun in the sky around new moon time, rising and setting at about the same time as the Sun.
Then the Moon gets farther and farther from the Sun in the sky each night until it gets back to full moon. The portion of the Moon that is lit up depends on the angle the Moon makes between the Earth and the Sun, which causes the various phases of the Moon.
During these phases, people on Earth can see the sunlight that strikes the Moon and is reflected from the Moon to us on Earth. So then we can see the Moon during the day. Again, if you see the Moon rising after sunrise, it is going from new moon to full moon, so each morning you see more and more Moon.
The Moon is “out” as many hours during the daytime as it is “out” during the nighttime. It’s just that we associate the Moon with the nighttime sky. Of course, it is brighter in the night sky and, hence, more noticeable.
We use the Latin word “luna” to refer to anything pertaining to the Moon. “Luna” is a Latin word etymologically related to “lucere,” which means “to shine.” Now, this “moonshine” is not the same as the illegal liquor by the same name.
Get a softball, a basketball, and a lamp. The lamp is the Sun. Face the lamp and hold the basketball (Earth) in front of it. As you move the softball (Moon) around the basketball, you will be able to see all the phases, and you will also see that the Moon is visible during the daytime hours. Mentally and physically put yourself in place of the basketball. As the softball Moon goes around your head, you will be able to see the Moon in the “daytime.” Remember that daytime for you is when your eyes can see the Sun.
We do need that ozone layer. It acts as a very thin shield high in the sky that protects us from the Sun’s harmful ultraviolet (UV) rays. Bad things can happen if too much UV light reaches the Earth’s surface: skin cancer and possibly eye damage (especially cataracts) and the weakening of our immune system, to name a few. Harm to our immune system means we have a harder time fighting off diseases.
Most of the ozone is in a layer that starts six miles above the surface of the Earth and goes up to about thirty miles above the Earth’s surface. The ozone layer is very good at blocking UV-B rays, which are the rays that cause sunburn and most skin cancers.
In the 1970s, scientists noticed that some of the ozone was going away, or being depleted. The main culprit was chlorofluorocarbons, or CFCs, which are used to keep things cold and to make foam and soaps. Some of these “bad guys” were released in the air, mainly from fire extinguishers, aerosol spray cans, manufacturing facilities, refrigerators, air-conditioning units, and dry-cleaning establishments. CFCs are very stable at low altitudes, so they stay in the atmosphere long enough to diffuse into the stratosphere. Up there, UV rays are strong enough to break them down and release chlorine. Each chlorine atom can attack and break apart thousands and thousands of ozone molecules. So just a little bit can do major damage.
Countries around the world knew that CFCs were really bad stuff; scientists noted as early as the 1970s that there was a seasonal depletion of ozone over the Antarctic continent. Little worldwide action was taken until 1987, when an international treaty, called the Montreal Protocol, sharply limited the production of CFCs. They were completely phased out in the developed world by 1996. A hole in the ozone layer over the Antarctic was discovered in 1985.
Three satellites and three ground stations have shown that the depletion of the ozone layer is decreasing. Nature is repairing the damage, and ozone levels started to rise ever so slightly in late 2009 in most parts of the world. It will be fifty years before all the damage is repaired. Humans have the capability to do damage to our planet Earth, but we also have the knowledge to protect our home, if we’ll use it.
Those thin-line clouds trailing behind jet aircraft are condensation trails, often shortened to “contrails.” Jet fuel, which is low-grade kerosene, is made up of carbon and hydrogen. When jet fuel burns with oxygen in the atmosphere, most of the exhaust consists of carbon dioxide (CO2) and water vapor. Generally, the water vapor is invisible. But cold air in the upper atmosphere can’t hold nearly as much water vapor as warm air can. So the water vapor condenses onto tiny particles, such as exhaust particles and dust in the air, and forms the clouds we see.
We create the same phenomenon when we go outside in the winter and exhale. We see our own breath, which contains a good deal of that invisible water vapor. The breath hits the cold air, and the water vapor condenses onto tiny particles in the air. We don’t see our breath in the summer because the warmer air holds the water vapor.
Contrails were noticed back in the 1940s, especially when the US Eighth Air Force sent hundreds of B-17 bombers from England to hit targets in Nazi Germany. Americans back home saw the contrails on Movietone newsreels seen in movie theatres.
The length of time that contrails persist depends on the altitude, temperature, water vapor content, pressure, sunlight, and shear winds. If the winds aloft are calm, the contrail will keep its shape and be seen from horizon to horizon. Sometimes, strong winds will spread out the contrail so it looks like those high-altitude cirrus clouds, often referred to as “mare’s tails.”
There has been speculation in some quarters concerning whether contrails are causing weather modification, a concept that isn’t entirely science fiction. Cloud seeding has been in operation in various locations around the globe since the 1950s. Shooting silver iodide crystals into clouds causes the clouds to give up their water content. The water vapor condenses onto the crystals. Cloud seeding has been used to relieve drought, increase snowfall, dissipate hurricanes, and suppress hail. The Chinese government promised clear skies for the August 8 opening of the 2008 Summer Olympics. They launched 1,100 rain dispersal rockets from twenty-one sites around Beijing. Sure enough, no rain fell on their parade.
People in rural areas tend to pay more attention to the skies. They generally report seeing more contrails these days than they did decades ago. That is no doubt true, as there has been a large increase in jet traffic, both civilian and military. In addition, much of that air traffic has been a higher altitudes, some as high as forty thousand feet, where winds aloft are less likely to disperse the contrails quickly.
There have been those stories, held by conspiracy theorists, that chemicals are being spread from planes for a certain purpose. They call them “chemtrails” rather than “contrails.” Some of the chemtrail conspiracy stories that float around the Internet claim that barium, aluminum salts, thorium, and silicon carbide are being released. Other accounts have the skies being seeded with electrically conductive materials as part of a super-weapons program. Other reasons stated are population control and alleviating global warming. Studies done in the United States, Canada, and Great Britain found no scientific evidence to support the allegation that high-altitude spraying is being conducted.
These operations may very well be going on, but thus far nobody has brought forth any proof or evidence. They remain, for now, conspiracy theories.
Our Moon’s surface bears the marks of many craters formed by collisions from meteorites, comets, and asteroids. The average speed of a body striking the Moon is about twelve miles per second. When one of these visitors strikes the solid surface of the Moon, the resulting shock wave fractures the rock and digs a cavity, or bowl-shaped hole, about ten to twenty times the diameter of the impacting rock. The collision shatters the asteroid into smaller pieces and generates heat that may either melt or vaporize them.
The Earth has many craters from collisions with asteroids and meteorites. But the Earth has a thick atmosphere that acts as a shield. As soon as an asteroid comes into contact with our atmosphere, the air in front of the asteroid packs together, and the resulting friction increases the temperature to thousands of degrees. The meteorite catches fire. We see it as a shooting star, or falling star, as the burning particles trail behind the meteorite. Most of these meteorites disintegrate before they have a chance to reach the surface of the Earth. Our atmosphere acts as a safety shield and a cushion to protect the surface we live on.
The Moon has no such protecting atmosphere. Scientists estimate that over a ton of meteorites hit the Moon every day, beating it up rather badly. The surface of the Moon reveals the evidence of millions of years of bombardment. Copernicus is a large crater, sixty miles across. On Earth, plate tectonics, wind, rain, glaciers, and surface changes have eroded most of the impact craters away. Similar processes do not exist on the Moon, so its craters have remained almost unchanged over billions of years. About the only thing that erases a crater on the Moon is a new impact that leaves a bigger crater.
The Earth actually has more impact craters than the Moon. The Earth’s diameter is four times that of the Moon, so it presents a bigger target for wayward asteroids and comets.
All the solid bodies of the solar system exhibit impact craters. Mercury looks much like the Moon. Venus’s atmosphere consists of thick carbon dioxide clouds, but remote robotic landers show that its surface is heavily cratered. Venus and Mars have had volcanic activity that filled in many of their craters.
