19 June 2019 ‘Comet Interceptor’ has been selected as ESA’s new fast-class mission in its Cosmic Vision Programme. Comprising three spacecraft, it will be the first to visit a truly pristine comet or other interstellar object that is only just starting its journey into the inner Solar System.
The mission will travel to an as-yet undiscovered comet, making a flyby of the chosen target when it is on the approach to Earth’s orbit. Its three spacecraft will perform simultaneous observations from multiple points around the comet, creating a 3D profile of a ‘dynamically new’ object that contains unprocessed material surviving from the dawn of the Solar System.
“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 unrivalled, 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.”
Comet Interceptor will be a new type of mission, launched before its primary target has been found.
The only way to encounter dynamically new comets or interstellar objects is to discover them inbound with enough warning to direct a spacecraft to them. The time between their discovery, perihelion, and departure from the inner Solar System has until recently been very short, historically months to a year: far too little time to prepare and launch a spacecraft. This timescale is, however, lengthening rapidly, with recent advances allowing observational surveys to cover the sky more deeply, coherently, and rapidly, such as the current Pan-STARRS and ATLAS surveys, and the Large Synoptic Survey Telescope under construction in Chile, LSST (www.lsst.org).
Long Period Comets are now discovered much further away, considerably more than a year pre-perihelion; e.g. C/2017 K2 (Pan-STARRS) was discovered beyond Saturn’s orbit in 2017, and will pass perihelion in 2022. From 2023, LSST will conduct the most sensitive search for new comets ever, providing a true revolution in understanding their populations, and making this mission possible.
Comet Interceptor will be launched with the ESA ARIEL spacecraft in 2028, and delivered to the Sun-Earth Lagrange Point L2. It will be a multi-element spacecraft comprising a primary platform which also acts as the communications hub, and sub-spacecraft, allowing multi-point observations around the target. All spacecraft will be solar powered. The spacecraft will remain connected to each other at L2, where they will reside until directed to their target. The mission cruise phase will last months to years.
Before the encounter, the spacecraft will separate into its separate elements, probably a few weeks pre-flyby. For very active comets, separation will be earlier, to maximize separation of the spacecraft elements, whilst for low activity targets, separation will occur only a few days before the encounter takes place.
The mission’s primary science goal is to characterise, for the first time, a dynamically-new comet or interstellar object, including its surface composition, shape, and structure, the composition of its gas coma. A unique, multi-point ‘snapshot’ measurement of the comet- solar wind interaction region is to be obtained, complementing single spacecraft observations made at other comets.
Additional science will include multi-point studies of the solar wind pre- and post-encounter over gradually-changing separation distances.
The proposed instruments for the main and an accompanying spacecraft are the following:
Spacecraft A: (ESA)
- CoCa: Comet Camera – to obtain high resolution images of the comet’s nucleus at several wavelengths.
- MIRMIS: Multispectral InfraRed Molecular and Ices Sensor – to measure the heat radiation being released from the comet’s nucleus and study the molecular composition of the gas coma.
- DFP : Dust, Field, and Plasma – to understand the charged gases, energetic neutral atoms, magnetic fields, and dust surrounding the comet.
Spacecraft B1: (JAXA)
- HI: Hydrogen Imager – UV camera devoted to studying the cloud of hydrogen gas surrounding the target
- PS: Plasma Suite – to study the charged gases and magnetic field around the target
- WAC: Wide Angle Camera – to take images of the nucleus around closest approach from an unique viewpoint
Spacecraft B2: (ESA)
- OPIC: Optical Imager for Comets – mapping of the nucleus and its dust jets at different visible and infrared wavelengths.
- MANIaC: Mass Analyzer for Neutrals and Ions at Comets – a mass spectrometer to sample the gases released from the comet.
- EnVisS: Entire Visible Sky coma mapper – to map the entire sky within the comet’s head and near-tail, to reveal changing structures within the dust, neutral gas, and ionized gases.
- DFP : Dust, Field, and Plasma (near-match of DFP sensors on spacecraft A) – to understand the charged gases, energetic neutral atoms, magnetic fields, and dust surrounding the comet.