By the late 1950s, a momentous new development in space exploration had been achieved: at that time it was not only possible to send spacecraft into orbit but also for humans to experience the great void of space at first hand. The space age had begun. It even became possible for people to set foot on another world. Soon scientists were looking beyond the Moon and sending probes to the planets in the remote reaches of the solar system.
The second half of the 20th century saw the first attempts by engineers and scientists to explore the solar system with the use of spacecraft. In 1957 the world was astounded when the Russians announced that their first satellite, Sputnik 1, was in orbit around the Earth. The Russians soon followed this achievement by becoming the first nation to send a living animal, the dog Laika, into orbit. On April 12, 1961, the Russian spacecraft Vostok 1 orbited the Earth in a flight lasting 108 minutes with the cosmonaut Yuri Gagarin (1934–68) on board. Gagarin thus became the first human to enter space.
As might be expected, the Americans were severely taken aback by these Russian space achievements, and in an attempt to claw back the initiative the US president John F. Kennedy (1917–63) announced a very ambitious project to put a man on the Moon by the end of the decade. There followed the so-called space race between America and Russia, with the early honors going to the Russians. In 1959 the Soviets achieved the first fly-past of the Moon, followed by the first hard landing on the Moon’s surface, and then the first orbit of the Moon. During the orbit, the probe took remarkable photographs of the Moon’s far side—it was the first time this view of the Moon had ever been seen because it always presents the same face to us as it orbits the Earth.
The financial cost of the space race was extremely high, and for this reason the Soviets could not hope to stay ahead of the Americans for very long. The Americans soon had satellites orbiting the Earth as well as piloted orbital flights. By the middle of the decade the American Apollo program was well under way. On July 20, 1969 Neil Armstrong became the first person to step on the surface of the Moon. The Apollo program achieved most of its aims, and out of the seven lunar missions (Apollo 11 to Apollo 17) only the ill-fated Apollo 13 did not reach the Moon. Apollo 13 remains as the greatest drama in the early history of space flight, after the dramatic return of the astronauts to Earth following a failure in the command module.
When samples of Moon rock were returned by the Apollo missions and the close-up views of the lunar surface were studied there was a great scientific return for the money spent on the space effort, but there was little of direct commercial value. After the Moon landing the next step in the exploration of space no longer involved piloted missions. Life support systems were very costly and heavy, and it was far more efficient and less dangerous to explore the solar system by means of robot spacecraft and to transmit the findings back to Earth.
Mars was the next target. In the 1960s several of the Mariner missions mapped practically the whole of the surface of Mars and produced strong evidence that the surface had supported sustained water flow at some time in the past. They were followed in the mid-1970s by the Viking landers which touched down on Mars to become the first craft to send back views from the planet’s surface. As early as 1877 the Italian astronomer Giovanni Schiaparelli (1845–1910) had studied Mars through his 20-centimeter (8 in) telescope and discovered what he thought were sets of lines crisscrossing the surface. The American astronomer Percival Lowell (1855–1916) examined this suggestion further, and by the end of the century he had produced an image of the red planet showing a network of canals, built perhaps to carry water from the poles to the Martian desert for irrigation. This imaginative interpretation of the geological features of Mars developed by Schiaparelli and Lowell was soon discredited, but when the Viking orbiter crafts produced the first detailed maps of the Martian surface they found valleys that could only have been created by running water at some time in the remote past. There were also plains and craters on the surface, as well as mountains and extinct volcanoes. One volcano, Olympus Mons, is far bigger at 15 miles (25 km) in height than any others in the solar system. Mars also has canyons even greater than the Grand Canyon in the USA.
At the end of the current decade we will know far more about Mars than we know at present. While Mars has been a primary target for planetary exploration, unfortunately many missions have proved unsuccessful, contact with the spacecraft being lost at launch, en route or crash-landing onto the surface.
However, orbiting crafts and landers are both providing more and more data to piece together the puzzle of Mars. The Reconnaissance orbiter was launched in August 2005 and is now providing more detailed mapping of the Martian surface from orbit.
The Opportunity and Spirit rovers have spent four years exploring Mars and their examination of surface rocks have provided the best evidence yet that Mars was once covered by oceans of liquid water. Each rover has traveled for several miles, and in 2006 Opportunity reached the edge of Victoria Crater, after spending many months exploring the smaller Endurance Crater. The rover had to shelter in a crevice while waiting for a large dust storm to clear. A safe path was found, and Opportunity entered into Victoria Crater. It is hoped that the crater will show evidence of how it was formed, possibly providing clues to the ancient surface history of Mars itself. The far rim of the crater, lying about 800 meters (2,625 ft) away and rising about 70 meters (230 ft) above the crater floor, can be seen in the distance. The alcove in front has been given the name Duck Bay.
