ESA has given the green light to launch a triple spacecraft mission designed to make the first flyby of a comet visiting the inner solar system for the first time. Part of the space agency's Cosmic Vision Programme, the "Comet Interceptor" mission will be set in a standby orbit, where it will wait for the discovery of an incoming comet to intercept.
Odd as it may seem to those of us of a certain age, deep space missions to visit comets have become a bit old hat. The first was ESA's Giotto probe that flew by Halley's Comet on March 13, 1986, but there have been many others since then, including a landing on comet 67P/Churyumov-Gerasimenko in 2016.
However, what all of these comet missions have in common is that they were all visits to what are known as short-period comets. That is, comets that came into the inner solar system, were captured by Jupiter's gravitational field, and set on new trajectories where they circle the Sun in highly elliptical orbits with periods of years, decades, or even centuries.
The plus side of this is that astronomers can predict the return of these comets and make detailed examinations of them from Earth or arrange unmanned probes to explore them. The minus side is that these comets have been baked and re-baked by their close encounters with the Sun so many times that their characteristics have been sharply altered.
What scientists want is to study a new comet because it would be very old. If that sounds like a paradox, what they really want is what they call a pristine or "dynamically new" comet. That is, one that has just been knocked out of its orbit in the Oort cloud on the far edge of the solar system where it has sat for the past five billion years and is heading in to visit the inner system for the first and maybe only time.
According to ESA, the problem is that sending a probe to such a comet means preparing a mission the objective of which hasn't been discovered yet. That seems a bit crazy and 20 years ago it would have been, but the development of new all-sky survey instruments like Pan-STARRS and the Large Synoptic Survey Telescope provide space agencies with an unprecedented early warning system for incoming comets.
But that still leaves the problem of getting the mission ready. It's for this reason that Comet Interceptor has been designated an F-Class, or "fast" mission, by ESA. This means that the new mission can go from green light to launch in eight years. Since the total mission will weigh in at under 1,000 kg (2,200 lb), it can act as a ride-along on an M-Class or "medium" mission. In this case, the agency's exoplanet-studying Ariel spacecraft in 2028.
Comet Interceptor will consist of three independent modules, A, B1, and B2, which will launch attached to one another. Each one is designed to study the target comet from a different perspective. The A module carries a high-resolution camera, a multispectral infrared instrument, and one for studying dust, magnetic fields, and plasma. B1 will be built by JAXA and will have a hydrogen imager, another plasma instrument, and a wide-angle camera. And B2 will map the comet nucleus, as well as carry a mass spectrometer and coma mapper.
The plan is that, still joined together, the three modules will be sent to the stable Sun-Earth Lagrange point L2 1.5 million km (932,00 mi) from Earth. There they will park until a target comet has been identified and its trajectory plotted. The modules will then proceed on an intercept course under their own propulsion before separating weeks before the flyby encounter.
The hope is that by studying such comets, a better understanding of the early solar system can be gained. In addition, such a mission could also intercept objects from outside the system, like 'Oumuamua, which flew by in 2017.
"Pristine or dynamically new comets are entirely uncharted and make compelling targets for close-range spacecraft exploration to better understand the diversity and evolution of comets," says Günther Hasinger, ESA's Director of Science. "The huge scientific achievements of Giotto and Rosetta – our legacy missions to comets – are unrivaled, but now it is time to build upon their successes and visit a pristine comet, or be ready for the next 'Oumuamua-like interstellar object."
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