There are fifty-seven known impact craters in North America alone, with more than 170 spread over the entire Earth. The Chicxulub crater in the Yucatán Peninsula of Mexico is not easy to see, but satellite images, local changes in the gravity field, and ringlike structures in the land around the impact site give clues to its size. Many scientists believe the resulting fires, tsunamis, and clouds of dust and water vapor contributed to the extinction of dinosaurs 65 million years ago. The oldest and largest impact crater recognized on Earth is the Vredefort crater in South Africa. It is two billion years old and one hundred miles across. A visit to the Barringer Crater near Winslow, Arizona, must go on your bucket list. A 150-foot-wide iron-rich meteoroid struck there fifty thousand years ago. It left a crater about three-fourths of a mile wide and 750 feet deep. The site has a beautifully constructed visitor center, extensive displays, and guided tours. You can walk down into the crater or all the way around it if you are so inclined. In 1908, a large meteoroid or comet hit in Siberia, Russia, near the Tunguska River. This “Tunguska event” was a powerful explosion, equivalent to 185 of the A-bomb dropped on Hiroshima, Japan, in 1945. The blast knocked over 80 million trees and killed a lot of reindeer. Recently, on February 16, 2013, a fifty-five-foot-wide rock lit up the skies over the Ural region of Russia. It was traveling at 40,000 mph. It broke up at five miles above the earth’s surface. The shock wave injured roughly 1,200 people.
The Moon has eight phases. When we see a big disk in the sky, that’s a full moon. It looks like a shiny quarter or perhaps a twenty-dollar gold piece. When we can’t see the Moon, that is the new moon phase. When the Moon appears to be getting bigger, it is waxing. When the Moon is getting smaller, it is said to be waning. When the Moon appears like the edge of a fingernail or a backward letter “C,” just coming out of its new moon phase, it is in the waxing crescent phase. Over several days, as more of the Moon is lit up, it reaches the first quarter, which is when it looks like half of the Moon is lit and resembles the letter “D.” The term “first quarter” means that it has moved a quarter, or one-fourth, of the way from one new moon to the next new moon. After first quarter, the Moon seems to grow more and get bigger; we call this the waxing gibbous phase. The name “gibbous” comes from the Latin word for “hump”; in this phase the Moon has sort of a humpback shape to it. The halfway point of the Moon’s cycle is the full moon, after which the Moon shrinks to waning gibbous, then last, or third, quarter, followed by waning crescent, then back to new moon.
Our Moon is not the largest moon in the solar system. Ganymede, a moon of Jupiter, is thirty-three hundred miles across, much larger than our Moon, which is twenty-two hundred miles in diameter. But our Moon is the largest moon in proportion to the planet it orbits. Because of its large comparative size, our Moon greatly affects our planet. The Moon is responsible for the tides (see question 97), and it also causes the Earth to weave back and forth in its orbit. The Moon’s tug on the Earth makes the Earth have one big wobble every twenty-six thousand years. This causes the Earth’s axis to point to different stars in the sky over the millennia.
“Luna” is the Latin word for month, which defines the time it requires the Moon to make one revolution, or orbit, around the Earth. The Moon is 240,000 miles from the Earth. It looks the same size in the sky as the Sun, but the Sun is four hundred times farther away. It is this same apparent size that allows us to see lunar and solar eclipses.
The Moon is about 4.5 billion years old, same as the Earth; they were formed at the same time. The Moon has no air or atmosphere, so its surface temperatures vary widely. Where it faces the Sun, the surface is 250°F. The part of the Moon’s surface that faces away from the Sun—the dark side—is 250°F below zero.
The side of the Earth that is closest to the Moon is subject to a greater tug of gravity than the center of the Earth. The side of the Earth facing away from the Moon experiences less of a tug than the center of the Earth. This has the effect of stretching the Earth a bit, and the parts that stick out are referred to as “tidal bulges.” The most noticeable effect is the tides, but the Earth itself is made slightly oblong by a few inches. This tidal bulge is in addition to the bulge created by the Earth’s rotation.
The Earth rotates once every twenty-four hours, faster than the Moon orbits it, which is about twenty-nine days. That tidal bulge wants to speed up the Moon and nudge it ahead in its orbit. The Moon resists by pulling back on the tidal bulge. That slows the Earth’s rotation. Because of conservation of energy, whatever one body loses, another must gain. The Moon gains by having a bigger orbit, which means moving slightly away from the Earth. The Moon is moving away from the Earth about an inch and a half per year.
The big red spot, officially known as the Great Red Spot, is a huge, long-lasting storm in the atmosphere in the southern hemisphere of Jupiter. It is about seventeen thousand miles across and is so big that three Earths would fit inside. It is a high-pressure storm, much like a giant hurricane, and it’s the biggest storm in our solar system. Hurricanes on Earth are low-pressure systems, but Jupiter’s red spot is a high-pressure system that rotates counter-clockwise with a period (time for one rotation) of about seven days. The Great Red Spot’s big red spot is actually pink and orange and was discovered in 1664, when Robert Hooke, English scientist and architect, saw it in his telescope. Italian-French astronomer Giovanni Cassini is credited with the discovery of the Great Red Spot at about the same time.
The colors are the result of chemical reactions occurring in the atmosphere. The best theory is that the storm dredges up material, mainly phosphorus, from the interior of Jupiter. That giant red spot has been raging for over three hundred and forty-five years.
Jupiter is a huge gas ball. It does not have a clear boundary between atmosphere and solid surface like Earth. The gases get thicker and thicker the closer they get to the center and most likely, at some point, change to liquid. There may even be a solid core at the center of Jupiter. Scientists simply don’t know for sure.
Jupiter is 2.5 times as massive as all the other planets combined, which is probably why it is named after Jupiter, the king of the Roman gods. Jupiter is 318 times as massive as the Earth, it has sixty-seven moons that we know of, and it is encircled by a ring system, just like Saturn. The planet’s four largest moons are called the Galilean moons, named after Galileo, who discovered them and plotted their positions in 1610. The largest, Ganymede, is bigger than the planet Mercury. You can see Jupiter and its four largest moons with a small telescope or a good pair of binoculars. However, the rings of Jupiter are too nebulous and flimsy to see from Earth.
A Jupiter day is about ten hours, the time it takes for one complete rotation. It takes Jupiter about twelve of our years to go around the Sun once. Jupiter is composed almost entirely of hydrogen. If Jupiter were bigger, its gravitational pull would be strong enough to crush this material into a small core, which would trigger nuclear fusion and turn Jupiter into a star, much like our Sun.
Two very good questions. First, let’s deal with the size. Yes, the Moon does appear to be larger when it is on the horizon and smaller when overhead (see question 62). This is a result of the Ponzo illusion, named after Italian psychologist Mario Ponzo, who published his findings in 1913.
The brain interprets the sky as being farther away near the horizon and closer near the zenith, or directly overhead. We notice this on a cloudy day, when the overhead clouds may be a few thousand feet away, but near the horizon they might be hundreds of miles away. Since the Moon actually stays the same size, our brain makes it look bigger when it’s near the horizon to compensate for the increased distance. The Moon on the horizon is interpreted by the brain as being farther away. Because it is the same apparent size as when it’s high up, the brain figures it must be physically larger. Otherwise, the distance would make it look smaller.
Try this fun activity: Hold the tip of a pencil eraser at arm’s length and aim it at the rising Moon. Note the size of the eraser and the size of the Moon. Try this again when the Moon is more overhead. You’ll find the relative sizes haven’t changed at all.
Now for the color question. Quite often, the Moon will appear yellowish, orange-colored, or reddish, especially when we have a full moon that is rising in the eastern sky in the early evening. Later in the evening, when the Moon is higher or more overhead in the heavens, the same Moon looks white or light blue in color. The reason for this change of color rests in the nature of light itself.
Remember that mnemonic device for learning the colors of the rainbow, ROY G BIV, for red, orange, yellow, green, blue, indigo, and violet? The ROY colors (red, orange, yellow) have the longest waves, and the BIV colors (blue, indigo, violet) have the shortest. We can think of light in terms of waves we see on water. Visible light is electromagnetic radiation that has the properties of frequency, wavelength, and velocity. Frequency is the number of vibrations per second. Wavelength is the distance from the crest of one wave to the crest of the next wave. Velocity is the speed of a wave, or how fast the wave travels. Light waves, unlike water waves, do not need a medium to travel through.
In a phenomenon known as scattering, particles in the atmosphere, such as smog, dust, moisture, and even oxygen, act as tiny mirrors that reflect light in every direction. The Moon looks orange or reddish due to this scattering of light by the atmosphere. When the Moon is on the horizon, the moonlight has to pass through much more atmosphere as it travels to our eyes than when the Moon is overhead. By the time the light from the Moon reaches our eyes, most of the blue and green wavelengths have been scattered. What is left is the red, orange, and yellow part of the spectrum. The same phenomena is responsible for our red sunrises and red sunsets.
