The European space mission that plans to ambush a comet

Artist’s impression of Comet Interceptor, which is due to launch in 2028 and will wait for its target for up to six years.Credit: Geraint Jones, UCL Mullard Space Science Laboratory

The European Space Agency (ESA) has approved a new mission, called Comet Interceptor, which will launch with no specific objective in mind – instead on the lookout for a visitor from the outer solar system, or even another star. . Comet Interceptor could give researchers their first glimpse of pristine materials far beyond the reach of the Sun, or even unveil the chemical makeup of alien worlds.

It will be the first probe to be stationed in space, ready to fly to a short-term target. “We are taking a big risk,” says Günther Hasinger, ESA’s scientific director. “But it’s a great reward.”

The mission, first proposed in 2019, will launch in 2028 with a new telescope, Ariel, designed to study the atmospheres of exoplanets. The two will travel to the second Lagrange point (L2), a point of gravitational stability 1.5 million kilometers from Earth – beyond the Moon’s orbit – where the recently launched James Webb Space Telescope also resides. .

Here, Comet Interceptor – the first of ESA’s ‘F-class’ rapid development missions – will remain floating in space, while scientists back on Earth search for a suitable target to visit. The goal is to find a virgin comet in a wide orbit taking hundreds of years, known as a long-period comet, to enter the solar system for the first time. Such a comet could originate from a vast region of icy objects called the Oort Cloud, which 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 in smaller orbits and are therefore more strongly weathered by the Sun.

A black and white image of comet 67P/Churyumov-Gerasimenko

Comet 67P/Churyumov-Gerasimenko, photographed by ESA’s Rosetta spacecraft. Rosetta and its lander, Philae, carried out extensive studies of the comet between 2014 and 2016.Credit: ESA/Rosetta/MPS for the OSIRIS team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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

The mission will include a main spacecraft and two smaller probes, one of which will be developed by the Japanese Space Agency (JAXA). Following mission 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 UK and OHB Italy in Italy.

Once the spacecraft is in position at L2, it can wait at least six years there for a suitable target to pass close enough to Earth orbit to visit. When this happens, Comet Interceptor triggers its thrusters and leaves L2 on a flyover trajectory. The main spacecraft will fly over the comet at a distance of about 1,000 kilometers to avoid any damage from nearby materials, while the smaller probes will dive closer, up to just 400 kilometers from the surface.

Rich rewards

The entire encounter will only last a few hours, but the scientific rewards are considerable and cannot be matched by remote observations with telescopes, including measurements of the comet’s composition, the gases and dust emitted, its temperature and first close-up images of such an immaculate icy object. This will give a window into the matter that formed at the dawn of the solar system, 4.5 billion years ago. “It’s a message in a bottle from the training period,” says Comet Interceptor project scientist Michael Kueppers of ESA Madrid.

More than a dozen long-period comets enter the inner solar system each year, although not all are accessible by Comet Interceptor. The team estimates an 80% chance that a suitable long-period comet will emerge in the Comet Interceptor epoch at L2. Such comets can only be spotted months before their closest approach into the inner solar system, so having a spacecraft ready at L2 makes the flyby easier than trying to arrange a short-term launch from Earth. .

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

An even more alluring possibility is offered, however. In the past five years, two objects have been spotted flying past our Sun that would have been ejected from other solar systems, ‘Oumuamua in 2017 and Comet Borisov in 2019. Telescopic observations have provided tentative glimpses of these Ephemeral visitors, and the sending of a spacecraft could tell researchers much more about their composition, water content and the system from which they originated.

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 unprecedented insight into the material of another solar system. “The interstellar object aspect is hugely exciting,” says planetary scientist Geraint Jones of University College London, who led the team that proposed the mission to ESA. “The chances of finding a suitable interstellar target are low. But we will remain vigilant. »

“This is the first time such a rapid response mission has been carried out,” Kueppers said. “We don’t expect to have a large number of potential targets. If we have a good goal, we aim for it.

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