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.

Plus près des comètes - 4

Quelques grains cométaires capturés dans l’aérogel du collecteur de Stardust. En impactant le gel à grande vitesse (6 km/s), ils y creusent un sillon en forme de cône, à la pointe duquel se trouve le grain (visible par exemple comme un point noir en bas de la deuxième image en partant de la gauche).

Crédit : NASA

Organic matter is abundant in unaltered cometary grains. Aromatic compounds (containing benzene rings) are found there as well of non-aromatic compounds such as hydrocarbon chains. And above all, an amino acid, glycine, was discovered in comet dust, which opens new perspectives for exobiology. This discovery, confirmed in 2015 by the Rosetta probe in the comet 67P/Churyumov-Gerasimenko, suggests that comets could have brought complex organic molecules to Earth.

Some grains of the comet 81P/Wild 2 are very similar to those collected in the atmosphere by airplanes flying at high altitude. They are probably genuine interstellar grains; but how were they incorporated into cometary nuclei?


Plus près des comètes - 3

Le noyau de la comète Wild 2, cible de la mission Stardust. Son diamètre est de 5 km, et il présente des cratères d’impact dû à des astéroïdes rencontrés au cours de son trajet.

Crédit : NASA


Plus près des comètes - 2

La sonde Stardust de la NASA. Elle se déplace vers l’avant gauche par rapport à la comète, et les panneaux solaires sont protégés de l’impact des poussières cométaires par les boucliers parallélépipédiques, dont le principe est dû à Fred Whipple.

Crédit : NASA

In January 2005, NASA launched another probe, the Deep Impact, intended to fly over the nucleus of comet 9P/Tempel 1 and to hit its surface to observe the nature of the material ejected during the impact. Deep Impact carried a 372-kilogram (820-pound) impactor named Smart. Dropped on July 3, 2005, Smart hit the comet a day later, while the probe observed the impact from 500 km away. The impact caused a very important ejection of dust: the nucleus was probably covered with very fine grains. However, it was difficult to infer unambiguously the properties of this surface. Further exploration was needed to truly understand the nature of a comet nucleus.

Plus près des comètes - 5

La sonde Deep Impact. Elle embarque deux caméras et un spectromètre infrarouge pour l’analyse des produits éjectés. L’impacteur Smart est doté de sa propre caméra, qui a fonctionné jusqu’à l’impact. La sonde communique grâce à son antenne orientable.


Plus près des comètes - 6

Le noyau de la comète Tempel 1 photographié par la sonde Deep Impact juste après la collision, le 4 juillet 2005. Sa taille est d’environ 7 km. Les jets de poussières partant de la région touchée diffusent la lumière du Soleil

Crédit : NASA