The particles that reflect light are very tiny, typically one-tenth the length of a wave of visible light. The shorter the wave of light, the more the scattering. A red wave is too long to effectively reflect off a particle that is a fraction of the length of that wave. We can think in terms of the “mirror” particle being too short. But a blue wave is short enough to bounce off the same particle. So blue light can’t travel through very much atmosphere before it is reflected in every direction. That’s why the BIV waves have all dissipated by the time the moonlight reaches our eye. Only the ROY colors remain, so we see the moon as reddish or orange.
When that same Moon is overhead a few hours later, the moonlight does not have as much atmosphere to travel through, so all the colors get through, and we see the Moon as more whitish or bluish.
Clouds are bunches of very tiny water droplets. The droplets are so light and small that they can float in the air. Clouds form when warm air rises, expands, and cools. The warm air has some moisture in it, but it is in vapor form and cannot be seen. The rising air cools about 3.5°F for every thousand feet in altitude, a pace referred to as the adiabatic cooling rate.
The lifted air is cooled to its dew-point temperature, the temperature at which air is completed saturated and can no longer retain its vapor. The relative humidity has reached 100 percent.
The expelled vapor condenses onto dust particles and oxygen molecules. Most of these particles come from cars, trucks, volcanoes, and forest fires. To “condense” means to change from a gas state to a liquid state. Billions and billions of these tiny droplets come together and form what we see as a visible cloud. At the highest altitudes the tiny droplets will freeze and form ice crystals.
So why are clouds white? Clouds are white because they reflect light from the Sun, which is made of the seven colors of red, orange, yellow, green, blue, indigo, and violet. Most of the time, clouds reflect those seven colors evenly, in about the same amount, to give white light. We see grayish, or very dark, clouds if the cloud cover is very thick; not as much sunlight is getting through the clouds. Thunderstorm clouds go so high in the atmosphere that these clouds take on a blackish appearance. The sheer thickness of the cloud layer blocks a greater amount of sunlight, and very little light reaches our eye, hence the “blackish” tint of the thundercloud.
Cloud height is determined by the type of cloud and the size of the tiny water droplets that make it up. Those fluffy white cumulus clouds with the flat bottoms generally have the heftiest water droplets, so those clouds are generally found below six thousand feet, but in some cases they can be found much higher. Cumulus clouds move about 10 to 20 mph but faster with thunderstorms.
Those very high thin wispy clouds, often called mare’s tails, are composed of very small ice crystals and are spotted at higher altitudes, beginning at about thirty thousand feet above the ground. These high cirrus clouds are pushed by the jet stream and can reach speeds of over 100 mph. The presence of a lot of cirrus clouds indicates that a change in weather is fast approaching.
Fog is a special kind of cloud. Most fog forms when warm moisture-laden air flows over a colder surface. If the air is full of moisture, it will condense onto particles in the air.
Besides the two main types of clouds, cumulus and cirrus, there are stratus clouds. “Stratus” comes from the Latin for “to spread out”; these stratus clouds stretch out over great distances in the sky. Stratus clouds yield a long, steady rain.
As a general rule, we tend to see many cumulus clouds in summer and a preponderance of stratus clouds in winter.
You most likely saw the International Space Station go over. But there are several other possibilities: shooting star, comet, airplane, or satellite.
What about a shooting star, also called a falling star or meteor? That’s a very fast streak of light that lasts a few seconds at most. The streak appears suddenly, goes only part of the way across the sky, and disappears. Not likely. It could also have been an airplane. They move slowly across the night sky, depending on altitude. Many times you can spot their red beacon light and even the position lights on either wing. Still, a satellite is most likely, because they move slowly. Many are going in a north-south direction, especially the reconnaissance, weather, and Earth-resources-sensing satellites.
The satellites used for television reception, like the ones used by DirecTV and Dish Network, have geosynchronous orbits, which means they stay over the same location on the Earth (see question 104). Their period, or time to make one revolution around the Earth, is twenty-four hours. You will recognize this as the time for the Earth itself to rotate once on its axis. They orbit at 22,000 miles above the Earth, which synchronizes their orbit to the rotation rate of the Earth. These satellites always follow along an extension of the equator. You wouldn’t see these satellites moving across the sky.
So most likely what you spotted was the International Space Station (ISS) traveling from west to east. The ISS has grown very big over the past few years. They’ve added module after module and solar panels galore. Its altitude is about 250 miles above the surface of the Earth, moving about 17,000 mph and taking about 1.5 hours to make one orbit of the Earth. There are other satellites in orbit around the Earth, but none as bright as the International Space Station.
You can see the ISS either in the early-morning hours before dawn or a few hours after sunset. It may be close to dark for the observer on Earth, but the sunlight strikes the surfaces of the ISS and reflects down to us. For an observer on the ground, it takes about 1.5 minutes for the ISS to go from horizon to horizon.
A really neat website for any sky observer is heavens-above.com. Set up a simple password and put in the coordinates of your house. The website will give a chart of when the ISS passes over your home, the time when it comes above the western horizon, when it reaches its highest point, and when it goes down below the eastern horizon. Also, all the observable passes of the Hubble Space Telescope are given.
Here is something for the curious mind and adventurous soul! Try to spot an Iridium flare. Sixty-six Iridium Communications satellites are up there. They’re not very big, only about thirteen feet long and three feet wide. Each satellite has three flat, door-size, highly polished aluminum surfaces (combo solar panel and antenna) that can reflect sunlight just like a mirror. The axis of the satellite is maintained vertical to the Earth’s surface. So software can predict the date, time, and any position on Earth where the bright light from the Sun hits those antennas and reflects to Earth. What we see here on Earth is a light or short streak of light, getting brighter until it reaches maximum brightness and fades again to nothing. The whole thing lasts only about five to twenty seconds.
The heavens-above.com website will tell you where to look in the sky, the exact altitude, and the azimuth (direction from north). The time of day will be right before sunrise or right after sunset, when the Sun is below the horizon for the observer but the satellite is high enough that the Sun’s rays strike it. People often report that brief flash of light as a UFO. Happy satellite hunting!
Yes, we can go to Mars. But the journey would be expensive, dangerous, and long. It doesn’t help that there don’t seem to be any strong arguments to go to Mars for economic reasons, either. For example, Mars has a lot of iron in its soil, but we have plenty of iron here on Earth.
We won’t go to Mars to get there first. That’s what the Moon race was all about in the 1960s, during the Cold War. We just had to beat the Russians to the Moon to prove our technological superiority. Yes, there was a lot of science that came out of the Moon race, and a lot of products, and processes, and spin-offs. But the Moon race was more political than scientific.
While it takes only three days to get to the Moon, a trip to Mars takes eight months one way. You’ll want to stay for a while, and it’s another eight months back, so a round trip to Mars and back would probably take at least two years. It’s going to take a very special person to handle two years in a confined and sterile environment, far from family and friends, with no access to medical facilities, accompanied by the same people every day.
You can’t go to Mars any old time you feel like it. A favorable period for sending a spacecraft to Mars is called a launch window. A launch window is a specific time period when any rocket or spacecraft must lift off to achieve a particular landing, mission, or rendezvous with another body. The launch window time is determined by the positions in their orbits of the launching platform, which is usually the Earth and the target, namely Mars. A launch window for Mars opens every twenty-six months. Launching a spacecraft to Mars is similar to throwing a dart at a moving target, except that you, the thrower, are standing on a rotating platform.
The spacecraft, launched from Earth, would actually go into an orbit around the Sun. The craft would then intercept the orbit of Mars about eight months after launch. Entry into the Martian atmosphere is tricky. Numerous unmanned missions have failed during this crucial stage, because so many things can go wrong: electronic glitches, lack of fuel, spacecraft speed, sandstorms, faulty trajectory, and rock outcroppings.
Another big problem is the communications lag between Earth and Mars. The radio time between the Earth and the Moon is only about 1.5 seconds. It can take up to about twenty minutes to send a radio call to Mars, and another twenty to get a response. So if the astronauts make a call to NASA for help or with a question, it could be about forty minutes before they get an answer, and that’s if NASA answers as soon as it gets the message.
Mars is no place to spend vacation time. It is a hostile environment, with an atmosphere that is almost all carbon dioxide and so thin it holds no heat. Average temperature at the equator is −50°F. The atmospheric pressure is too low and the temperature too cold for regular liquid water to exist.
My guess is that we will continue to explore Mars with unmanned spacecraft for many years to come. Since 1960, according to the Planetary Society, almost forty craft have been sent to Mars. The Mars Reconnaissance Orbiter, sent in August 2005, mapped Mars from low orbit. The Mars Phoenix Mars Lander, launched in 2007, touched down on Mars in May 2008. It landed near one of the polar ice caps and dug down 1.5 feet searching for ice and water.
