Source: NOVA: "Finding Life Beyond Earth"
In this video segment adapted from NOVA, learn how comets could have delivered important building blocks for life—amino acids—to Earth billions of years ago. Watch video that features real satellite imagery as well as simulations to see how scientists are learning more about the chemical makeup of comets and the early solar system. Learn about NASA's Stardust mission, which brought samples of comet dust back to Earth in 2006 and led to a major find for astrobiology—the discovery of the amino acid glycine in comet material.
Comets are solar system objects that are sometimes called “dirty snowballs” because they are made of ice, dust, and rock. Compared with planets, comets are small—comet nuclei range in size from a few hundred to a few thousand meters. Most comets exist in the outer edges of the solar system, where it is very cold. When a comet travels close enough to the Sun in its orbit, the heat causes materials within the comet to sublimate—to change directly from a solid to a gas. Jets of gas and dust stream out of the comet, creating a thin atmosphere around the comet, called a coma, and a long tail that can extend for millions of kilometers.
Scientists are interested in studying comets because they are time capsules—ancient remnants from the formation of the solar system. Within comets, there may be material that has been unchanged since the comets were first formed. Such preserved raw material is a record of the conditions of the early solar system. By studying comets, scientists can better understand the origins of the solar system and life on Earth. But how can scientists study material from objects located so far from Earth?
NASA's Stardust mission was able to successfully send a spacecraft to collect material from a comet and return it to Earth for laboratory analysis. Launched in 1999, the mission had a seven-year time line that included traveling billions of kilometers, orbiting the Sun three times, collecting particles from a comet, and returning the samples back to Earth. Comet Wild 2 (pronounced Vilt) was chosen as a target because it has only orbited the Sun a few times—and thus is in relatively pristine condition—and its orbit placed it in a favorable location for the mission. The path of the spacecraft enabled a flyby of the comet at a relatively low velocity while the comet was actively outgassing (releasing gas and dust). In order for the spacecraft and the comet to meet at similar speeds, which would prevent the spacecraft or the material samples from getting damaged by high-speed collisions, the speed of the spacecraft was increased using a gravity assist from Earth. This maneuver used the relative movement and gravity of Earth to give an energy boost to the spacecraft while it flew by the planet, altering the spacecraft's path and increasing the size of its orbit. The spacecraft's journey to Comet Wild 2 took seven years—five years to rendezvous with the comet and then two years to return the sample capsule back to Earth.
During the comet encounter, the spacecraft deployed a special grid that was filled with aerogel—a very low-density material nicknamed “frozen smoke.” As the spacecraft flew past the comet at a distance of 240 kilometers (149 miles), tiny particles of gas and dust were captured in the aerogel. The aerogel slowed down the particles gradually so that they were not damaged. Afterward, the grid was folded into the return capsule, which stored and protected the particles. In 2006, the return capsule detached from the spacecraft and parachuted back to Earth with the samples.
Thousands of tiny cometary dust particles were returned to Earth, providing an incredible opportunity for scientists to learn about the origins of the solar system. Although the total mass of the samples is only a thousandth of a gram, the analysis of the samples will require many years of research. A major challenge will be ruling out the possibility of contamination from sources on Earth. The comet samples have already yielded many exciting findings, including the discovery of the amino acid glycine. Amino acids are molecules that are used to build proteins and are essential to life. The discovery of an amino acid in cometary material supports the hypothesis that comets could have delivered some of the raw ingredients for life on Earth and suggests that the basic building blocks for life may be prevalent in space.
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