The Phoenix lander is currently en route, and will search likely sites for signs of water. Two further Scout missions are planned to arrive in 2013 and 2018 and they will eventually provide much better exploration data. The geological survey of the whole of Mars will be complete within a few years, by which time the scientists will attempt much closer surveys of the more interesting areas. Geologists on Earth want a sample of Martian rock and the Mars Science Laboratory, to be launched in 2009, will help to provide one. It is hoped that a sample can be brought back to Earth by 2014.
Venus was the next planet to be explored by probes. The planet had traditionally been thought to be the one most similar to Earth, albeit with a slightly smaller mass and a much warmer climate. The Russian Venera probes reached the surface of Venus in the 1970s to early 1980s and were able to determine conditions underneath the thick layer of cloud that obscures the planet. The temperature was a searing 470 °C (880 °F), and the atmosphere was 96 percent carbon dioxide and about 4 percent hydrogen. It was also about 90 times denser than the Earth’s atmosphere. None of the Venera probes lasted longer than about two hours, succumbing quickly to the extreme heat and pressure at the surface. There had been much volcanic activity on Venus, producing large quantities of sulfur in the atmosphere. Indeed, the dense clouds that enveloped the planet were found to consist mostly of sulfuric acid. Radar surveys showed many gently rolling hills on the surface, many volcanoes and two “continents.” There were many craters including a huge impact crater on the surface, christened Klenova, which was 88 miles (142 km) across. It was a great disappointment to discover that the surface of the planet of love could hardly have been a more hostile environment.
In 1974 the Mariner 10 probe reached Mercury and went into an orbit around the planet that enabled three close approaches to be made. The probe mapped a remarkable 45 percent of the surface during these orbits. The surface proved to be heavily cratered like the Moon. Unfortunately the planet was no more hospitable than Venus. The very thin atmosphere consisted of hydrogen, helium, sodium, potassium and a trace of oxygen. As a result of the thin atmosphere the daytime temperatures reached about 350 °C (662 °F), but at night they fell to about 170 below zero. Features examined included the Caloris Basin, about 808 miles (1300 km) in diameter and surrounded by a ring of mountains. It was also possible for the probe to measure the length of the day on Mercury. The year on Mercury has long been known to be the equivalent of 88 Earth days, but the solar day turned out to be 176 Earth days—two Mercurian years! The first fly-by of NASA’s Messenger mission, launched in 2004, was achieved in January 2008, with two more fly-bys to come before it enters Mercury’s orbit in 2011.
The development and launch of the very sophisticated Voyager probes in the 1970s meant that exploration of the outer planets of the solar system became possible. The two Voyager probes visited Jupiter in 1979 and Saturn in 1980 and 1981, with Voyager 2 traveling to study both Uranus and Neptune during the late 1980s. These missions were followed by the Galileo spacecraft’s visit to Jupiter, and Cassini’s sojourn at Saturn in later decades. To save fuel, these probes were designed to reach the outer reaches of the solar system by using the gravitational pull of the other planets to produce a sort of slingshot effect, helping to propel them on their journey. Very sharp and interesting images of the gas giants and their satellites (moons) were transmitted back to Earth. Clearly visible on the surface of Jupiter—the biggest planet in the solar system—is the Great Red Spot. This feature is so large that it was seen in the 17th century. It is about 15,500 miles (25,000 km) long by 7, 456 miles (12,000 km) wide—large enough to swallow two Earths. It takes about six days to make a rotation, and although it changes shape over a period of time it has been a permanent feature on Jupiter for over 300 years. It is best described as a colossal hurricane or typhoon. The energy needed to maintain it comes from inside the planet, and it is part of an ever-changing atmosphere around Jupiter.
In 1993 there was great excitement when it was realized that the comet Shoemaker-Levy 9 was approaching Jupiter and that it would strike the planet in 1994. The comet was broken into more than 20 pieces by the gravitational field of Jupiter and the fragments struck the planet between the 16th and the 24th of July 1994. The Hubble Space Telescope and other large telescopes were brought to bear on the event and it was also observed and recorded by the world’s astronomers. The impacts punched dark, giant asymmetric holes in Jupiter’s atmosphere, which took several months to dissipate.
