Space Missions to Comets

Among the first proposals for a space mission to a comet was a big NASA/ESA mission sending a probe to a rendezvous with comet Tempel-2 and another probe to flyby Halley's comet, the first comet proven to fly on a closed orbit. While NASA had to drop out of the common project, ESA picked up their end and made it her own flyby mission Giotto to 540 km sunward of the comet Halley nucleus. Soon after, a decision was reached to redirect two Venus missions of Interkosmos to Halley's comet too, as VEGA 1 and 2. The fly-by speeds were near 70-80 km/s. When it was found that the Venus missions could be retargeted in a way to intercept comet p/Halley, many dust-related instruments were hastily added to the payload. Among them were dust counters and the impact mass spectrometers PUMA, many of the instruments developed in close cooperation with those flown on Giotto. Since no dust protection could be added to the space crafts, they had to stay away from the nucleus quite a bit more than Giotto did. Some instruments could be flown only on one of the two sister crafts. VEGA 1 was the first to fly by at 10,000 km and at 79 km/s followed by VEGA 2, next was Giotto at 540 km on the sunward side of the nucleus. At a considerably larger distance two Japanese spacecrafts flew by Halley's comet. During its flyby Giotto was apparently hit by a large dust particle, while all other spacecrafts seem to have survived unharmed. Even before the first results from these missions were presented, NASA had announced her new Comet Rendezvous and Asteroid Flyby mission (CRAF) in 1985. In the development phase it was the sister spacecraft to Cassini of the new Mariner Mark II class. It was fully equipped for its purpose, with all kinds of imagers for all wavelengths from the infrared to the UV, with a whole suite of instruments to analyze the dust and gas released from the comet. Penetrators would be sent to its nucleus to provide the ground truth for the remote sensing instruments. When the cost for the combined CRAF/Cassini program grew out of bound, CRAF was cancelled in 1992 in favor of the Cassini mission, which took advantage of a rare planetary constellation offering a low energy flight trajectory.

In the wake of the CRAF cancellation two developments evolved, which became important for the further in situ investigation of comets: NASA created her Discovery program and ESA redefined her Comet Nucleus Sample

Return mission after NASA had dropped out into a comet rendezvous mission, which was named Rosetta.

Since many instruments on CRAF had reached enough maturity, numerous proposals were put forward centering on former CRAF investigations. As many individual missions cannot have the same science return as the combined mission, and all together cost more, none of them made it. Instead, three other comet-related, genuine Discovery proposals were successful. It had taken 12 years after Giotto for the first, Stardust, to be launched. Stardust is a dust-focused mission, which attempts to do in situ measurements of both, interstellar and cometary dust by means of the dust impact mass spectrometer CIDA and also to return complementary samples back to the Earth for laboratory analysis. A camera and a dust flux monitor are also on board. The second cometary mission selected was CONTOUR, again with a dust mass spectrometer, dust counters and also a gas mass spectrometer. While Stardust is aimed at a very detailed analysis, CONTOUR addressed the diversity of comets when visiting three of them with the identical payload (at different flyby speeds, however). (Unfortunately CONTOUR disappeared after orbit injection and was lost on August 15, 2002). The third cometary mission is Deep Impact. This mission is focused on the attempt to observe the subsurface material of a cometary nucleus. While all other missions rely on the dust and gas, which are released from the nucleus under the influence of the Sun's heat input, the 10.2 km/s impact of a daughter probe (largely made of copper) will create a crater of estimated depth and diameter of 25 and 100 m, respectively. The process will be observed from the Earth and from the mother spacecraft on its flyby with optical instruments covering the infrared and visible (4.8-0.3 |m) light.

Yet another NASA mission has visited a comet: Deep Space 1. Its prime purpose, however, is to test a long list of new technologies for interplanetary space research, among them the long-term use of an ion engine for propulsion, autonomous navigation, and attitude control.

By far the most complex mission is ESA's "comet chaser" Rosetta, launched on March 2, 2004 to Comet 67P/Churyumov-Gerasimenko. It is built around a spacecraft hosting a grand suite of instruments for remote observations of the comet nucleus during the rendezvous (after a 10-year cruise phase), as well as the plasma environment it creates. The payload includes optical instruments from the UV to the IR as well as mass spectrometers for neutral gas ions as well as for solids (dust). A landing device with a complementary set of instruments will be deposited on the nucleus and will take measurements over several weeks. With an instrument on both the orbiter and the lander, one attempts to investigate the interior structure of the nucleus.

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