Introduction

It is basically impossible to summarize the entire history of astronomy and space exploration in just 250 milestones, but I’m not going to let that stop me from trying! I work in a field that has a rich and exciting history. Chronicling that history is a daunting task, but, from the perspective of a space enthusiast who was lucky enough to pursue a career in space sciences, it is an embarrassment of riches. In the last 50 years alone, we have been witness to one of the most profound and important bursts of human exploration in history: the Space Age. People have left the planet (some are living off-planet right now!), and a dozen have walked on the Moon. Using robotic proxies and giant telescopes—some launched into space—we have been able to see, up close, the alien landscapes of all the classically known planets, visit asteroids and comets, and view the cosmos in all its glory.

All of this has been made possible because we have, as Sir Isaac Newton put it best, “stood on the shoulders of giants.” No appreciation for the wondrous discoveries of modern astronomy and space exploration would be complete without a thoughtful consideration of the foundations of modern science and experimentation that were built by our ancestors. Many of their achievements were attained at great personal or professional cost, and many others were not recognized as important until decades—even centuries—later. Where it has been impossible or impractical to recognize the specific individuals responsible for these contributions, I have included entries that at least acknowledge the importance of key groups of people in setting the stage for future achievements. Examples include the sky maps still preserved in the caves of some of the earliest humans; the Sumerians’ contributions to the birth of cosmology 5,000 to 7,000 years ago; the still-mysterious Stone Age civilizations responsible for constructing ancient sky observatories like Stonehenge; the careful chroniclers of celestial events from the Chinese Xià, Shāng, and Zhōu dynasties (2100 BCE–256 BCE); and the various schools of mathematics and astronomy from early Egyptian, Indian, Arab, Persian, and Mayan societies that have exerted such a strong influence on modern astronomy, astrophysics, and cosmology.

It is, of course, possible to identify and recognize specific individuals who have played critical roles in the development of scientific thought in general, or physics and astronomy in particular. No accounting of the history of the science upon which modern astronomy is based would be complete, for example, without mentioning the lasting contributions of ancient philosophers, mathematicians, and astronomers, such as Pythagoras, Plato, Aristotle, Aristarchus, Eratosthenes, Hipparchus, and Ptolemy. In more recent times, scientists like Nicolaus Copernicus, Galileo Galilei, Johannes Kepler, Isaac Newton, Albert Einstein, Edwin Hubble, Stephen Hawking, and Carl Sagan have all become household names, famous for incredible advances in modern physics, astronomy, and space science. These giants are featured prominently in many entries in this book.

But many others—perhaps famous only in textbooks—have been responsible for important advances or discoveries, and their works also represent critical milestones. These eminent scientists include Christiaan Huygens, discoverer of Saturn’s “thin, flat ring” and large moon, Titan; Giovanni Cassini, who discovered Jupiter’s Great Red Spot, Saturn’s moon Iapetus, and the true nature of Saturn’s rings; Edmond Halley, whose eponymous periodic comet returns to the inner solar system every 76 years; the last of the pretelescopic giants of astronomy, Tycho Brahe, whose data enabled Johannes Kepler’s discovery of the laws of planetary motion; Charles Messier, a prolific comet hunter who first catalogued more than 100 of the most famous nebulae in the sky; the mathematician Joseph-Louis Lagrange, who predicted the existence of the special gravitational balance points in space that are now named after him; William Herschel, the discoverer of Uranus and several of its moons; the spectroscopy pioneers Joseph von Fraunhofer, Christian Doppler, and Armand Fizeau, who provided the foundations for astronomers to measure both the compositions and velocities of celestial objects; the discoverers of radioactivity, Pierre and Marie Curie, and their colleague Henri Becquerel; the inadvertent father of quantum mechanics, Max Planck; Harlow Shapley, one of the first astronomers to truly grasp the phenomenal size of the Milky Way; liquid-fueled rocketry pioneer Robert Goddard; the astrophysicist and “cosmic web” codiscoverer Margaret Geller; and Eugene Shoemaker, the planetary scientist who helped recognize the importance of impact cratering on our planet and others. Thus I have tried to chronicle these and other important contributors to the advancement of astronomy, astrophysics, planetary science, and space exploration who might not have quite reached the pinnacle of scientific fame among the general public.

