The 1990s were marked by marvels that scientists are still trying to understand: the discovery of planets outside our solar system ... the mysterious dark energy that appears to dominate our universe ... embryonic stem-cell research that has sparked so much hope and concern over the past decade.
Today's installment of our 50-year science timeline, drawn up to mark the 50th birthday of the Council for the Advancement of Science Writing, focuses on the advances of the '90s. This list isn't set in stone, so feel free to weigh in with your own suggested milestones. We'd also love to hear what milestones you're hoping to see in the next 50 years.
33. Witnessing a cosmic crash: Comet Shoemaker-Levy 9 smashes into Jupiter during one of the most widely watched astronomical events of the century. This was the first time astronomers predicted a planetary impact in advance. The event also had an impact on our own planet, pushing along efforts to catalog near-Earth asteroids and assess the threat they may pose.
34. Quantum computing quest: U.S. mathematician Peter Shor demonstrates a theorem for a procedure that could be used to crack the RSA cryptographic code using a computer based on quantum interference phenomena. Such a quantum computer was discussed in 1980 by Paul Benioff, and a year later by Richard Feynman. Since then, researchers have worked to construct quantum computing devices. In 2007, Canada-based D-Wave said it built the first practical quantum computer, but other researchers doubted whether the device was truly a quantum computer. In 2009 Google announced that D-Wave's technology was being incorporated into its new image recognition system.
35. Math milestones: More than 350 years after Fermat's Last Theorem was proposed, British mathematician Andrew Wiles proves the claim that xn + yn = zn works for whole integers only if n is less than 3. The hard-won proof earns Wiles a $50,000 prize. Eight years later, reclusive Russian mathematician Grigory Perelman proves another long-running puzzle, the Poincare conjecture - but turns down a $1 million prize as well as the Fields Medal, mathematics' highest honor.
36. Alien planets: Astronomers detect the first extrasolar planet circling a normal star, 51 Pegasi. The discovery built upon 1992's detection of "pulsar planets," and pioneered techniques that have been used to find more than 400 extrasolar planets to date. The findings have led scientists to conclude that planets are much more common in the universe than previously thought.
37. First cloned mammal: Researchers announce the birth of Dolly the Sheep, the first mammal to be cloned from the adult cell of another animal. The achievement was followed by a string of other cloned species - ranging from dogs and cats to champion racing mules and rhesus monkeys. Dolly also touched off a long-running political and religious debate over human reproductive cloning.
38. Big bounce on Mars: Mars Pathfinder probe lands on Mars, marking a new era of interplanetary exploration two decades after Viking. Pathfinder blazed a trail for the even more wildly successful Mars Exploration Rovers, Spirit and Opportunity (which were both launched in 2003 and landed, like Pathfinder, cushioned by airbags). The Pathfinder mission also served as an early milestone in public interest in science as mediated by the Internet.
39. Dark energy: Two teams of astronomers studying distant supernovae determine that the expansion of the universe is accelerating, supporting a theoretical twist that Albert Einstein once called the "biggest blunder of my life." The discovery of the acceleration factor has sparked one of the biggest mysteries of contemporary cosmology: What is dark energy?
40. RNA interference: Biomedical researchers Andrew Fire and Craig Mello publish a study showing how small RNA molecules influence genetic pathways in C. elegans worms, opening up a new field of research into RNA interference. RNAi-based therapies could address a wide variety of illnesses, including AIDS, cancer, Huntington's and Alzheimer's disease.
41. Human embryonic stem cells: First human embryonic stem cells are isolated. Such cells can transform themselves into virtually any tissue in the body, raising hopes for new cell-based therapies. Because embryos were destroyed in the process of extracting the cells, the process touched off a years-long ethical and political debate, highlighted by federal funding limits in 2001. In 2007, two teams of researchers used genetic modification to transform ordinary skin cells into cells that appear to function like embryonic stem cells. The use of these reprogrammed cells, known as induced pluripotent stem cells or IPS cells, may resolve the ethical concerns.
Go back to the 1960s ... the 1970s ... the 1980s ... or go ahead to the 2000s. You can also see the full 50-year timeline on CASW's Web site, and you can check out Science's top breakthroughs of the past year. Don't forget to leave your comments below, on the past half-century of discovery as well as the half-century ahead.
The 50-year timeline was prepared in consultation with CASW's board, of which I am a member. Join the Cosmic Log team by signing up as my Facebook friend or following b0yle on Twitter. And pick up a copy of my new book, "The Case for Pluto." If you're partial to the planetary underdogs, you'll be pleased to know that I've set up a Facebook fan page for "The Case for Pluto."