In 2011, NASA sent a sophisticated Mars Science Lab rover vehicle to the Red Planet. On August 6, 2012, NASA successfully landed Curiosity in Gale Crater on the surface of Mars. NASA asked the general public to name the spacecraft. Its website received hundreds of possible names, including Adventure, Pursuit, Vision, Wonder, and Perception. A sixth-grader in Kansas submitted the winning entry, Curiosity. Clara Ma wrote, “Curiosity is the passion that drives us through our everyday lives. We have become explorers and scientists with our need to ask questions and to wonder.”
If you ever get an opportunity to peer at Saturn through a telescope, even a small one, you will be looking at one of nature’s most majestic sights. You will be inspired, as Galileo was in 1610, when he became the first person to observe the beautiful ring structure of Saturn.
Four robot spacecraft have visited Saturn. The latest, named Cassini, went into orbit around Saturn in July 2004. Cassini sent back thousands of color pictures of the rings and moons of Saturn, and it carried a detachable vehicle, Huygens, which parachuted onto the surface of Titan, a moon of Saturn. Stunning pictures of this alien world were sent back to Earth by the wheelbarrow-size Huygens.
There are between five hundred and one thousand rings, with gaps in the rings, and they are mainly composed of ice, along with some rock and dust. Collectively, they are 175,000 miles wide but only about three hundred feet thick. The smallest particles are smaller than a grain of sand, and the largest are about the size of a bus. Each little particle, no matter what its size, could be thought of as a moon.
Saturn has sixty-two moons, some major and some minor. Some of the minor moons are only a mile or two across. Titan is the largest moon and is much bigger than our own Moon.
We know that the Moon pulls on the Earth and the Earth pulls on the Moon. It is that gravitational tug on each other that causes the tides here on Earth (see question 97). But tidal forces affect solid objects as well. When an object, such as a Moon, gets too close to the planet it orbits, the tidal forces will tear that object apart and shatter it into thousands of pieces. A mathematical rule, known as the Roche limit, determines how close an object can get to a planet without being torn apart. The Earth’s Roche limit is about 12,000 miles. Not to worry, though; our Moon is 240,000 miles away.
Saturn is almost one hundred times more massive than our Earth, so its Roche limit extends out quite far from the planet’s surface. Billions of years ago, moons, rocks, comets, and any other debris that got too close to Saturn were pulverized and trapped in orbits, which formed the rings. Even today, asteroids and other objects are continually bombarding the solar system, and any of that stuff that comes too close to Saturn gets caught in the ring system.
The other gas giants, Jupiter, Uranus, and Neptune, also have rings around them, but their rings are quite faint and not easily seen. They are more like wispy circles. Saturn has the most material inside that Roche limit, and hence has the most elaborate set of rings.
There are many gaps in the rings. The biggest gap is the Cassini Division. From Earth, it looks like a thin black gap in the rings. It was first seen in 1675 by Giovanni Cassini from the Paris Observatory.
Not very close. The temperature on the surface of the Sun is about 10,000°F. Compare that with your typical commercial pizza oven of around 700°F.
The Sun is 93,000,000 miles from the Earth. If you drove a car at 65 mph, without stopping, it would take 160 years to get there. The Sun is 400 times farther away from us than the Moon.
If you used an aluminum spacecraft, you could get to within 8 million miles of the Sun. Proportionally, that’s equivalent to being on the nine-yard line in football. The temperature would be about 1,220°F, the melting temperature of aluminum. The trick to getting close to the Sun is to radiate or reflect heat as fast as it is absorbed. One way to do this is to paint the side of the spacecraft toward the Sun silver or white. Paint the side away from the Sun a black color. White reflects and black absorbs. Another device is to make the spacecraft long and thin, like a needle, and point the “needle” toward the Sun. That technique will dump more heat. Less surface area of your craft is exposed to the Sun. You’re up to the twelve-yard line . . . and sweating. A third mechanism is to coat your spaceship with the kind of tile that was used on the space shuttle. That reinforced carbon-carbon heat shield will withstand temperatures of up to 4,700°. Now you can get to about 1.9 million miles from the Sun. You’ve made it to the two-yard line; you are in the red zone.
Getting any closer would be a hellish trip, no pun intended. Cosmic radiation on that two-yard line would kill passengers inside the spacecraft within hours.
The Mercury Messenger spacecraft was launched by NASA in 2004. It was a daring and complex mission, in which the refrigerator-size vehicle made one pass by Earth, two passes over Venus, and three passes by Mercury before going into orbit around Mercury in March 2011. Mercury Messenger came within 30 million miles of the Sun. That’s not even to the fifty-yard line, to use our football analogy. Mercury Messenger employed a large ceramic cloth sunshade. The Mercury Messenger spacecraft photographed the entire surface of Mercury, studied the soil composition, mapped the magnetic fields, and tested for any atmosphere. In November 2010, NASA announced the finding of water, ice, and organic compounds in permanently shadowed craters near Mercury’s North Pole.
But for humans, getting really close to the Sun is not practical or wise. We can just lie out in the backyard and get a real nice sunburn in less than an hour.
It takes about five months, or 150 days, to get to Venus, the planet named after the Roman goddess of love and beauty. A straight, direct shot from Earth to Venus is not the way to go. It would require too much propulsion power and rocket fuel. No country has such a powerful rocket. There is a much easier way. A spacecraft is put into a path, or orbit, around the Sun so that at some later time, the orbit of the spacecraft intersects the orbit of Venus.
Space navigators use Newton’s laws, Kepler’s laws, and a Hohmann transfer technique to get spacecraft to Venus and Mercury and to the planets of Mars, Jupiter, Saturn, Uranus, and Neptune.
Newton’s three laws of motion are the foundation of mechanics in physics. The first law is the law of inertia, the second is the law of acceleration (force equals mass times acceleration, or F = ma), and the third is the law of action and reaction; all are employed in determining the forces, paths, and techniques used in space travel.
Johannes Kepler developed three laws that describe the motion of the planets across the sky. The first and third are vital to space travel. The first is the law of orbits: All planets move in elliptical orbits, with the Sun at one focus. The third law relates the time it takes to complete an orbit versus the distance away from the Sun.
A German engineer and rocket enthusiast, Walter Hohmann, wrote a book in 1925 detailing how a spacecraft can transfer from one orbit to another by using two engine burns at precisely the right time, direction, amount of thrust, and duration.
You can’t go to Venus just any old time. A window of opportunity presents itself every nineteen months. That is when Venus is fifty-four degrees behind the Earth in its orbit. The time of day for launch is also important. To go to Venus, the spacecraft must be launched within a few minutes of 7:40 AM local time. This time places the spacecraft on the launch pad in the precise position (Sun off to the tangent east) to make the initial thrust to put the craft on its way to Venus (see question 91).
Despite the obstacles, nearly thirty spacecraft missions have been sent to Venus by the United States, Russia, and the Europeans. The United States’ Magellan spacecraft used radar to map over 98 percent of the surface of Venus. It started its mission in 1990. The European Space Agency (ESA) launched the Venus Express in November 2005, and it went into orbit around Venus in April 2006. The Venus Express is loaded with scientific instruments and successfully photographed lightning in the clouds of Venus. Japan launched the spacecraft Akatsuki in 2010, and it failed to go into orbit around Venus. Akatsuki is in orbit around the Sun. The spacecraft will be in the correct position in its orbit in 2015 to make another orbit-insertion burn.
Venus is the second planet from the Sun. Because its size, density, gravity, and composition are similar to those of Earth, Venus is sometimes referred to as our “sister planet.” But it is a mean sister. The temperature on the surface is hotter than a pizza oven’s, about 900°F.
Venus is much like a car parked out in the Sun on a summer day. Sunlight comes through the windows and is absorbed by the materials in the interior. The seats, dashboard, etc., radiate that heat as infrared light. Infrared waves are a lot longer than visible light rays, and infrared light can’t go through the window glass. So the heat is trapped in the car and the temperature goes way up. The thick carbon dioxide and the sulfuric acid clouds of Venus are like the glass in the car window. Both represent the runaway greenhouse effect (see question 199).
The air pressure on Venus is ninety times that on Earth. Such high pressure crushed, for example, Russian spacecraft that had landed on the planet. The Russians sent ten probes to the surface of Venus, and while all could be called successful to some extent, the longest time that any Russian lander survived was just over two hours.
