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Prepare to be hit… at 70 km/s

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BIRA-IASB is building the electric field sensors for ESA’s Comet Interceptor mission that will visit a fresh comet entering the inner solar system for the first time. These sensors are a delicate piece of equipment. They are very lightweight, built with Aluminum walls that are only 0.3 mm thick in order to comply with the mass limitations on the spacecraft. However, there is one major question: will the probes survive the comet flyby as it zaps through the cloud of dust particles ejected by the comet, at speeds that may reach up to 70 km/s?
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The Comet Interceptor mission

The European Space Agency is working on the Comet Interceptor mission that will perform a comet flyby. After the successful encounter of Rosetta with comet 67P/Churyumov-Gerasimenko, there is still a lot to learn about comets! While 67P is a comet with moderate activity, that has spent some time in the inner solar system already, it is time to visit a “new” comet, one that enters the inner solar system for the first time.

The spacecraft will wait in a parking spot in space (around the Sun-Earth L2 point) until survey telescopes on Earth (e.g. LSST) detect a new comet when it is still far away from the Sun. At that point, Comet Interceptor will start its cruise phase to a comet encounter point near 1 Astronomical Unit. This will be a flyby mission, in which the relative speed between comet and spacecraft can be anywhere between 10 and 70 km/s.

The COMPLIMENT Langmuir probes

A fresh comet is expected to be very active, with a complex plasma environment, and actually quite different 67P. Comet Interceptor will consist of a mothership and two daughter spacecraft that will be released upon comet approach, enabling multi-point measurements at a comet for the first time.

One of the plasma instruments on the mothership is COMPLIMENT, a pair of electric field sensors. Each of them is an Aluminum sphere with 8 cm diameter, mounted on a 1 m boom. They will measure the electric field and waves that naturally occur in the plasma. The instrument will also determine some of the plasma characteristics. BIRA-IASB builds these sensors, while our colleagues at LPC2E (Orléans, France) and IRF (Uppsala and Kiruna, Sweden) take care of the electronics.

Dust impact risk

The comet will zip by the spacecraft at high speed. Dust particles released from the comet will create hypervelocity impacts on the spacecraft and on the COMPLIMENT sensors. A hypervelocity impact releases so much kinetic energy that it can create significant damage.

We have developed a model that estimates the accumulated damage throughout the flyby. Upon hitting the probe sphere walls, very small particles can create tiny pits (sub-micron to micron scale, 0.001 mm). Typically the impactor and the excavated material vaporize and create a small plasma cloud, which can be measured – that is scientifically interesting, because COMPLIMENT can thus provide measurements of the dust size distribution of these tiny grains.

Impact by larger particles (several tens of microns, > 0.050 mm) may puncture the sphere walls, degrading the probes functioning. Still larger particles may be fatal. The risk depends on:

  • the amount of dust
  • the dust size distribution
  • the flyby speed
  • the flyby distance

The latter will be chosen to keep the spacecraft relatively safe, but one obviously wants to get close to the comet for obtaining detailed images and to sample the entire plasma environment.

Perspectives

Further refinements of the dust impact model are needed to prepare the instrument science operations and to assess the risk of fatal particle hits.

Want to know more?

Snodgrass, C., and Jones, G.H. (2019). The European Space Agency’s Comet Interceptor lies in wait. Nature Communications, 10, A5418. https://doi.org/10.1038/s41467-019-13470-1 Open Acces Logo

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Figure 1 Comet Interceptor mission concept: voyage to a dynamically new comet (Credit: Snodgrass & Jones/Nat Commun)
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Figure 2 Top view of a COMPLIMENT Langmuir probe sphere showing the predicted accumulated damage after a 70 km/s flyby of a comet with a dust production of 1000 kg/s with a closest approach at 1000 km. Click here to see a movie (Credit: BIRA-IASB)