Comet Interceptor is the first F-class mission of the European Space Agency’s Science Programme. Its main goal is to perform a flyby of a pristine long-period comet (LPC). The selection of such target looks to maximise the chances that this would be a “dynamically new comet”, which are objects originating from the Oort cloud who have seen their orbit perturbed into a cometary one recently and are on their first low perihelion passage. These objects are prime candidates for the study of the composition of the Early Solar system, as they are supposed to have endured without significant volatile depletion since the time of the formation of the Solar System.
The mission will be composed of three spacecraft: a mothercraft and two companion probes that are to be released in the final days prior to the comet flyby. Both the mothercraft and one of the probes are developed by ESA, while the second companion probe will be provided by JAXA. Given that it is unlikely that a suitable candidate is available for a direct transfer, it will launch on a transfer trajectory to a SEL2 orbit, where it will wait for a suitable cometary candidate to be discovered. It will then initiate a transfer towards its encounter with the comet, which it will reach close to its ecliptic crossing and at heliocentric distances between 0.9 and 1.1 au.
For navigating the main spacecraft towards its destination, the spacecraft will use ground based radiometric observations throughout the mission. Also, optical measurements will be used for relative navigation with respect to the comet in the Approach to Encounter phase. The preparation of navigation activities presents unique challenges due to the uncertain nature of the target object. The encounter can present a range of values for different parameters that are relevant for the relative optical, such as relative velocity between target and main spacecraft, Earth distance, Phase angle through the final Approach to Encounter phase and others. As such, knowledge analyses have been performed for a number of missions to synthetic targets to explore the sensitivity of navigation to the conditions of the encounter with the comet.
A second characteristic of this kind of mission is that at the time of departing the SEL2 parking orbit there may remain a high ephemeris uncertainty for the target comet. This is due to the high distance to the Earth at which the target comet needs to be discovered in order to allow the spacecraft time to transfer to the comet’s elliptic plane crossing point. To further complicate the ephemeris prediction, the comet dynamics around perihelium are affected by non-gravitational accelerations on the comet, caused by the sublimation of frozen materials as the comet gets closer to the Sun, sublimation which is the cause behind the formation of comet comas. The uncertainty of the ephemeris prediction will thus gradually decrease as the comet comes closer to its perihelium, and telescope observations are used to further refine its orbit. Also, further refinement is expected once the non-gravitational accelerations start to have observable signatures in the comet motion. This requires that some stochastic manoeuvres are planned through the cruise to incorporate the improvement on the ephemeris.
This paper will present the highlights of the navigation analyses performed by ESOC’s Mission Analysis team in preparation for this mission. The main focus will be the navigation of the comet encounter which has been studied for a set of synthetic targets which attempt to represent the variability of encounter conditions. Both the evolution of prediction uncertainty through the Approach to Encounter and the resulting dispersions at the time of flyby are to be covered. The coupling of this observation campaign with previous comet encounter ephemeris prediction will be explored, along with the presentation of the necessary adaptations implemented to produce an accurate representation of the relative dispersion between the main spacecraft and companion probes around the Comet Encounter.