Closer to comets

After its failure to explore Halley’s comet, NASA decided to resume the study of comets, while the European Space Agency was working on the Rosetta project. NASA’s technological probe Deep Space 1, launched in 1998 to test ion propulsion, had recorded an image of a second comet nucleus, that of comet 19P/Borrelly, 15 years after Giotto. NASA launched another probe, Stardust, in 1999.

The Stardust spacecraft approached within 250 kilometers (155 miles) of the nucleus of comet 81P/Wild 2, photographed it and collected in its tail about 10,000 dust grains, braked and trapped in a gel-filled collector. After this operation was finished, the collector was sealed and placed in an airtight capsule. Then the probe began its journey back to the Earth. At an altitude of 110 km, the capsule detached, entered the atmosphere, was slowed by two parachutes and landed smoothly. This was on January 15, 2006, seven years after the launch. After several years of work, the results of the grain analysis were published. The grains exhibited a wide variety of minerals and of extremely heterogeneous organic elements. These were the remains of the interstellar material from which comets were formed, 4.6 billion years ago, in the proto-planetary disk that surrounded the young Sun. Part of this material has undergone significant changes. Certain grains formed of refractory materials have been subjected to high temperatures in the inner parts of  the protoplanetary disk. Other grains formed of more volatile materials were kept at low temperatures in the outermost areas of the disc, and therefore contain interstellar matter little or not processed. The coexistence of these different types of grains in the same comet shows that there has been a vigorous mixing of the material inside the primitive protoplanetary disk.