Venus’s carbon dioxide atmosphere is denser than the air that you and I breathe. Surface conditions on Venus are horrific. The surface is bone-dry, and the heat of 900°F causes water to boil away. Much of the surface is barren plains punctuated by thousands of volcanoes and four large mountain ranges.
Venus reflects much sunlight off its clouds, so it shows up in the night sky in the west or in the morning sky in the east. It is so bright that gunners in World War II were known to have shot at it, thinking it was an enemy aircraft. Venus is often reported as a UFO.
Venus takes 243 days to rotate on its axis and 225 days to go around the Sun. Hard to believe, but its day is longer than its year. While all the other planets rotate counterclockwise as viewed from above the solar system, Venus rotates clockwise.
The Moon actually does fall. Strange as it may seem, the Moon is falling all the way around the Earth as it orbits our planet. But to get a more complete explanation with deeper understanding, we must look at the concept of forces.
A force is a push or a pull. One of the most common forces is gravity. We know that gravity acts on an object on Earth by pulling it straight down toward the center of the Earth.
However, just because there might be a force on an object, this doesn’t mean that the object will go in the direction of the force. Pretend a bowling ball is moving straight down the middle of the lane. You run up to the bowling ball and give it a sideways kick toward the gutter. The bowling ball does not go straight into the gutter and does not continue in the middle-of-the-lane path it originally had. Instead, the ball changes direction slightly so it has a diagonal motion, and it continues rolling at an angle. This is because the ball already had some forward motion to it before you kicked it, so the force of your kick combined with the preexisting force somewhat changes the ball’s direction.
Now pretend you drop a baseball from the rim of a tall cliff. It will fall straight down. The only force acting on it is gravity. If you throw the baseball straight out, horizontally, it will move horizontally, but at the same time, it will start to fall some. Remember, gravity is still pulling it down. The baseball falls at an angle that constantly changes as the force of the throw diminishes. The path of the ball is an arc, actually called a parabola.
Next time, you throw the baseball harder. It goes farther, falling at a shallower, more gradual, angle. The force of gravity was the same, but your throw gave the baseball greater forward speed, or velocity, so the deflection is much less, resulting in a shallower angle.
If you threw that ball so hard that it traveled about one mile before it hit the ground, it would have to fall about six inches more than before. Why is that? The Earth is curved. So as the ball is traveling out that one mile, the Earth is curving away underneath it.
Now you throw the baseball even harder, perhaps hard enough to make a major league roster! If you throw the ball straight out, so it travels six miles, the Earth would curve away about thirty feet. Throw it sixty miles out, and by the time the baseball drops, the Earth will have curved more than a half mile away.
Finally, you ate your spinach, you feel like Superman, and you throw the baseball so hard that the Earth curves away underneath the ball so much that the ball never gets any closer to the ground. It goes all the way around in a circle and might hit you in the back of the head. You have just put the baseball into orbit around the Earth.
In actual practice, you couldn’t do this at the surface of the Earth, because the baseball would run into too much air resistance. You’d have to get it up quite high, about 100 miles, before you threw that baseball straight out. This is the mechanism we use to put satellites around the Earth. Our natural satellite, the Moon, stays in orbit for the same reason. All objects, in the absence of air resistance, fall at the same rate, so the mass or weight of the satellite does not make any difference. For Earth, the speed you need to throw a baseball or satellite so that the curve of its drop, or fall, matches the curvature of the Earth is five miles a second, or 18,000 mph.
Gravity gets weaker the farther out you go, which is why the Earth’s gravity has a much weaker pull on the Moon than it does on Earth-observation satellites, the International Space Station, and Hubble Space Telescope. Those satellites are about 200 to 500 miles above the surface of the Earth, whereas the Moon is roughly 240,000 miles from the Earth. The Moon orbits the Earth much more slowly than those low-Earth-orbit satellites. The speed of the Moon in its orbit around the Earth is about 2,300 mph. It takes a full month for the Moon to make an orbit around the Earth. Those artificial satellites go around the Earth at a speed of around 17,500 mph, taking only ninety minutes to orbit the Earth.
Here is a special case. If a satellite is put up about 22,500 miles from the Earth, it takes twenty-four hours to orbit the Earth—the same time it takes the Earth to rotate, which means the satellite’s following a geosynchronous orbit, so it stays above the same spot on the Earth (see question 104). So we can use a small dish antenna and pick up those TV channels without having to move the antenna.
Very big! The Milky Way Galaxy is one of billions of galaxies in the known universe. The Milky Way is a barred spiral galaxy—a galaxy with a central bar of bright stars across its center—in what is called the Local Group within the Virgo Supercluster of galaxies. It is estimated that our Milky Way Galaxy contains about 200 billion to 400 billion stars.
The disk of the Milky Way Galaxy is about one hundred thousand light-years in diameter. A light-year is a unit of distance, not a unit of time. It is the distance that light travels in one year. Light travels at about 186,000 miles a second. So multiply the number of seconds in a year, roughly 31,500,000, by 186,000 miles per second, and a light-year comes out to be about 6 trillion miles. That is 6 million million miles, or, written out, 6,000,000,000,000 miles. And the diameter of our galaxy is about one hundred thousand of these!
Our solar system, with the Sun as the primary star, is located about one-third of the way out in one of the spiral arms of the Milky Way Galaxy. If the entire galaxy were reduced to eighty miles in diameter, our solar system would be only 0.1 inch in diameter. We are indeed small stuff in a big place!
Go out in the country, away from the city lights, look up, and note the hazy band of white light running from the southwest to the northeast. You are looking edgewise at the heart of the Milky Way. The Milky Way Galaxy appears brightest in the direction of Sagittarius, which is toward the Galaxy’s center.
Be aware that every star you see with the naked eye in the night sky is in our own Milky Way Galaxy, with one exception. The Andromeda Galaxy in the constellation Andromeda looks like a white, fuzzy patch of cotton candy. The galaxy, also known as M31, can be seen right off the square of stars in the constellation Pegasus. Use any star chart or the Internet to find its position.
The Andromeda Galaxy is one of about fifty galaxies in the Local Group and is a spiral galaxy. It is about three million light-years away. When you look up at the Andromeda Galaxy, the light you see tonight left the galaxy three million years ago, which makes looking at the night sky, in one sense, like being in a time machine (see question 246).
The Milky Way’s center is home to a very dense object believed to be a supermassive black hole. Most every observed galaxy has one or possibly more.
There is a television ad that says the universe has more stars in it than there are grains of sand on the Earth. According to the best estimates, there are one hundred stars for every grain of sand on Earth.
The gravitational pull of the Moon affects tides on the Earth. As the Moon’s gravity pulls on the water of the Earth, the ocean bulges on both sides of the planet. The Earth’s water bulges on its Moon-facing side because it’s being pulled by the Moon’s gravity, leading to a rise in the tide. But the Moon’s gravity also creates a high tide on the opposite side of Earth, because the Moon is pulling on the Earth, too. The inertia of the ocean water causes it to remain at rest, creating this second bulge as the Earth moves away from its water. The solid Earth is being pulled toward the Moon, but liquid water on the far side does not have as much pull or tug.
Points on the sides of the Earth between the two tide bulges have a low tide. Because the Earth takes twenty-four hours to rotate once, any point on Earth will have a high tide followed by a low tide in roughly six-hour intervals. During one twenty-four-hour period, each position on Earth will have about two high tides and two low tides. Because the Moon is moving around the Earth, the actual time between tides is about six hours and thirteen minutes. If the Moon were somehow stopped in its orbit, the time between tides would be about six hours. But during one rotation of the Earth, the Moon has moved ahead into its orbit about one-thirtieth of the distance of one orbit, or about twelve degrees. So the Moon is not in the same position in its orbit as it was twenty-four hours ago.
The Sun affects ocean tides, too. The Sun’s gravity is much stronger than the Moon’s, but the Sun is much farther away from the Earth, so it has a weaker effect on the tide. The Moon’s gravity causes about 56 percent of the tide, and the Sun’s gravity about half the Moon’s effect.
When the Sun, Moon, and Earth are all lined up, gravity’s pull is the greatest. These tides occur twice a month, during the full moon and the new moon. They are called spring tides. Despite the name, they have nothing to do with the season.
When the Moon is in its first quarter and its third quarter, the Moon and the Sun are at ninety degrees from each other, so the gravitational pull of one counteracts the other’s. The lower-than-normal tides that result are termed neap tides.
