The unusual thing about Comet Interceptor is that it won't know its target prior to launch. The probe will travel to the Lagrange Point 2 (L2), a gravitationally stable point 930,000 miles (1.5 million kilometers) from Earth in the direction away from the sun.

L2 is one of five points between Earth and the sun where the gravitational pulls of the two bodies create balanced conditions. A spacecraft in this region orbits the sun in sync with Earth while shielded by it from the sun's glare, which makes this region a sought-after destination for astronomy missions. (NASA's James Webb Space Telescope is one of the residents at L2.)

For Comet Interceptor, L2 will be just a temporary destination. The spacecraft will wait there for a unique target to arrive in the inner solar system, either a comet from the outskirts of the solar system, or an object from even farther afield, from interstellar space, such as the famous 'Oumuamua, which passed 15 million miles (24 million km) away from Earth in 2017.

ESA made headlines in 2014 with its Rosetta mission, which placed the Philae lander on the surface of Comet 67P. In 1986, ESA's Giotto probe made the first-ever close observations of a comet when it flew past the famous Haley's Comet. These comets, however, are so-called short-period comets that visit the inner solar system regularly and have flown close to the sun many times before. Each encounter with the sun changes the comet's chemistry, ESA said in a statement yesterday, making it less and less representative of the chemical state of the young solar system.

"A comet on its first orbit around the sun would contain unprocessed material from the dawn of the solar system," Michael Küppers, a Comet Interceptor study scientist at ESA, said in the statement (opens in new tab). "Studying such an object and sampling this material will help us understand not only more about comets, but also how the solar system formed and evolved over time."

ESA expects that Comet Interceptor will not have to wait too long for an exciting target to appear, as new comets are currently discovered at a rate of at least one per year. Such a timeframe would be too short to build and launch a dedicated spacecraft. Comet Interceptor, however, wil be able to meet the visitor promptly.

Once Comet Interceptor reaches its target, the three spacecraft will separate and image the body in sync from multiple angles to create a three-dimensional profile, ESA said in the statement.

ESA will build the main spacecraft and one of the auxiliary probes, while JAXA, which has landed spacecraft on two separate asteroids with the Hayabusa 1 and Hayabusa 2 missions, will be responsible for the second smaller satellite.

Each of the probes will be equipped with different instruments to analyze the surface composition, shape and structure of the comet as well as the dust and gas of its coma, the tail-like cloud emanating from the surface.

The three satellites together will weigh less than 2,200 pounds (1,000 kilograms), ESA said.

Read here:

https://www.space.com/esa-comet-interceptor-mission-2029-launch