And then there are the forgotten—or, at least, the undeservedly neglected—men and women who have made discoveries, developed new theories, performed paradigm-shifting experiments, or simply slogged away to find some critical scientific needle in a haystack, but who, for whatever reason, have not received the public attention or scientific accolades that befit their contributions. These more obscure geniuses include the sixth-century Indian mathematician and astronomer Aryabhata; the venerable eighth-century calendar expert Bede of Jarrow; the tenth-century Arabic star mapper ‘Abd al-Rahmān al-Sūfī; the heretical Giordano Bruno, burned at the stake in 1600 for asserting the existence of other habitable worlds; the seventeenth-century Danish astronomer Ole Røemer, who made the first accurate measurement of the speed of light; English astronomer and predictor of the 1639 transit of Venus, Jeremiah Horrocks; German physicist Ernst Chladni, who correctly deduced the extraterrestrial origin of meteorites in 1794; Arthur Eddington, a British astrophysicist who was among the first people to understand the insides of stars; and American radio engineer Karl Jansky, who in 1931 had an idea for an experiment that led to the invention of radio astronomy.

The unsung also include a number of extremely influential female astronomers who often had to work harder than their male colleagues to overcome the biases and prejudices of a male-dominated field. These noteworthy women include Caroline Herschel, younger sister of William Herschel and an accomplished late-eighteenth-century British comet hunter and star mapper; the world’s first female professor of astronomy, Maria Mitchell; and the early-twentieth-century female “human computer” colleagues at Harvard, including Annie Jump Cannon and Henrietta Swan Leavitt, who developed the classification scheme for stars still widely in use today and who discovered so-called standard candle stars used to estimate distances in the universe. I’ve tried to mention many other important but often overlooked astronomers, physicists, philosophers, and engineers throughout this book, though I’m afraid I’m still not giving them the credit they deserve. As a professional astronomer and planetary scientist, I’m embarrassed to admit that even I hadn’t heard of some of these amazing scientists prior to doing the research for this book.

I noticed partway through the research that the number of individuals being singled out for mention was decreasing over time, especially in the entries after the 1950s—the start of the Space Age. This reflects, in my opinion, a recent trend in astronomy and space exploration—and perhaps all scientific fields. Science and exploration used to be fairly individualistic enterprises, usually practiced by wealthy men who worked alone, often under a monarch or patron of some kind and often in fierce competition with other wealthy gentleman scientists. There were exceptions, of course: notable collaborations (such as that between Tycho Brahe and Johannes Kepler, or among Pierre and Marie Curie and Henri Becquerel) and research groups (for example, al-Tūsī’s thirteenth-century research team at the Marāgheh Observatory in Iran, or the sixteenth-century Kerala school of mathematics in India) certainly existed. But overall, before World War II, most of the scientific advances in my field were primarily made by individuals.

In contrast, as technology advanced in the latter half of the twentieth century, more and more advances in physics, astronomy, and space exploration began to fall under the realm of what many now call Big Science. Big Science is a group or team enterprise; individuals have expertise in specific parts of the project, but the project spans such a wide range of disciplines that no one team member is expert in all of it. An early relevant example in physics was the US Army’s Manhattan Project of the 1940s, aimed at developing the first atomic weapons. Experts were needed with engineering, materials, and aeronautics expertise, and the army also needed to find scientists who were the world’s leaders in understanding nuclear reactions at extremely high temperatures and pressures. Of course, many of those scientists were astronomers who had been, just a few years earlier, developing those skills by figuring out how stars shine. Other early Big Science projects that relied on teams of individuals with astrophysical or space science expertise included the development of military radar systems and of rockets, such as intercontinental ballistic missiles for suborbital flight and Earth-orbiting satellites for military and civilian use.

The civilian history of astronomy-related Big Science has been dominated by the achievements of the US National Aeronautics and Space Administration (NASA), formed in 1957. This book is chock-full of NASA milestone achievements in human and robotic space science and exploration, and very few of those achievements can be directly associated with an individual. Indeed, my own experience with NASA robotic astronomy and planetary science missions—using the Hubble Space Telescope or working with instruments on orbiters around the Moon, Mars, and asteroids and on the Mars rovers Spirit, Opportunity, and Curiosity—has reinforced my realization that most cutting-edge modern astronomy and space exploration work requires large teams of people to succeed. The ranges of expertise required are impressive. A Mars rover mission, for example, requires planetary scientists (including physicists, chemists, mathematicians, geologists, astronomers, meteorologists, and even biologists), computer scientists and programmers, an enormous variety of engineers (including those specializing in software, materials, propulsion, power, thermal, communications, electronics, systems, and others), and management, financial, and administrative support staff. Similar ranges of expertise are needed to build, launch, and operate space telescopes, space shuttles, big particle detectors and colliders, and the International Space Station (by some estimates, the most expensive and complex project ever attempted by humans). Further, these kinds of Big Science projects can each cost hundreds of millions to tens of billions of dollars or more over their lifetimes. Individuals are usually not singled out when these kinds of projects succeed or fail, because the collective efforts of the team were required to get the job done. The Soviet Union’s success in space exploration projects in the 1960s and 1970s was the result of a similar team-oriented (though more military-run) formula. Recently, the nineteen-nation European Space Agency and the nations of Canada, Japan, Brazil, South Korea, India, and China have become bigger players in international astronomy-oriented Big Science projects in addition to smaller astronomy and space exploration projects of their own.

