Catching a comet

There’s an assumption with space missions that they must be years in the planning before they’re approved for launch. In the case of Comet Interceptor, a new UK-led project to observe a long-period comet up close for the first time, that assumption couldn’t be more wrong.

The first in a new type of F-Class (the F stands for ‘fast’) mission, Comet Interceptor was selected by ESA in June 2019, with the request for proposals having gone out less than a year before. Rather than years in the making, the entire concept for the mission is barely 18 months old.

“We had the idea a few months before [ESA’s call for proposals in July 2018], but actually, no, we started then,” mission proposal lead Prof Geraint Jones told The Engineer, shattering those aforementioned assumptions.

Jones is head of the Planetary Science Group at UCL’s Mullard Space Science Laboratory. Together with deputy mission lead Dr Colin Snodgrass from the University of Edinburgh, he tailored a plan based on the parameters of ESA’s request. Those included a launch mass of less than 1,000kg and a budget of €150m.

The chosen entry would also be hitching a ride with ESA’s Ariel space telescope, set to launch in 2028 to the L2 Lagrange Point, 1.5-million kilometres from Earth in the opposite direction to the Sun. This was ideal for Comet Interceptor, as its final target is currently unknown. The spacecraft will lie in wait at L2 until a suitable long-period comet – or even an interstellar object such as Oumuamua – is identified from Earth. It will then travel to intercept its target, splitting into three separate craft that will help create a 3D profile of the celestial body.

"It’s part of the whole concept that we sit at L2 and wait for the right target"

“The really nice fit with our mission is the fact we’re being delivered to Lagrange point L2 and we can just sit there and wait,” Jones explained. “For almost every other mission to a small body you have to identify your target beforehand and then you’ve got launch windows where you can only launch at certain times of the year.

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Further reading

• UK-led Comet Interceptor set to launch in 2028
• UK space: A new world of opportunity
• Teledyne e2v to supply sensors for giant astronomical camera

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“Other missions could be delivered to L2 and then just leave when they needed to go to their targets, but it’s not the most efficient way of doing it. For our proposal, it’s part of the whole concept that we sit there and wait for the right target to come along.”

The mission’s goal is to characterise the comet’s surface composition, shape and structure, as well as the make-up of its gas coma. Each of the three spacecraft will carry a unique sensor suite, comprising cameras, imagers and spectral analysers. When we spoke to Jones, the team was still in consultation with ESA over the exact payload, with the space agency due to make its final decisions at the end of October. According to the professor, this type of wrangling is typical as the costs and benefits of individual instruments are weighed up. What’s important is that the core scientific objectives of the mission are maintained.

“The whole point that we’re doing this for is ultimately for the science,” he said. “So, we do want to be assured that whatever does get descoped or removed potentially, that it doesn’t impact the science too much.

“We do have an input, but ultimately it’s ESA’s decision. We’re working closely with them. For the proposal, we had to provide a science traceability matrix showing all the scientific objectives and goals that we have, and which instruments would meet them. So, if any area looks like it might be reduced in terms of scientific return, we can point out the implication so that’s also taken into consideration.”

Most space missions tend to rely on tried-and-tested equipment where possible. For Comet Interceptor, the accelerated timeline makes this especially pertinent. Its instrumentation is influenced by previous ESA endeavours, including the Rosetta mission that captured the world’s attention back in 2014 when its Philae lander touched down on comet 67P/Churyumov–Gerasimenko. Comet Interceptor’s plasma package and mass spectrometer have taken cues from Rosetta, while echoes of other missions can be found throughout the payload.

“Every instrument has got strong heritage or is largely a re-flight of a previous instrument,” Jones explained.

“So the main camera, CoCa [Comet Camera], led from the University of Bern, large elements of that are actually based on the CaSSIS [Colour and Stereo Surface Imaging System] camera that is on the ExoMars Trace Gas Orbiter that’s orbiting Mars now: another ESA mission.”

It’s not just ESA’s experience that the mission will be tapping into. One of Comet Interceptor’s three spacecraft will be developed by JAXA (Japan Aerospace Exploration Agency) and is set to carry a Lyman-alpha hydrogen imager, as well as a wide-angle camera and plasma suite. Members of Jones’s team had already been working with the Japanese agency on imaging hydrogen around comets, so the collaboration was a natural fit.

“Although it’s a much reduced-cost mission compared to the usual-sized missions that ESA launches, it still has to reach a minimum threshold of design maturity and JAXA clearly has the expertise to do that,” said Jones. “It has flown a small satellite in deep space arguably before anyone…it’s fantastic to have them on board.”

As well as liaising with JAXA, ESA and numerous space scientists around the world, Jones himself is heading up development of the EnVisS (Entire Visible Sky coma mapper) instrument. Based on the JANUS camera that will fly on board ESA’s JUICE (JUpiter ICy moons Explorer) mission in 2022, EnVisS will map the entire sky around the comet’s head and near tail, revealing changes in the structure of its dust and gases.

Comet Interceptor’s multinational team includes scientists from the length and breadth of Europe, as well as India, Japan, the US, Canada, Russia and Chile. The last of those countries is likely to play a pivotal role in the mission’s outcome, as Chile is home to the Large Synoptic Survey Telescope (LSST). Currently under construction high in the Andes, the LSST will use three primary mirrors and the world’s largest digital camera (3.2 gigapixel) to photograph the entire visible sky anew every three nights or so. When it’s fully operational in 2023 it will be the most likely source of targets for Comet Interceptor.

“LSST in particular is expected to basically revolutionise things,” said Jones. “So, we probably will find more comets, but what’s most important for us is that they’re found further out.”

With Comet Interceptor not launching until 2028, it’s entirely possible that the mission’s target may already be identified by then, either by the LSST or other comet-hunting telescopes such as Hawaii’s Pan-STARRS.

“That’s what we’re hoping for,” said Jones. “We’re preparing a plan for choosing targets as well. We may have – if we’re lucky – two or three and have to decide between them. If there’s an interstellar object, for example, I would expect that to trump all the others.”

Intercepting an interstellar object is particularly enticing, as astronomers have only recently confirmed their existence. Two years ago, the oblong-shaped Oumuamua set pulses racing when it was detected by Pan-STARRS, with suggestions it may even have alien origins. Although that theory has been debunked, the mysterious tumbling asteroid has raised a whole host of new questions for astronomical science.

On 14 October 2019, a second interstellar object, Borisov, was confirmed.  Its shape and behaviour align with conventional comets, with only its hyperbolic speed indicating origin outside our solar system. This only serves to highlight the strangeness of Oumuamua, as well as how little we know about these interstellar visitors. Whether Comet Interceptor gets to observe one up close remains to be seen, but the possibility is certain to keep stargazers excited over the coming years