1961

American astronomer Frank Drake devises a formula to estimate how many civilizations elsewhere in our Milky Way galaxy are capable of communicating over long distances in space. The formula, now known as the Drake equation, requires astronomers to calculate a string of challenging probabilities, such as the odds that intelligent life arises on a planet. Thus far, we know of only one such civilization: our own.

1964

American physicists Murray Gell-Mann and George Zweig propose that the protons and neutrons in an atomic nucleus are each made of a trio of still smaller particles, fancifully named quarks by Gell-Mann. Today, physicists view quarks, electrons, and neutrinos as the basic constituents of matter.

1965

American physicists Arno Penzias and Robert Wilson find a faint glow that pervades the universe: the cosmic microwave background radiation. This glow, which fills space at a temperature of less than 5 degrees Fahrenheit above absolute zero, is strong evidence that the entire universe has cooled down uniformly since its birth in the Big Bang about 13 billion years ago.

1967

English radio astronomers Jocelyn Bell and Antony Hewish discover steady signals from the first known pulsar, a rapidly rotating neutron star that sweeps the galaxy with beams of radio waves and other forms of light. Astronomers have since found more than 1,000 of these dense stellar cinders, which can spin hundreds of times every second.

1974

English physicist Stephen Hawking determines that black holes emit particles and slowly evaporate over time, a quantum-mechanical process now known as Hawking radiation. As a result, in the distant future of the universe all black holes will shrink and ultimately explode when they become very small.

1981

American astrophysicist Alan Guth proposes that the infant universe underwent a period of extraordinarily rapid expansion, called inflation, for less than one billion trillion trillionth of a second. This expansion explains the smoothness of the cosmos on the largest scales. Inflation also suggests that galaxies and clusters of galaxies condensed from quantum fluctuations in the fabric of space-time during the Big Bang.

1987

Canadian astronomer Ian Shelton, working at an observatory in Chile, finds an unexpected flare of light on a photograph of the Large Magellanic Cloud, a nearby galaxy. The new "star" is Supernova 1987A, the brightest supernova seen from Earth in 383 years. Astronomers study the explosion to learn new details about the deaths of giant stars.

1990

American planetary scientist Carl Sagan and his colleagues use theGalileo spacecraft to test whether a space probe can find unambiguous signs of life on a planet--in this case, Earth--without landing on its surface. Sagan also led a team of astronomers and artists who created gold-plated phonograph records containing sounds and images from Earth, to travel aboard the Voyager spacecraft far beyond our solar system.

1993

An American team of planetary scientists, Eugene and Carolyn Shoemaker, and astronomer David Levy find a comet captured by Jupiter's gravity. Astronomers watch in 1994 as Comet Shoemaker-Levy 9, broken up into a long chain of fragments, plows into Jupiter in the first recorded collision of objects in our solar system. The explosive impacts create dark Earth-sized plumes of gas in Jupiter's atmosphere.

1995

Americans Alan Hale, a professional astronomer, and Thomas Bopp, an amateur astronomer, independently discover a large new comet beyond the orbit of Jupiter. Comet Hale-Bopp brightens as it approaches the Sun and becomes visible to the unaided eye during much of 1996 and 1997, making it perhaps the most-watched comet in history.