The European Space Agency’s Comet Interceptor lies in wait

The European Space Agency (ESA) recently selected Comet Interceptor as its first ‘fast’ (F-class) mission. It will be developed rapidly to share a launch with another mission and is unique, as it will wait in space for a yet-to-be-discovered comet.

experienced heating on many orbits near to the Sun. Oort cloud objects were scattered to this distant reservoir during the formation and early evolution of our Solar System, and have been preserved there ever since. They are therefore some of the most pristine 'building blocks' from the era of planet formation, and, when finally scattered back into the inner Solar System to feel the warmth of the Sun, produce some of the most spectacular bright comets. The high activity levels, and therefore brightness, of such comets means that they have been popular telescopic targets over the years, but are difficult to explore with spacecraft. However, the primary reason they have yet to be visited is that their appearance is fleeting-they are typically discovered only a few months to a year before they pass perihelion, before returning to the distant outer Solar System, with orbital periods of tens of thousands of years, which mean they will be seen only once by humanity. This has always been incompatible with the timescales to design, build and launch a space mission, which can take decades.

ESA's new fast approach
Comet Interceptor will be developed on a relatively rapid timescale for ESA, to take advantage of extra space available on the launcher of the medium (M-class) mission Ariel, which is due to launch in 2028 9 . Such a timescale is still not fast enough to react to any comet we currently know, but the specific opportunity offered by the F-class call, i.e. a free launch to a halo orbit around the Sun-Earth L2 Lagrangian point, enables an unique approach: to design and launch a mission without a known target, that can wait in space for a suitable comet to be discovered. The L2 point is where the gravitational pull of the Sun and Earth balance to produce a stable point in space 1.5 million km away from Earth, on the planet's night side. It is the favoured parking location for space observatories, as an orbit can be maintained there with little fuel, while keeping a constant distance from Earth and stable illumination. L2 is also the ideal parking location for Comet Interceptor, as both station keeping there and departure for the comet, once it is identified, take relatively little fuel (Fig. 1).
The F-class call also encouraged proposals that would employ a multiple spacecraft architecture, i.e., a small 'swarm' of similar craft, or a 'mothership' that releases smaller probes. Comet Interceptor follows the latter approach, which is also ideal for encountering a DNC: multiple spacecraft mean that the comet will be seen from different angles during the fly-by, building up a 3D picture of the nucleus, coma, and interaction with the solar wind. This will allow differences in time (e.g. due to the changing activity of the comet) and in space (e.g. due to inhomogeneities in the outgassing pattern) to be separated, which has not been possible in any previous fly-by mission, or even with Rosetta, which could only sample one area of the coma at a time. In addition, the mothership and probes architecture of Comet Interceptor solves one of the difficulties in visiting a DNC: the relatively high and unpredictable activity levels. The dust grains in the coma present a significant hazard as the spacecraft pass through at 10-80 km s −1 . Comet Interceptor will be made up of a mothership (spacecraft A, Fig. 2) that will perform a relatively distant fly-by, at around 1000 km from the nucleus, where the dust flux will be manageable and the safety of the spacecraft can be assured, and two smaller probes (spacecraft B1 and B2), which will make closer approaches to perform high-risk/high-return measurements, but may not survive the whole encounter. This is a radically different approach to mission design, and the usual highly conservative approach of spacecraft engineering, that enables scientific measurements that could not previously be attempted. The approach builds on the many recent advances in small spacecraft technology, both in developments for easier access to Earth orbit in the 'new space' industry, and experience in deep space exploration, especially with the highly successful Japanese Hayabusa 2 mission to asteroid 162173 Ryugu 10 . One of Comet Interceptor's small probes, B1, will be supplied by the Japan Aerospace Exploration Agency (JAXA), and will build directly on heritage from Hayabusa 2 (spacecraft A and B2 will be built by ESA).

Intercepting an unknown target
The most unique aspect of Comet Interceptor is the fact that it must be designed without knowing its target, or the precise geometry of the fly-by encounter. The project is possible because large sky surveys are now finding incoming comets with greater warning times, of a few years at least, and the Large Synoptic Survey Telescope (LSST), currently under construction in Chile 11 , is expected to greatly increase our ability to discover comets years before they reach perihelion. Simulations of LSST performance, based on the best current understanding of the underlying population of Oort cloud comets from the Pan-STARRS survey 12 , Launch Wait at L2 Cruise to comet Sun Point of interception of comet's path Fig. 1 Sketch of mission phases. The spacecraft launches from Earth and waits at L2 until a suitable comet is discovered, before departing on an interplanetary cruise to be in the right place at the right time to intercept the comet as it crosses the ecliptic plane. The smaller probes are released just before arriving at the comet. The trajectory of the spacecraft is in green, while the comet's orbit is in red.
suggest that~5 years between discovery and interception is likely, and the target comet may be found before the mission is launched, but it still will not be known before the design must be fixed. This means that the mission must be designed to be as flexible as possible, and able to cope with a wide range of targets and encounter geometries. Such an approach naturally enables a great deal of choice later in the project, and leaves open the exciting possibility of encountering an interstellar object (ISO) like 1I/'Oumuamua. Comet Interceptor would be well equipped to characterise an ISO, as all indications from 1I/'Oumuamua suggest that it is a cometlike body, albeit with a very low dust production 13 . However, the probability of a reachable ISO being discovered during the right period appears to be very low-the lack of observable dust coma around 1I/'Oumuamua means that any similar body would not be discovered until very shortly before its closest approach, even with LSST. The recent discovery of the second ISO, 2I/Borisov, gives slightly more reason to be hopeful-this object looks like a typical comet 14 , and was discovered inbound.
In any case, Comet Interceptor will demonstrate the possibility of this approach, opening the possibility of a dedicated ISO mission to be proposed later in the coming decade, once LSST has provided better statistics on the true arrival rate of such objects. Such a mission would probably need to be of a larger class, with more mass allocated to fuel to enable a rapid response over a wider possible intercept range, but presents a scientifically compelling case-the only way to study material from another stellar system in situ. Additionally, Comet Interceptor will be a proof of concept for the use of a similar 'rapid reaction' spacecraft waiting in orbit for the discovery of close-approaching Near Earth Asteroids, which have been proposed for both planetary defence and commercial resource utilisation approaches.
More information about the mission, and updates on its development, can be found at Comet Interceptor Mission home: http://www.cometinterceptor.space.