The European Space Agency (ESA) has approved a new mission, called Comet Interceptor, which will launch without any specific target in mind — instead lying in wait for a visitor from the outer Solar System, or even from another star. Comet Interceptor could give researchers a first glimpse of pristine material from far beyond the Sun’s reaches, or even unveil the chemical make-up of alien worlds.

It will be the first probe to be parked in space, ready to fly to a target at short notice. “We are taking a significant risk,” says Günther Hasinger, ESA’s director of science. “But it’s a high reward.”

The mission, first put forward in 2019, will launch in 2028 along with a new telescope, Ariel, designed to study the atmospheres of exoplanets. Both will travel to the second Lagrange point (L2), a point of gravitational stability 1.5 million kilometres from Earth — beyond the orbit of the Moon — where the recently launched James Webb Space Telescope also resides.

Here, Comet Interceptor — the first of ESA’s ‘F-class’ quick-development missions — will remain floating in space, while scientists back on Earth search for a suitable target for it to visit. The goal is to find a pristine comet on a wide orbit taking hundreds of years, known as a long-period comet, that is entering the solar system for the first time. Such a comet could originate from a vast region of icy objects called the Oort Cloud, that exists far beyond Neptune in the outer Solar System. No mission has visited such an object before. Other missions, such as ESA’s Rosetta spacecraft, have visited short-period comets, which spend more time in the inner Solar System on smaller orbits and are thus more heavily altered by the Sun.

“Comet Interceptor is going to give us a first real glimpse of a primordial body,” says Alan Fitzsimmons, a comet researcher from Queen’s University Belfast, UK, who is not involved in the mission. “We have no idea what it will look like. That will truly be new, never-seen-before science.”

The mission will comprise a main spacecraft and two smaller probes, one of which will be developed by the Japanese Space Agency (JAXA). Following the mission’s approval last week, ESA will now select a prime contractor to develop the main spacecraft, from one of two competing designs from Thales Alenia Space in the United Kingdom and OHB Italia in Italy.

Once the spacecraft is in position at L2, it can wait there for at least six years for a suitable target to pass close enough to Earth’s orbit to visit. When that occurs, Comet Interceptor will fire its thrusters and leave L2 on a fly-by course. The main spacecraft will fly past the comet at a distance of about 1,000 kilometres to avoid any damage from material nearby, while the smaller probes will dive closer, down to as little as 400 kilometres from the surface.

Rich rewards

The entire encounter will last just hours, but the scientific rewards are considerable and cannot be matched by remote observations with telescopes, including measurements of the composition of the comet, the gas and dust emitted, its temperature, and the first close-up images of such a pristine icy object. That will give a window on material that formed at the dawn of the Solar System, 4.5 billion years ago. “It’s a message in a bottle from the formation period,” says Michael Kueppers at ESA in Madrid, Comet Interceptor’s project scientist.

More than a dozen long-period comets enter the inner Solar System every year, although not all of those would be reachable by Comet Interceptor. The team estimates an 80% chance that a suitable long-period comet will emerge in Comet Interceptor’s time at L2. Such comets can be spotted only months before their closest approach into the inner Solar System, so having a spacecraft ready at L2 makes a fly-by easier than trying to organize a launch at short notice from Earth.

In the unlikely event that a suitable long-period comet does not turn up, the mission will be repurposed to visit another target, such as 73P/Schwassmann–Wachmann 3, a short-period comet that is thought to have broken into pieces.

An even more alluring possibility is on offer, though. In the past five years, two objects have been spotted flying past our Sun that are believed to have been ejected from other solar systems, ‘Oumuamua in 2017 and comet Borisov in 2019. Telescopic observations provided tentative glimpses of these fleeting visitors, and sending a spacecraft could tell researchers much more about their compositions, water content, and the system they originated from.

If such an object is spotted while Comet Interceptor is at L2, and if the object passes close enough to be visitable, then the spacecraft could be sent to intercept it instead, giving us an unprecedented glimpse of material from another solar system. “The interstellar-object aspect is extremely exciting,” says planetary scientist Geraint Jones at University College London, who led the team that proposed the mission to ESA. “The chances of finding a suitable interstellar target are small. But we’ll be keeping an eye out.”

“This is the first time that such a rapid-response mission has been done,” says Kueppers. “We don’t expect to have a large number of potential targets. If we have a good target, we’ll go for it.”