There’s been a lot of speculation recently about interstellar visitor 3I/ATLAS - much of which is probably caused by low quality data given that we have to observe it from either Earth, or in some case Mars. In either case it’s much further away that what would be the ideal. But that might not be the case for a future interstellar object. The European Space Agency (ESA) is planning a mission that could potentially visit a new interstellar visitor, or a comet that is making its first pass into the inner solar system. But, given the constraints of the mission, any such potential target object would have to meet a string of conditions. A new paper, available in pre-print on arXiv, by lead author Professor Colin Snodgrass of the University of Edinburgh of his colleagues, discusses what those conditions are, and assesses the likelihood that we’ll find a good candidate within a reasonable time of the mission's launch.
Comet Interceptor (CI), as the mission is known, is a ESA F-Class mission, meaning it's intended to be developed and launched quickly. It will then sit in a parking orbit at the Earth-Sun L2 Lagrange point until a suitable target Dynamically New Comet (DNC) - a type of comet that would be entering the inner solar system for the first time. Alternatively, if the mission gets really lucky, it could also observe an interstellar object as it passes through our solar system on its one-way journey, but the likelihood of that happening within a reasonable distance at the precise time CI is waiting is astonishingly low.
DNCs, though, are more common. The paper notes 132 of them from 1898 to 2023, though they have their own quirks to deal with. Many are extremely faint, and are only discovered a few months or years before their arrival in the inner solar system. That’s where another new mission comes into play - the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory is expected to find many more DNCs than have ever been seen before, and hopefully give enough warning that the CI mission team can analyze a potential target for its suitability.
Fraser discusses the Comet Interceptor mission.But even if LSST does find a reasonable candidate, there’s no guarantee that a comet will brighten to a point that it would be interesting as it gets closer to the Sun. Alternatively, there’s also no guarantee that it wouldn’t simply break apart before CI gets close enough to inspect it. Given that the mission can only choose one potential target, these unknowns add an element of chance large enough to make most inveterate gambler blush.
So it’s better to game theory out the possible scenarios to have a better idea of what to expect when actually selecting a real target. That analysis started with some basic constraints on the mission. There was a limited “delta-v” - the energy required to reach the comet, given the constraints of the spacecraft carrying the necessary fuel out to the L2 point. The authors calculated this at 1.5 km/s - not particularly fast by interplanetary mission standards.
CI would have to intercept the comet somewhere between .9 an 1.2 au - somewhere right around the Earth’s orbital path - and, importantly, it must cross the ecliptic plane where Earth actually is in order to be within range. The spacecraft must also keep the Sun at an angle between 45° and 135° to ensure its solar panels work. And, perhaps most importantly, the fly-by of the comet can’t take place at more than 70 km/s, as the resulting damage from dust could obliterate the smaller probes CI would release to study the comet’s coma. To go along with that, there’s a sweet spot of outgassing, where the target comet must produce enough gas to be interesting, but not enough to destroy the probe. Halley’s comet seems to be a reasonable upper limit on the outgassing required, according to the paper.
ESA's video explaining the Comet Interceptor mission. Credit - ESA YouTube ChannelThe authors tackled their analysis of historical comets from two different angles. First they took a look at the most scientifically interesting examples. Constraints for this approach included only looking at comets on their way into the solar system rather than on the way back out, and having a brightness of magnitude 10, which the authors used as a proxy for activity. After selecting for those, they had nine potential candidates left. But on further inspection, literally none were actually reachable given the engineering constraints of the mission. Most were either too far from crossing Earth’s path, or required too much delta-v to reach.
So they tried another selection mechanism - filtering for feasibility first. Which in practice meant they selected for ones that could be reached within the mission’s 1.5 delta-v budget, while keeping the activity requirements. This narrowed it down to three comets, all of which were found within the last 25 years. The most interesting was C/2001 Q4 (NEAT), which was found back in 2001, about 2.5 before it reached its perihelion. It had good activity levels, and could be reached within the 1.5 km/s delta-v budget. The only problem was a relatively high fly-by speed of 57 km/s - meaning the probes CI would release to study the coma could be damaged, and that there would be only limited time with the comet to collect valuable data.
Realistically, the likelihood of finding an ideal candidate in the 2-3 year window of CI’s mission is not great. As such, the mission operators will likely have to find one that’s “good enough” and collect whatever data they can on it. That’s an inherent limitation of these types of mission where the final target isn’t known until after the mission has been designed and launched. But, with some luck, CI will be able to find a good candidate, most likely with the help of LSST, when it launches in 2029. Maybe, if it’s really, really lucky, it will even find an interstellar visitor to rendezvous with - if that happens my money’s on naming that particular interstellar object Rama.
Learn More:
C. Snodgrass et al - Considerations on the process of target selection for the Comet Interceptor mission
UT - ESA Gives Green Light on its Comet Interceptor Mission
UT - Could We Launch a Mission to Chase Down Interstellar Comet 3I/ATLAS?
UT - Researchers at SwRI Produced a Mission Concept for Exploring Interstellar Objects Like 3I/ATLAS
