Designing the Comet Interceptor comet flyby

The challenge

The Comet Interceptor spacecraft will wait at the L2 Lagrange point until one or a few potential target comets are identified. At some point, a decision must be made to allow enough time for the spacecraft to cruise to the intercept point. 

However, little will be known about the chosen target at that moment. Before selection, a detailed trajectory design phase ensures the flyby is technically possible – confirming, for instance, an intercept point between 0.9 and 1.2 astronomical units from the Sun. While the flyby velocity and geometry will be determined in advance, factors like the comet outgassing rate, the dust production rate, and the solar wind conditions remain uncertain.

The challenge is daunting: the comet flyby can reach relative speeds up to 70 km/s - ten times the speed of a bullet – while conditions near the comet nucleus remain unknown. Fortunately, one key parameter can still be adjusted until just before the flyby: the closest approach distance. 

A close pass would give the cameras a detailed view of the comet nucleus and its ephemeral atmosphere and would allow the instruments to sniff the composition and other details of the neutral and ionized comet environment, but ... then the risk of comet dust impacts is high. 

Senior comet scientists recall very well Giotto’s flyby of comet 1P/Halley in 1986, where a dust particle damaged instruments and knocked the spacecraft out of its proper orientation. A similar event would severely limit the mission’s science return.

Finding the optimal flyby distance

To find the sweet spot between achieving the best science and minimizing the risk, the mission scientists developed a method to find the optimal flyby distance despite the remaining uncertainties in the comet and solar wind properties. 

This is done by simulating a wide range of possible flyby configurations, based on the expected probability distributions for the gas and dust production rates and the solar wind conditions, for different flyby distances, and by assessing both the potential science return and impact risk for each scenario.

As long as the gas production remains below 10²⁹ molecules/s - not more active than 1P/Halley - the optimal flyby distance for the mother spacecraft is a few hundred to 2000 km. The daughter probes are designed for higher dust impact risk and can venture even closer to the nucleus. 

The ideal target comet would have a gas production rate of 10²⁸-10²⁹ molecules/s, a low dust-to-gas ratio, high solar extreme ultraviolet radiation flux, and a slow flyby speed. 

The flexibility offered by tuning the flyby distance is important to be able to do good science for a range of different comet activity levels, as it is unlikely that more than 1 or 2 suitable target comets will pop up in the few years that Comet Interceptor can wait. 

Fortunately, the study shows that the science return and the risk are not highly sensitive to the actual flyby distance (except very close to the nucleus). A navigation precision of a few 100 km is sufficient and technically achievable as Comet Interceptor will refine its trajectory based on images acquired upon approach and will manoeuvre to aim at the desired flyby distance.

Reference

• J. De Keyser, N.J. T. Edberg, P. Henri, H. Rothkaehl, V. Della Corte, M. Rubin, R. Funase, S. Kasahara, C. Snodgrass, Optimal choice of closest approach distance for a comet flyby: Application to the Comet Interceptor mission, Planetary and Space Science, 2024, 106032, https://doi.org/10.1016/j.pss.2024.106032.