Just as challenging as identifying key individuals has been identifying key events in the history of astronomy and space exploration. Some, like the formation of the Earth and planets, or the first humans in space, or the first humans to land on the Moon, are no-brainers. But most events fall into a continuum of importance that varies from one person to the next (more on that in a bit). Pinning down the exact dates for some of these events and putting them into a simple chronologic listing is also difficult, either because they are only best guesses of prehistoric occurrences (for example, when life emerged on Earth), or because they occurred over a wide span of time (for example, the formation of the first stars and galaxies), or because they are predicted to occur at some uncertain future time—such as the end of the universe! In cases where the chronological timing of key events is uncertain or broad, or both, I have indicated uncertainty by placing a c. (the Latin abbreviation for circa, meaning “about”) in front of the date listed.

The timing of historical and especially of modern events is usually much more accurately known, but there is still a significant challenge in identifying which events—out of a seemingly infinite number of scientific discoveries, theories, inventions, and missions in astronomy and space exploration over the last few centuries, and particularly over the last 50 years—to include in such a compendium. It is perhaps inevitable, then, that a bias would creep into any attempt to focus on just a subset of these incredible achievements, and I will be the first to confess that such a bias exists in my own compilation of milestones: I am a solar-system snob. My passion at work is to study planets and moons and asteroids and comets—what, to many other astronomers, are effectively just little bits of leftover debris that didn’t happen to fall into the newly forming Sun 4.5 to 5 billion years ago. It’s true that the Sun is 99.86 percent of the mass of the solar system (and that Jupiter is most of the rest), but it’s also true that the remaining 0.14 percent is incredibly interesting—partly because life developed and thrives on at least one speck of that debris, and may have existed (or perhaps still exists) on others. When my astrophysics or cosmology friends lament the fact that I have to focus my research on such insignificant, nearby objects, it’s easy to counter with the fact that the latest discoveries in extrasolar planet research are showing that solar systems are probably common around other stars, too. Our solar system may be one of millions—or, more likely, billions—in our galaxy. And yet we do not know if any of them harbors life, as ours does. That makes us very special, even if we are very small.

As you voyage through this history of astronomy and space exploration, you may detect this bias, among my collection of milestones, toward discoveries and theories and adventures related to our nearest neighbors in space: our solar system. To me it’s a good bias to have, partly because solar system objects are what we know the most about scientifically, and partly because it is important to get to know one’s neighborhood in order to understand and appreciate the bigger community. That is, the physics and chemistry and celestial mechanics and geology and spectroscopy and engineering and other skills needed to explore our solar system—with telescopes, robotic spacecraft, high-speed computer simulations, cutting-edge laboratory experiments, or human exploration crews—provide the foundation for exploring our neighboring stars, our Milky Way galaxy, our nearby galaxies, and the cosmos, now or in the distant future. To me, these pivotal moments are most worthy of being called milestones in space exploration: when a point of light is resolved into a truly unique world (and there are more than 50 relatively large and millions of smaller unique worlds in our neighborhood), or when we visit these worlds for the first time, either virtually through the eyes of our robotic emissaries or in person. Our solar system is sort of like our playground. By getting to know the worlds around us, we are dipping our toes into what Carl Sagan famously referred to as the “shores of the cosmic ocean,” preparing for that time when we will someday wade farther out into the water.

Finally, it’s important to point out that this collection of milestones in the history of astronomy and space exploration is certainly not exhaustive or complete. Pragmatic limitations on the length and size of this book restricted the collection to just 250 entries, representing only a fraction of the people, historic discoveries, and paradigm-shifting events that have characterized this exciting field over the course of time—indeed, over the entire history of space and time. Different authors might certainly have assembled a different set of milestones, but all would have faced the same dilemma: how to decide which ones not to include? When I set about outlining this project, I decided to try to cover not only the many phenomenal achievements of the Space Age but also to include and acknowledge a sampling of the many fundamental achievements from scientists of antiquity, spanning the ancient empires of Mesopotamia, China, India, Egypt, Europe, and the Americas. As well, I wanted to make sure that some of the major achievements from the Middle Ages, Renaissance, and more recent history, from preindustrial times to the Industrial Revolution, were also captured. In attempting to balance the timeline, I may have shortchanged many deserving people, discoveries, or events from more modern times, for which I ask your forgiveness and indulgence. As I wrote in the beginning, it is basically impossible to summarize the entire history of astronomy and space exploration by choosing just 250 milestones. But let’s not let that stop us!