The Galileo probe was able to make a spectral analysis of the atmosphere of Jupiter. It was found to be composed of 86 percent hydrogen and 13 percent helium. The remaining 1 percent consisted of traces of methane, ammonia and water vapor. The Galileo spacecraft dropped a probe into the planet, revealing wind speeds of 373 miles per hour (600 km/hr) and small concentrations of helium, neon, oxygen, carbon, water and sulfur. Although the sample was taken at only one point on Jupiter’s vast surface, it is likely to be typical of the whole planet. Jupiter is one of the large planets known as gas giants. It has no solid surface, being composed of gas and liquid, although it has a rocky core.
The close-up views of the planets’ satellites often proved to be just as exciting as the planets themselves. In the case of Jupiter, its larger moons Io, Europa, Ganymede and Callisto all have interesting features. Io was found to be extremely active volcanically, with molten lava and great plumes of sulfur ejecting from numerous volcanoes and rising to heights of up to 300 miles (500 km) above the surface. Europa by contrast was found to have an icy surface. The images showed brown streaks on the surface typically about 12.5 to 25 miles (20–40 km) wide and the Galileo spacecraft was able to see a cracked surface with the appearance of ice floes, suggesting evidence of liquid water beneath the ice.
Ganymede is the largest satellite of Jupiter and the largest in the solar system, with a diameter greater than that of Mercury. It has an iron-rich core with a permanent magnetic field, a rocky mantle with a thin atmosphere and an ocean of liquid water deep beneath the surface. Ganymede takes only 7.2 days to circle around Jupiter in a synchronous orbit. The probes were able to detect changing magnetic fields and electric currents near the surface, and the most likely cause of this would be an ocean of salt water. There are deep furrows in the icy crust and these can be explained in terms of a system of tectonic plates similar to the Earth’s crust. The evidence for liquid water on Europa is even stronger and the possibility that both Ganymede and Europa could support some form of primitive life has created great excitement.
Callisto is the outermost of the true satellites of Jupiter, and it takes 16.7 days to complete an orbit. It has a thin atmosphere of nitrogen and carbon dioxide, and like Ganymede and Europa it may have liquid water under its icy surface. One of Callisto’s most striking features is a set of concentric rings left behind by a huge impact millions, or perhaps billions, of years ago. The crater was named “Valhalla” and the impact was so great that the outer rings are 1864 miles (3000 km) in diameter, and in some places huge spires of rock up to 100 meters (328 ft) in height have been thrown up. The source of heat to retain liquid water in such a cold part of space is probably created by radioactive decay inside the crust and the mantle of the moons. However, the surface of the planet is very cold; probes have measured 155 °K (−118 °C) at noon and 80 °K (−193 °C) at night. With the advantage gained by the use of space probes, scientists believe Jupiter has at least 63 satellites. This number will doubtless rise when more sensitive probes make the journey to Jupiter and the outer planets. Many of the satellites are not true moons but captured asteroids—identified by the fact that they are irregular ovals rather than true spheres.
The planet Saturn has also revealed some of its secrets to the space probes Voyager and Cassini. The number of known satellites of Saturn is now 60, but only seven of these are large and spherical. New details of the ring system were revealed, showing the small “shepherding” satellites called Prometheus and Pandora confining the particles on what is known as the F band to a fixed orbit. The rings consist mostly of lumps of ice, but some of the particles have a rocky core. The thickness of the rings proved to be a mere 10 meters (33 ft); they are not visible from the Earth when they are viewed edge on.
Saturn’s largest moon is Titan. It was discovered by Christiaan Huygens (1629–95) in 1655. Titan is heavy enough to retain an atmosphere, of which 90 percent is nitrogen, and the rest is mainly methane and other hydrocarbons. The Cassini spacecraft showed that the surface of Titan is partially liquid and free of craters, and that it is still undergoing dynamic changes. There has been much discussion about the possibility of Titan supporting life, due to the presence of complex organic molecules on its surface and in its atmosphere, but one of the main stumbling blocks is the very low surface temperature of only 95 degrees above absolute zero. For the necessary chemical reactions to create life, much higher temperatures are probably needed.
Probes have also visited Uranus and Neptune. In 1986 Voyager 2 found Uranus to be a very featureless world, but the Hubble Space Telescope discovered a system of belts and zones. The rings of Uranus had been discovered in 1977 during an occultation (eclipsing) of a star by the planet. Voyager enabled a further study, showing them to be different from those around Jupiter and Saturn, and perhaps formed more recently. Voyager discovered ten additional small moons including some small ring-shepherding satellites similar to those of Saturn. The most interesting moon is Miranda; the Voyager 2 image shows a world that underwent a shattering collision millions of years ago, Miranda has re-formed into a spherical shape, but with deep valleys and cliffs twice the height of Mount Everest.
