A chance encounter between a recently launched spacecraft and a comet that began disintegrating last month could help us learn more about these icy objects.
Geraint Jones at University College London and his colleagues have calculated that the European Space Agency鈥檚 Solar Orbiter spacecraft will pass behind comet C/2019 Y4 (ATLAS) at a distance of about 30 million kilometres in a matter of weeks.
The spacecraft, which launched on 10 February, may pass through the comet鈥檚 two long tails, allowing it to perform unprecedented studies of debris from the object.
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鈥淎t the end of May, there鈥檚 a chance that Solar Orbiter may cross the ion tail,鈥� says Jones. 鈥淎nd then a few days later, on 6 June, it鈥檒l cross the comet鈥檚 orbital plane, and that鈥檚 where the dust tail is.鈥�
The ion tail of a comet consists of the electrically charged particles, or ions, pushed behind it for many millions of kilometres by charged particles from the sun, the solar wind. The dust tail, meanwhile, comprises grains of dust that have been pushed off the comet and follow its orbit.
Solar Orbiter is equipped with a suite of instruments designed to study the sun, including those to take the first-ever images of its poles. Some of these instruments could also study the tails of comet ATLAS.
ESA was unaware of this opportunity before the craft launched, so is now looking into what can be achieved.
鈥淭he decision should be taken in the next days,鈥� says Yannis Zouganelis, the deputy project scientist for the Solar Orbiter mission. 鈥淚f positive, the measurements would start as soon as possible and last for as many days as needed to cover the entire period of interest.鈥�
The spacecraft is currently in a commissioning phase, testing out its different instruments, and this isn鈥檛 expected to be completed until 15 June 鈥� too late for the rendezvous. But there is a chance some will be all set before that.
鈥淭hree of our instruments will most likely be ready and able to make measurements,鈥� says Zouganelis. 鈥淭hese are the magnetometer, the [radio] waves instrument and the energetic particle detectors.鈥� A fourth instrument, designed to study the solar wind, could also be fully tested in time.
The spacecraft could probe the structure of the comet鈥檚 ion tail and see if a shock wave believed to form as the comet鈥檚 head pushes through the solar wind also passes behind it.
It could also measure the mass of grains in the dust tail, and it could even detect pristine material emanating from the comet鈥檚 broken innards. 鈥淭he ions would be potentially coming from inside the nucleus,鈥� says Jones. It isn鈥檛 thought that any of the material could damage the spacecraft.
Whatever happens, Solar Orbiter looks set to become one of just a handful of spacecraft to pass through the tail of a comet. NASA and ESA鈥檚 Ulysses spacecraft, which launched in 1990, passed through at least three comet tails. However, these crossings went unnoticed until after they occurred. 鈥淣ow we know what to look for,鈥� says Jones.
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