Montezuma observing a comet.
© DEA / G. DAGLI ORTI/Getty Images
A sixteenth century illustration showing Montezuma observing a comet. The European Space Agency has a different plan.
Scientists at the European Space Agency (ESA) are working on a new "fast" mission to make the first flyby of a pristine comet - meaning one that has never before passed close to the sun.

Since the first comet mission, all the way back in 1978, numerous space agencies have made more than a dozen comet flybys, including one rendezvous and landing.

But never before has a mission attempted to visit a comet on its first plunge toward the sun, when its never-before-heated surface is almost unchanged from when it formed at the dawn of the solar system, some 4.5 billion years ago.

The recently approved mission, called Comet Interceptor, will also be unique in what it does as it nears its target.

Rather than simply flying by, it will split into four parts, each of which will whizz past the comet on a slightly different trajectory.

Three of these will be tiny instrument packages, which will view the comet from different angles. This will allow scientists back on Earth to create detailed 3D models not only of its surface, but of the gas, dust, and plasma surrounding it.

The fourth will be the mother ship, which will collect data from the smaller probes and relay it back to Earth.

"It's a novel concept," says Fabio Favata, head of the Strategy, Planning, and Coordination Office in ESA's Directorate of Science.

Details of the mission have yet to be determined, but the use of the word "fast" in its description doesn't mean it will be traveling at warp speed.

Rather, the term applies to the mission's rapid-development timeline. If all goes according to plan, there will be little more than eight years from proposal selection to launch - an extremely rapid pace for a space mission.

It's also low budget, with an estimated cost of about €150 million, Favata says.

The plan, he adds, is for the spacecraft, expected to weigh in at about 800 kilograms, to share a ride with ESA's exoplanet-studying ARIEL space telescope, scheduled for launch in 2028.

Both missions will be deployed to a location known as Lagrange Point 2 (L2), about 1,500,000 kilometres above the Earth, directly outward from the sun. There, the Earth's and sun's gravities conspire to allow objects, once placed there, to remain indefinitely, with only minor course corrections.

ARIEL will use L2 for its observations. Comet Interceptor will park there, waiting for astronomers to spot a suitable, incoming comet - or, if they are extremely lucky, something like 'Oumuamua, the cigar-shaped interstellar object that shot through the solar system in 2017.

That way, rather than having to rush to build and launch a spacecraft when a comet or interstellar interloper is spotted (a more-or-less impossible task, given the fact that most comets aren't spotted all that far in advance), Comet Interceptor will be ready and waiting.

And even with relatively limited lead time, it should be able to move as far as 200 million kilometres ahead or behind the Earth's position in its orbit, allowing it to intercept comets that don't come anywhere close to the planet itself.

"How much lead time we will get depends on when the comet is discovered," Favata says.

What will be moving fast is the object Comet Interceptor needs to intercept.

How fast depends on the object's precise origin and the angle at which it is coming in.

"If it goes the same direction as the Earth, the encounter speed will be moderate," Favata says.

"If it's retrograde, it could be up to tens of kilometres per second."

One of the advantages of using the three detachable instrument packages, Favata adds, is that it allows them to get closer to the comet than would be safe for the mother ship.

Comets are known to eject dust as the sun warms them, and if the mother ship were to hit too much such high-speed dust, it might be too badly damaged to return data to Earth. If one of the three instrument packages were to be destroyed, however, there would be two more to back them up.

Thus, Favata explains, "the smaller spacecraft can be sent closer because they can take a higher risk". Though he notes that these simple probes "ought to be thought of as autonomous scientific instruments, rather than spacecraft".

As the boom in the use of tiny CubeSats in Earth orbit indicates, the use of such miniaturised space probes isn't going to be one of the mission's major technological challenges.

"But it's certainly one of the novel elements," Favata says.

Meanwhile, other scientists are excited because the mission will allow a chance to compare the comet to the only other near-pristine outer solar system object ever to be visited by a spacecraft: Ultima Thule, a 6.5-billion-kilometre-distant object that NASA's New Horizons spacecraft flew past on 1 January 2019.

Current theory says that most comets formed closer to the sun than objects such as Ultima Thule, says John Spencer, a planetary scientist with the Southwest Research Institute (SWRI) in Colorado, US, and a member of the New Horizons team.

Interactions with the giant planets then hurled these proto-comets out into the Oort Cloud, a far-flung region of the solar system, a trillion or more kilometres away. There, they have remained ever since, awaiting the gravitational nudges that eventually send them plunging toward the sun to become comets.

If Comet Interceptor finds that such an incoming comet resembles Ultima Thule, Spencer says, it will mean that the processes that formed both objects in the Kuiper Belt and the Oort Cloud were similar, even though they are located in quite different parts of the solar system.

But, he says, "it's possible they will look different, either because formation processes were different closer to the sun, or because Oort Cloud objects experienced more impacts before they were ejected - or evolve very rapidly on their first approach to the sun, before we can get the spacecraft to them".

Whatever the answer, he says, "we can't wait to find out".