The planet Neptune also has a ring system. The largest moon is Triton, discovered in 1846 and with a circular but retrograde motion around its planet. The tidal forces on Triton are so great that in the near future it could break into many small pieces—this would create a very spectacular ring around Neptune to rival those around Saturn.
For many years after its discovery in 1930 by Clyde Tombaugh (1906–97), Pluto was considered to be the most distant planet in the solar system. But in 2006 a formal decision was made to downgrade this icy little world with a diameter of 1,490 miles (2,390 km) and a mass of about 2 percent of that of the Earth from the status of a main planet to that of a dwarf planet. This decision was made in the light of the identification of several new solar system bodies similar in density, orbit and size to Pluto. The largest of these, Eris, was discovered in 2005 by a team led by the American astronomer Michael Brown (b. 1965). With an estimated diameter of at least 1,550 miles (2500 km), it is larger than Pluto. Ceres, the largest of the asteroids in the belt that lies between Mars and Jupiter, is also now classified as a dwarf planet. None of these dwarf planets has yet been visited by any space probe, but this will be rectified in 2015, when the Dawn Mission visits Ceres, and the New Horizons probe reaches Pluto.
One of Pluto’s three moons, called Charon, has a diameter of nearly 1243 miles (2000 km) and is only about 12,426 miles (20,000 km) away, so it may claim to be a double planetary system. Pluto’s orbit passes inside the orbit of Neptune, and it takes 248 Earth years for Pluto to orbit around the Sun.
Beyond the orbits of Neptune and Pluto are many chunks of rock and ice, numbering several billions. These chunks of rock and ice are called comets. Some of them are located in a ringed area called the Kuiper Belt and others are found in an approximately spherical region called the Oort Cloud. When the orbit of one of these comets takes it near the Sun, the Sun’s heat melts the ice, and as it evaporates it forms the familiar spectacular “tail” sometimes visible to the naked eye. The Kuiper Belt was named after the American astronomer Gerard Kuiper (1905–73), who proposed the existence of the belt in 1951. It is believed that there are about 200 million comets in the Kuiper Belt. The Oort Cloud, where most comets are thought to exist, was discovered by the Dutch astronomer Jan Oort (1900–92) in 1950.
The space age has also seen the development of instruments that can be used closer to home but which can be employed to study objects in deep space. In 1990 the space shuttle Discovery launched the Hubble Space Telescope (HST). It was a successful launch, but soon afterward there was great dismay when it was discovered that the 2.4-meter (7.8 ft) objective mirror was flawed and a haze surrounded all of the star images. It was three years before the defect could be corrected, but the mirror was successfully upgraded in 1993 and the telescope was enhanced again in 2002. After the second upgrade the resolution of the telescope was a tenth of a second of arc. This meant that the telescope had the power to see something the size of a 1-centimeter (0.4 in) diameter coin at a distance of 12.4 miles (20 km). The HST is due for a fifth and final upgrade that is planned to extend its life by several more years.
Throughout the history of astronomy, new instruments have led to new discoveries. The same was true for the HST. Free from the Earth’s restricting atmosphere and with clear views in all directions over a wide range of the optical spectrum, the HST was quickly making new discoveries. More detail was seen on planets, and at the limits of observation sharper images were seen of proto-planetary systems around other stars, star clusters, nebulae, galaxies and quasars. The whole sky came under scrutiny, and when other specialized telescopes joined Hubble it was mapped in infrared and ultraviolet wavelengths. In spite of the great cost of putting a telescope into orbit around the Earth, the HST was seen as one way forward for astronomy and cosmology. For centuries the twinkling of the stars, caused by the Earth’s atmosphere, restricted the sharpness of the images observed with earthbound telescopes. And for centuries it was assumed that there was no solution to the problem. Then, in parallel with the development of space observatories, along came the advanced technologies of active optics and adaptive optics applied to ground-based observation. The former technique applies computer technology to adjust the mirror every few seconds according to changes in temperature and to keep the focus of the mirror sharp. The latter uses sensors to follow the observed twinkling of the stars so that the software can minutely readjust the shape and direction of the mirror to correct the variation. At present the ground-based telescopes can resolve to about 0.3 arc seconds, three times coarser than that of the HST, but because they are built on the ground they can be constructed much larger and more cheaply than the HST, and it is only a question of time before they are producing sharper images.