Tides are also more pronounced once a year when both the Sun and the Moon are closer to the Earth. This happens around January 4 of every year. The Nazis fire-bombed London on December 29, 1940, knowing that there would be little water in the Thames River at low tide, making London’s fire hoses ineffective. (Yes, hoses; they couldn’t rely on hydrants, which required a pumping station, so they literally carried the fire hoses through the mud and placed them in the river, relying on fire-engine-driven pumps to pull the water out of the river.)
The famous beach assaults in World War II, such as in Italy and Normandy and against the Japanese in the Pacific, were planned for high tide. The idea was to get men and material as far inland as possible. Arguably the most difficult and brilliant amphibious landings were designed by General Douglas MacArthur to outflank North Korean troops that had trapped UN forces in the Pusan perimeter. The Inchon landings on September 15, 1950, were tricky because of the tides, seawall, mud flats, and monsoons.
Tides are most pronounced along the coastline and in bays, where there is a unique topography. The tides at the Bay of Fundy between Nova Scotia and New Brunswick have a range of fifty feet between high tide and low tide. An understanding of tides is important for the fishing industry, for coastal navigation, and for the building industry along the coast.
Halley’s Comet is returning in July 2061, about fifty years from now. Edmond Halley was an English astronomer and a friend to the most famous scientist of all time, Isaac Newton. In 1705, he used Newton’s law of gravitation to find the orbits of comets from their known positions in the sky over a period of hundreds of years. Halley determined that the comets sighted in 1531, 1607, and 1682 had almost the same orbit. He used gravitational calculations to predict the return of this same comet in 1758. Indeed, the comet was sighted on Christmas night of 1758, but Halley did not live to see his prediction come true. He had died some sixteen years earlier, in 1742. Halley is honored by having the comet named after him. The last time Halley’s Comet “came around” was 1986. Because of its orbit and position relative to the Earth, Halley did not put on a good show for anyone in North America.
Going back through the records, it has been shown that Halley’s Comet has been sighted almost every seventy-six years as far back as 240 BC. The closest sightings at the time of Jesus were 12 BC and again in 66 AD. So Jesus never saw Halley’s Comet.
When Halley’s Comet was seen in England early in 1066, it was viewed as a bad omen. Later that year, William the Conqueror invaded England and defeated Harold II at the Battle of Hastings. Halley’s Comet is prominently displayed in the famous Bayeux Tapestry that commemorates the Norman king William’s victory over the Saxon king Harold.
Mark Twain said, “I came in with Halley’s Comet in 1835. It’s coming again next year [1910], and I expect to go out with it.” And he did.
A comet has been described as a “dirty snowball.” The nucleus is composed of ice, dust, and frozen methane. The nucleus of Halley’s Comet is potato-shaped, ten miles long and six miles across. When it comes close to the Sun, it develops a temporary atmosphere called a coma (from the Greek word for “hair”), a dense cloud of water and carbon dioxide that surrounds the nucleus. Two long tails, one a dust tail and the other an ion tail, stretch back millions of miles. The tail of dust is created by the comet material being burned off the giant snowball. The dust tail forms a curved path that streams behind the comet in its orbit around the Sun. The ion tail points directly away from the Sun following the magnetic lines of force that emanate from the Sun.
Most comets have cigar-shaped orbits and can only be seen with the naked eye when they are close to the Earth and Sun. More than four thousand comets have been cataloged. Meteor showers occur when the Earth passes through the debris left in the path of a comet. The Perseid meteor shower occurs every year between August 9 and 13, when the Earth passes through the orbit of Comet Swift-Tuttle. Halley’s Comet is the source of the Orionid meteor shower every October.
Once the planets were in motion, nothing was needed to keep them in motion. The best current thinking is that our solar system—the Sun and eight planets we know (nine if you include Pluto, which is no longer officially a planet)—was formed out of a swirling disc of gas and dust some four to five billion years ago. The main body of this mass heated up to a temperature at which nuclear fusion could start. That became the Sun. The outer, smaller clumps of dust and gas in this huge disc formed the planets. Scientists believe that other star systems are formed in much the same way.
So once this rotation is in motion, no force or push or pull is needed to keep it rotating. That neat idea follows Isaac Newton’s universal first law of motion, the law of inertia: An object in motion tends to stay in motion, and an object at rest tends to stay at rest.
The planets move in circles around the Sun. More precisely, the orbits are ellipses, or squished circles, if you will. Venus is the planet with the most circular orbit, and Mercury has the most elongated, or egg-shaped, orbit, if we exclude Pluto.
It is generally recognized that life needs both a source of energy and a solvent. For us Earthlings, the source of energy is the Sun and the solvent is water. In order for life to thrive, the planet must lie within a narrow range of distances from the star or sun it is going around. If the planet is too close to the star, the solvent (water) will boil away. If it is too far away from the sun, the solvent will freeze. For our Sun, the habitable range seems to be between the orbits of Venus and Mars. It just so happens that Earth is nearly halfway between these planets. We hit the jackpot!
Mercury is too close to the Sun for people to live. Any water would evaporate immediately. Jupiter is too far away, and it’s a gaseous planet, so it doesn’t have a solid surface anyway. Mars is not the nicest place to live; it’s too cold most of the time. Venus has dense carbon dioxide clouds dripping sulfuric acid and is about as hot as a pizza oven. Not a nice place to visit! We humans are very fortunate to have this “pale blue dot” to live on.
Short answer: possible, but not probable. However, asteroid, meteor, and comet impacts have to be taken seriously. Natural disasters, such as the recent Indian Ocean tsunami, can kill tens or hundreds of thousands of people. But an asteroid, meteor, or comet collision with Earth could kill millions or even billions of people. A loss of a million people is a terrible disaster; a billion, or a thousand millions, is on another order of magnitude altogether.
The February 15, 2013, meteor blast in the Ural Mountain region of Russia, near the city of Chelyabinsk, just might have been a wake-up call to the power of an exploding meteor. More than a thousand people were injured and 7,200 houses damaged from the shock wave of the blast. The meteorite was estimated to be more than fifty feet in diameter and have a weight about 11,000 tons.
A recent warning from NASA was issued on December 25, 2004. Looking ahead, an asteroid called 2004 MN4, with a diameter of 1,350 feet, has a one in three hundred chance of hitting Earth on April 13, 2029. Astronomers keep tracking some seven hundred of these “near-Earth asteroids.” NASA stresses that the risk probably will be reduced to zero as they “keep an eye” on its orbit.
Two fine movies, Deep Impact and Armageddon, both released in 1998, deal with the subject of comets or asteroids striking the Earth. In both movies, crews in spacecraft are sent to intercept and destroy the invader. If the time ever comes for us to actually need to do something like this, the hope is that we Earthlings will send unmanned devices to do the job.
Scientists have identified more than 200 proven sites of meteor impact on planet Earth. Let’s look at three of the best known.
A comet or asteroid five to ten miles wide struck in the Yucatán Peninsula in Mexico 65 million years ago. This impact is believed to have caused the extinction of the dinosaurs and thousands of plant and animal species. The best proof came when scientists Luis and Walter Alvarez discovered a thin layer of iridium in soil that covers the entire Earth. The metal iridium is rare on Earth but abundant in meteorites.
A second example of meteor impact is the Barringer Crater, which was created fifty thousand years ago when a nickel-iron meteorite 150 feet across slammed into the desert floor in Arizona. The crater is about three-fourths of a mile wide and 750 feet deep. When Europeans first came across the crater, they discovered thirty tons of meteoritic iron scattered around the crater. The force generated by the impact was equal to the explosion of 2.5 million tons of TNT.
In the third instance, a large meteor or piece of an asteroid or comet exploded in the atmosphere in the Tunguska Region of Siberia in Russia on June 30, 1908. The blast devastated an area the size of Rhode Island.
On January 12, 2005, NASA launched the Deep Impact spacecraft, which arrived at comet Tempel 1 on July 4, 2005. The satellite probe smashed into the comet with a force of 4.5 tons of TNT and blasted a crater in it, which allowed scientists to study the composition of this space visitor. NASA scientists determined the ingredients that make up our solar system’s primordial “soup.”
So we’re far along on having the technology to nudge a comet or asteroid into a different orbit and away from the Earth, and it’s possible that we might one day soon be able to blast it with a nuclear bomb or deflect it with lasers.
In summary, an asteroid or comet hitting the Earth? Put this way down on the bottom of your list of things to worry about!
Constellations are groupings of stars that help us make sense of which stars are which. When you look at the night sky, you can count about fifteen hundred stars with the naked eye. Constellations break up the night sky into manageable bits. They help make sense of all those pinpoints of light.
