The European Space Agency (ESA) has approved a new mission called Comet Interceptor, which will launch without any specific target – instead waiting for a visitor from the outer solar system, or even from another star. The Comet Interceptor could give researchers an initial glimpse into pure materials far beyond the sun’s reach, or even unveil the chemical makeup of space worlds.
This will be the first probe to be parked in space and ready to travel to a target in no time. “We’re taking a big risk,” says Gunter Hasinger, director of science at the European Space Agency. “But it’s a big bonus.”
The mission, first introduced in 2019, will be launched in 2028 along with a new telescope, Ariel, designed to study the atmospheres of exoplanets. Both will travel to Lagrange Point II (L2), a gravitational stable point 1.5 million kilometers from Earth – beyond the Moon’s orbit – where the recently launched James Webb Space Telescope is also located.
Here, Comet Interceptor – ESA’s first F-class rapid development mission – will remain afloat in space, while scientists are back on Earth searching for a suitable target to visit. The goal is to find a pure comet with a wide orbit that takes 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 bodies called the Oort Cloud, which is found far from Neptune in the outer solar system. No expedition had visited such an object before. Other missions, such as the European Space Agency’s Rosetta spacecraft, have visited short-period comets, which spend more time in the inner solar system in smaller orbits, and thus are dramatically altered by the sun.
“The Comet Interceptor will give us our first real glimpse into a primordial body,” says comet researcher Alan Fitzsimmons from Queen’s University Belfast, UK, who was not involved in the mission. “We have no idea what it will look like. This will truly be a new science that has never been seen before.”
The mission will consist of a main spacecraft and two smaller probes, one of which will be developed by the Japan Aerospace Agency (JAXA). Following mission approval last week, the European Space Agency will now select a main contractor to develop the main spacecraft, from one of two competing designs from Thales Alenia Space in the UK 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 to Earth’s orbit to visit it. When that happens, the Comet Interceptor will fire its thrusters and leave L2 on a fast track. The main spacecraft will fly through the comet at a distance of about 1,000 kilometers to avoid any damage from nearby materials, while the smaller probes will dive even closer, to less than 400 kilometers from the surface.
The entire confrontation will only last for hours, but the scientific rewards are great and cannot be compared to remote observation with telescopes, including measurements of the comet’s composition, gas and dust emitted, its temperature and the first close-ups of such a pristine icy body. That would give a window into the material that formed at the dawn of the solar system, 4.5 billion years ago. “It’s a message in a bottle from the formative period,” says Michael Kuipers of the European Space Agency in Madrid, Comet Interceptor project scientist.
More than a dozen long-period comets enter the inner solar system each year, although not all of these comets are accessible by the Comet Interceptor. The team estimates there is an 80% chance of a suitable long-range comet appearing at the time of the Comet Interceptor at L2. Such comets can be spotted just months before they approach the inner solar system, so having a spacecraft ready at L2 makes flying them easier than trying to organize a launch in short order from Earth.
In the unlikely event that a suitable long-range comet does not appear, the mission will be redirected to visit another target, such as 73P/Schwassmann-Wachmann 3, a short-range comet believed to have shattered into pieces.
However, there is a more attractive possibility on offer. In the past five years, two objects flying near our sun that are believed to have been expelled from other solar systems have been observed: Oumuamua in 2017 and Comet Borisov in 2019. Telescopic observations have provided the first glimpses of these passing visitors, sending a signal that the spacecraft could tell researchers a lot about. their compositions, water content and the system from which they originate.
If such an object is spotted while the Comet Interceptor is at L2, and if the object passes close enough to be visitable, the spacecraft could be sent to intercept it instead, giving us an unprecedented glimpse into material from another solar system. “The aspect of the interstellar body is very exciting,” says planetary scientist Geirant Jones at University College London, who led the team that proposed the mission to the European Space Agency. “The chances of finding a suitable target among the stars are slim. But we will keep an eye on that.”
“This is the first time this rapid response mission has been done,” Kueppers says. We don’t expect to have a large number of potential targets. If we have a good goal, we will strive for it.”