We’re about to find many more interstellar interlopers—here’s how to visit one

A long-period comet is an icy visitor from the farthest reaches of the Solar System that has spent little time getting blasted by the Sun’s heat and radiation, freezing its physical and chemical properties much as they were billions of years ago.

Long-period comets are typically discovered a year or two before coming near the Sun, still not enough time to develop a mission from scratch. With Comet Interceptor, ESA will launch a probe to loiter in space a million miles from Earth, wait for the right comet to come along, then fire its engines to pursue it.

Odds are good that the right comet will come from within the Solar System. “That is the point of the mission,” Snodgrass told Ars.

But if astronomers detect an interstellar object coming toward us on the right trajectory, there’s a chance Comet Interceptor could reach it.

“I think that the entire science team would agree, if we get really lucky and there’s an interstellar object that we could reach, then to hell with the normal plan, let’s go and do this,” Snodgrass said. “It’s an opportunity you couldn’t just leave sitting there.”

But, he added, it’s “very unlikely” that an interstellar object will be in the right place at the right time. “Although everyone’s always very excited about the possibility, and we’re excited about the possibility, we kind of try and keep the expectations to a realistic level.”

For example, if Comet Interceptor were in space today, there’s no way it could reach 3I/ATLAS. “It’s an unfortunate one,” Snodgrass said. “Its closest point to the Sun, it reaches that on the other side of the Sun from where the Earth is. Just bad timing.” If an interceptor were parked somewhere else in the Solar System, it might be able to get itself in position for an encounter with 3I/ATLAS. “There’s only so much fuel aboard,” Snodgrass said. “There’s only so fast we can go.”