Constellations were named thousands of years ago by different cultures in different lands. Wonderful mythologies and numerous stories evolved around the characters they saw in the starry heavens. The constellations we are familiar with today come from the ancient Babylonian, Greek, and Roman civilizations. There are eighty-eight of them in the night sky. The official boundaries of these constellations were finally established in 1929 by the International Astronomical Union (IAU), the organization of professional astronomers. Every star in the night sky is now in a defined constellation.
In my hometown in Wisconsin, I can see about fifty-three constellations over the course of the year. If you want to see all eighty-eight, you have to live near the equator. At the North Pole, you can see only about half of them in a year’s time.
People in the northern part of the United States can see five major constellations on any night of the year: Ursa Major (the Big Dipper, or Great Bear), Ursa Minor (the Little Dipper, or Little Bear), Draco (the Dragon), Cepheus (the King), and Cassiopeia (the Queen). We refer to these as circumpolar constellations, because they seem to circle Polaris, the Pole Star.
These five constellations are good starting points to learn where other stars and constellations are located. For example, start with the Big Dipper, a part of the larger constellation of Ursa Major. The two end stars in the bowl of the Big Dipper point to Polaris, which is the last star in the handle of Ursa Minor.
There is a whole slew of these memory aids to help stargazers find their way around the night sky. For more information, there are many good books in the library that have star charts. There are also excellent websites to investigate; one of the best is skymaps.com.
Certain constellations are seen during different seasons, so spotting the constellations in the night sky each year is like seeing old friends. For example, Orion (the Hunter), is a winter constellation. As the leaves and temperature fall, go outside early in the evening and look to the east. Up comes Orion chasing Taurus (the Bull) across the night sky.
The names of stars are a blend of Arabic, Greek, and Latin words. One of the first star catalogs was put together by the Egyptian astronomer Claudius Ptolemy, who lived under Roman rule. Ptolemy, in about 100 AD, compiled the names of stars the Greeks had used, along with some Latin words. The Arab world had some very sharp astronomers, so the text was translated into Arabic, then into Latin, and finally into English and other modern languages. The names we use today are derived from that Arabic, translated to English. New star names these days come from the IAU.
Sirius, in the constellation Canis Major, means “scorching” or “searing” in Greek. It is an appropriate name because Sirius, the Dog Star, is the brightest star you and I will see from Tomah.
Castor and Pollux are stars in the Gemini Constellation. Gemini means “twins,” and Castor and Pollux were twin brothers. Castor is generally understood to have been a Greek warrior. Arcturus, in the constellation Bootes, means “bear watcher.” Arcturus follows the Great Bear as it revolves around Polaris, the Pole Star. Our own star, the Sun, simply goes by the name “Sun.” But sometimes it is referred to by its Latin name, Sol.
About fifteen hundred stars can be seen by the naked eye, meaning not using telescopes or binoculars. These stars were all given names way back in ancient times. Constellation names depend on the culture that labeled them. Ursa Major, which we call the Great Bear, is called the Plow, or Plough, in England. The heart of Ursa Major is the seven bright stars that make up the Big Dipper. These seven stars are known in Hindu astronomy as the Seven Great Sages. In Dutch, it is called the Saucepan. In Finnish, its label is the Salmon Net. The handle of the Big Dipper has three stars, and the bowl has four stars. The middle star of the handle is actually two stars that revolve around each other. They are a binary pair. The brighter of the two is Mizar, and the dimmer is Alcor. The ability to see or resolve these two stars, which appear very close together, was often quoted as a test of eyesight. It is in Arabic writings, English literature, and Norwegian prose. The Plains Indians of North America used the binary system as an eyesight test for their young ones.
Try looking for the double star in the handle of the Big Dipper. Choose a moonless clear night when the humidity is low. Take along a pair of binoculars. That’s what I do . . . now!
Yes, the Sun is a big ball of fire, but quite a different kind of fire from a match, fireplace, or bonfire. Instead, it is a nuclear furnace, in which four hydrogen atoms are fused into one helium atom, leading to temperatures in the Sun’s outer layers that are roughly two to four million degrees. The surface is about 10,000°F. Over 600 million tons of hydrogen are converted into helium every second. The missing four million tons of mass are converted into pure energy in the form of heat and the motion of atoms, according to Einstein’s famous equation, E = mc2.
The Sun, ultimate source of all life and energy here on Earth, is nearly a million miles across and is about 333,000 times the mass of the Earth. If we could stand on the Sun, we would weigh about twenty-eight times what we weigh here on Earth. The Sun is a Class G, or yellow, star and is about halfway through its stable part of its life. Scientists who have analyzed the Sun’s light have found about sixty elements present.
Galileo, observing sunspots through his thirty-power telescope in 1610, noticed that the Sun rotates. The Sun is not a solid body like, say, a billiard ball. It is more like a fluid, and all parts or areas of the Sun do not rotate together. The equator takes twenty-five days to rotate once, but the polar regions need more than thirty days to rotate.
The Sun is in a state of equilibrium, remaining the same size by a balance of two forces. Radiation, in which streams of protons are trying to push the gases outward, tries to make the Sun bigger. Opposing this force is the Sun’s tremendous gravity, which is trying to make the Sun smaller. Overall, the inward force of gravity is balanced by the outward force of gas and radiation pressure.
The thermonuclear reactions that occur in the Sun can be created here on Earth by a hydrogen bomb, or H-bomb. The first thermonuclear, or “sun,” bomb was detonated in 1952. When it was discovered that the temperatures created by a fission atomic bomb were four or five times the temperature at the center of the Sun, we knew we were just a step away from a fusion hydrogen bomb. Scientists are now working on fusion power, but it will probably be at least a few decades before we have a practical and economical fusion reactor.
We’ve all seen these beautiful rings around the Moon and the Sun. We called them Moon dogs or Sun dogs when we were growing up on the farm. I don’t know how the “dog” got in there.
High, thin, cirrus clouds contain millions of tiny ice crystals. The ice crystals are kept aloft by rising air currents. Each ice crystal is an elongated hexagonal, or six-sided, shape and acts as a miniature lens. Light enters one crystal face and exits the opposing face refracting, or bending, twenty-two degrees, which corresponds to the radius of the Moon or Sun halo. Most of the ice crystals are about the same size and shape, so the Moon ring is always about the same size. The ring can even appear to have colors such as in a rainbow.
A friend of mine who’s an expert of weather folklore says that a ring around the Moon means bad weather is coming. The high, thin, wispy cirrus clouds that cause the ring normally precede a warm front by one or two days. Normally, a warm front is associated with a low-pressure system, which means stormy weather.
When a low-pressure system is over a region, the air begins to rise. Rising air cools, and the moisture condenses into precipitation and clouds. So a low-pressure system means an increased chance of clouds, rain, and snow. A nice way to remember: L is for low or lousy. A high-pressure system usually brings good weather; H is for high or happy.
Once in a great while, under conditions of rising air currents, the majority of the ice crystals will have a different orientation. Then the ice crystals produce a halo of about forty-six degrees, which makes the rings dimmer than the twenty-two-degree halos. This rare type of ring is seen around the Sun more often than around the Moon.
Not all satellites do—only the ones called geosynchronous. From our Earthling point of view, a satellite in geosynchronous orbit appears to hover over one point on Earth. The satellite is in a high orbit when it circles the Earth once a day, the same amount of time it takes the Earth to rotate on its axis. A receiving dish on Earth can point to the satellite at one spot in the sky, and the dish does not have to move to track the satellite. There are now about a couple hundred satellites in geosynchronous orbit located above the equator.
A set of rules, called Kepler’s laws, determines how far a satellite must be above the Earth’s surface for a geosynchronous satellite. The closer an orbiting object is to the Earth, the faster it goes and the less time it takes to orbit. The space shuttle (when it was in service) and the International Space Station (ISS) are 250 miles high and take about ninety minutes to orbit. A geosynchronous satellite, at 22,200 miles away, takes twenty-four hours, or one day, to orbit. In comparison, the moon is 240,000 miles up and takes about thirty days to orbit.
Early satellite dishes for home use were about eight to twelve feet across. We still see a few of those around. About ten years ago, technicians put up a very powerful satellite transmitter. So the receiving dish could be much smaller, typically about eighteen inches across.
I highly recommend the website heavens-above.com, which shows the exact location of the ISS and Hubble Space Telescope, as well as the positions of the planets. It will show the night sky for your location, discuss meteor showers, and list Iridium flares that you can see, too. Iridium flares are bright flashes of light in the sky seen in the early morning or early evening, caused by reflections of the Sun off the shiny flat antennas of some sixty-six satellites owned by Iridium Communications.