There is a technique used in exploring the heavens called interferometry. This is a method of enhancing the resolution by combining the electromagnetic radiation detected by two or more telescopes. It has been used in the radio waveband for many years, but it is now being applied to shorter-wavelength optical observations taken by telescopes such as the twin Keck Telescopes on Mauna Kea in Hawaii. Each of these telescopes has an array of 36 hexagonal mirrors, all independently moveable, and the combined total is equivalent to a telescope with an objective mirror of 85 meters (279 ft).
The HST was just the first of NASA’s “Great Observatories” in space. It was followed in 1991 by the Compton Telescope which detected hard X-rays and gamma rays from space, and the Chandra Observatory in 1999. These telescopes detect photons from the very highest frequencies of the electromagnetic spectrum. Light at these frequencies is unable to penetrate the Earth’s atmosphere and, therefore, telescopes for detecting them can only operate from above the atmosphere. When X-rays strike metallic surfaces they tend to penetrate them, unless they strike at a very shallow angle in which case they are reflected. Special telescopes have been designed to focus X-rays using concentric nested paraboloid and hyperboloid mirrors, and much of the sky has now been mapped at these frequencies. Gamma rays are even more difficult to focus. They can, however, be controlled using crystals and tiny directional holes called collimators. Fortunately the gamma rays have very high energy levels and they are easy to detect.
The last of the Great Observatories is the Spitzer Space Telescope, launched in 2003, which maps the sky at infrared frequencies. It studies the light from planets, comets and interstellar dust clouds in the infrared part of the spectrum. The wavelength coverage of the space telescopes is augmented by two observatories that detect ultraviolet photons (the Extreme Ultraviolet Explorer [EUVE] and the Far Ultraviolet Spectroscopic Explorer [FUSE]), both launched in the 1990s.
The Sun is the brightest object in the sky and although we know a great deal about it there is still much to learn by studying it from space. The Solar and Heliospheric Observatory spacecraft, or SOHO, was built by a consortium of 14 European countries and it was launched in December 1995 to study the Sun. It has been able to plot temperatures and convection currents inside the Sun at temperatures of up to one million degrees Celsius. It can see right into the core of the Sun where nuclear fusion of hydrogen into helium is taking place. Originally the mission was only expected to last for two years, but such has been its success that it has been continually extended to 11 years so that a complete sunspot cycle can be studied.
From its orbit high in space SOHO has also studied the Sun’s surface and phenomena such as the solar wind, the stream of particles (mainly electrons and protons) that emanate from the Sun. SOHO has also discovered over 1300 new comets. A few of them have elliptical orbits similar to Halley’s Comet, but the majority travel far into space without returning.
The Chandra X-ray Telescope has the distinction of being the heaviest payload ever launched by a space shuttle. It is named after the Indian-American Nobel prizewinner and astrophysicist Subrahmanyan Chandrasekhar (1930–95), who was the first to recognize that there was an upper limit to the mass of a white dwarf star. The Chandra X-ray Telescope was put into an elliptical orbit around the Earth in 1999 and it is still sending back valuable information about the X-ray universe. The X-ray universe provides more information about high-energy particles in space, which can originate from gases at multi-million degree temperatures; or from regions with intense magnetic and gravitational fields, such as occur very close to a black hole. X-rays are well outside the visible spectrum; they need special grazing incident mirrors for focusing, and the images are reproduced in false colors to enhance salient features.
The USA celebrated Independence Day 2005 by making the first contact between an artificial object and a comet. The spacecraft Deep Impact, with a mass about the size of a small car, struck the comet Tempel 1 on July 4 and the event was observed and photographed by many telescopes. The speed of the impact was 25,000 miles per hour (40,234 km/hr). From the impact data, astronomers were able to make deductions about the nature of the comet’s surface, its mass and its chemical composition.
When we hear of the latest developments in astronomy and cosmology it looks increasingly as though the only exciting discoveries still to be made are with the use of space probes and orbiting telescopes or high-budget earth-bound telescopes. There are, however, still opportunities for the amateur astronomer, especially since even quite sophisticated telescopes and detectors can be purchased relatively cheaply.
Amateur astronomers play an important role in the detection of both supernovae and comets, both of which are discovered by painstakingly charting the sky and looking for changes in the Milky Way and local galaxies. For example, one amateur, the Reverend Robert Owen Evans (b.1937), has more supernovae to his name than any other astronomer, and it was the amateur David Levy (b.1948) who co-discovered the comet Shoemaker-Levy on its course to impact with Jupiter.