A black hole is a place in Washington, DC, where all our tax dollars go. Just kidding! A black hole is perhaps the strangest object in the universe. It is the remains of a massive dead star that has run out of fuel and collapsed.
There are two main, competing processes that shape stars. Fusion reactions are similar to tiny hydrogen bombs going off and tend to make the star bigger. At the same time, gravity tends to crunch all the solar material to the center. These two forces are balanced throughout a star’s life, which typically lasts for billions of years. The size of a star is determined by this balance between gravity, making it smaller, and explosive forces, making it bigger, which shifts only at the end of a star’s life, when the ultimate fate of any star is determined by its mass.
Here’s what happens to a star the size of our Sun. When nearly all the hydrogen is converted to helium, gravity will dominate and the Sun will collapse, ignite the nuclear ashes of helium, and fuse them into carbon. The Sun will then expand to the size of the orbit of Mars, at which point it will be a red giant. After a few million years, the helium will be all burned out, the red giant will collapse, and the Sun will become a cool cinder, called a black dwarf. Our Sun will never be a supernova. It is just too small.
When all the star’s hydrogen is used up, the fusion reactions stop, and the star’s gravity takes over, pulling material inward and compressing the core. This compression generates heat, eventually leading to a supernova explosion, which blasts material and radiation out into space.
The story is quite different for a star more than ten times the mass of the Sun. A massive star can become a supernova. Once nuclear fusion is done, the collapse doesn’t stop. The star not only caves in on itself, but the atoms that make up the star collapse so there are no empty spaces. What is left is a core that is highly compressed, very massive, and very dense. Gravitation is so strong near this core that light can’t even escape. The particles within the core have collapsed and crushed themselves out of visible existence. The star disappears from view and is now a black hole.
So if we can’t see a black hole, how do we know they exist? By their gravitational effects. Though they’re not visible, we can detect or hypothesize about their presence by studying surrounding objects. Astronomers can see material swirling around one or being pulled off a nearby visible star. The mass of a black hole can be estimated by observing the motion of nearby visible stars.
The core, or nucleus, of Galaxy NGC 4261, for example, is about the same size as our solar system, but it has a mass 1.2 billion times as much as our Sun’s. Such a huge mass for such a small disk indicates the presence of a black hole. The core of this galaxy contains a black hole with huge spiral disks feeding dust and other material into it.
What happens inside a black hole? Much is unknown. When astrophysicists talk about the workings of black holes, they speak a different language: event horizons, singularities, gravity lenses, and ergospheres—strange stuff! And yet, incredibly, Albert Einstein predicted the existence of black holes way back in 1915 in his General Theory of Relativity.
A shooting star actually comes from our own solar system that has only one star, namely the Sun. A shooting star is a meteor that passes through our atmosphere and becomes extremely hot due to friction. It gives off light.
Meteors enter our atmosphere at speeds of between 25,000 mph and 160,000 mph. Those tremendous speeds cause a lot of friction between the meteor and air. The meteor burns with an extremely high temperature and emits light, much like the filament in a lightbulb. We label that streak of light across the night sky a meteor.
They’re called meteoroids when they travel though space. The term “meteor” is used to describe the object when it makes a lighted visible path through the atmosphere. If a piece is big enough to survive these tremendous temperatures and actually hit the Earth, it is termed a “meteorite.” You can find meteorites in museums.
Thank God we have an atmosphere. Not only does our atmosphere provide us oxygen for breathing, but it also acts as a buffer zone, protecting us from meteor impact. Very few meteors reach the Earth’s surface, with most turning into vapor before they hit Earth. It’s a different story on the Moon. The Moon has no air, so it has no atmosphere. And so the Moon’s surface is pockmarked with meteor impacts. The Earth does have some notable craters of its own, however. The Tunguska object was a meteor impact in Siberia in 1908 that flattened trees up to fifteen miles away.
Most meteors are observed when the Earth passes through a part of its orbit where a comet passed before. Comets are debris left over from the formation of the solar system. They have large elliptical, or oval, orbits. Comets swing around the Sun in predictable cycles. Probably the most famous is Halley’s Comet, which visits us about every seventy-six years.
As comets go around the Sun, they shed an icy, dusty debris stream. The Earth passes through this junk, and bits and pieces of the comet ignite in the searing friction of the atmosphere. They become so hot they give off light, hence the name shooting star. Most are dust- or pea-size. They are traveling at thousands of miles per hour. Moving faster than sound travels, they give off a sonic boom.
Meteor hunters go out looking for them. The first recovery from the April 14, 2010, meteor was made by a dairy farmer near Livingston, Wisconsin. He said he was “drinking a beer in his chair” and the meteorite exploded above his house and a piece hit his shed and bounced right next to him. He found it in his driveway. A meteor collector from Illinois paid him two hundred dollars for it. That’s good “beer money.” The meteor, witnessed by hundreds of people in the Midwest, could have been a part of the Gamma Virginid meteor shower that began on April 4. It could also have been a rogue one. The April 14, 2010, meteor was believed to be traveling at 36,000 mph.
The best collection of meteorites in the world is at the Field Museum in Chicago. Visit the Barringer Crater in Arizona, where a meteor one hundred and fifty feet in diameter struck fifty thousand years ago. The crater is about three-fourths of a mile across and more than 500 feet deep. Astronauts used it for training for lunar surface exploration in the 1960s. The largest meteorite found in the United States is the fifteen-ton Willamette Meteorite, found in Oregon.
The rocks and dust that fall as meteors come from the Earth’s passing through the debris of a comet. Meteor showers are an increase in the number of meteors at a particular time of the year. They are named after the star constellation from which most seem to fall. The best-known and most prolific meteor shower is the so-called Perseids, seen in the direction of the constellation Perseus; this occurs around August 12 of every year. A viewer can see several dozen of these falling stars in an hour.
The first known case of a human hit by a meteorite was Ann Hodges in Alabama. In 1954, an eight-pound stony-iron meteorite crashed through her roof, bounced off her radio, and badly bruised her. According to rumors, she went to church that very day!
George Mallory was an English mountaineer who was part of three British expeditions to climb Mount Everest. He and his partner, Andrew Irvine, disappeared on the northeast ridge in 1924. Someone had asked Mallory, “Why do you want to climb Mount Everest?” He reportedly replied, “Because it’s there.” Mallory’s body was found on May 1, 1999.
That answer may also be a good enough reason to travel into space, orbit the Earth, voyage to the Moon, and venture on to Mars and beyond. We have forever looked to the heavens and wondered what was there. Humans have an insatiable appetite to explore.
However, the human race’s first steps into space, in the 1960s, were not motivated primarily by a quest to explore the heavens. Instead, the reasons were mainly political. We, meaning the United States, were locked in a fierce struggle with totalitarian Soviet Communism in what was known as the Cold War. Which system of government was the best, socialism or free enterprise? Who had the best technology? The rest of the world was watching.
The Soviet Union beat the United States into space by launching the first man-made artificial object to orbit the Earth, called Sputnik, on October 4, 1957. The United States responded with Explorer in January 1958.
The Russians had many firsts: the first human into space (Yuri Gagarin), the first space walk, the first space station, first satellite to orbit the moon, and first pictures taken of the far side of the moon.
But the United States caught up with and surpassed the Soviet Union by landing three men on the moon in July 1969. Soon after, space missions became less competitive. The next “first” was when the United States and Russia cooperated in 1975 by having three American astronauts and two Russian cosmonauts link up during Earth orbit.
We may not have realized it at the time, but many good things would come out of that space race. The technology needed for spaceflight has produced thousands of spin-offs that contribute to our national security, economy, productivity, and lifestyle. Foremost might be the Earth satellites used for weather forecasting and Earth Resource Satellites (ESA) to monitor crops, flooding, pollution, insect infestations, crop yields, and the health of forests.
It is difficult to find any area of everyday life that has not been improved by these technological advancements. Microcomputers, design graphics, compact discs, whale identification practices, the development of earthquake prediction systems, air purification methods, smokestack monitoring, devices to measure radiation leaks, scratch-resistant lenses, flat-pane television, high-density batteries, the GPS system, and noise-abatement techniques all result from NASA research.
So why do we explore space? Perhaps it was Edmund Hillary—the New Zealand mountaineer and explorer who, along with his Sherpa guide, Tenzing Norgay, were the first humans to reach the summit of Mt. Everest, in 1953—who said it best: “It is not the mountain we conquer, but ourselves.”