The James Webb Space Telescope is about to have a new neighbor. This week, the European Space Agency greenlit a scheme to park a comet-intercepting spacecraft at the second LaGrange point, or L2.
Called the Comet Interceptor, the craft will launch in 2029 and then hang out with the Webb until an as-yet-undiscovered comet happens to fly into its sphere — then, it will do what the name suggests: Intercept the comet. If successful, Comet Interceptor will shed light on the formation of our Solar System, the interstellar medium, and may even help answer an issue closer to home: How to prevent a comet from hitting Earth.
The Comet Interceptor mission will be the first to (hopefully) visit and study a long-period comet, and preferably, one in “pristine” condition. This kind of comet is one that is on its first trip inward toward the Sun. A similarly attractive option would be a drifting interstellar object like ‘Oumuamua, which astronomers discovered in 2017 and is on its way out of our Solar System.
While most astronomers agreed that ‘Oumuamua was probably the remains of a disintegrating comet or even a chunk of a dwarf exoplanet, a cadre of scientists led by Avi Loeb of Harvard University argue that it could be an alien spacecraft. Comet Interceptor could have settled the debate, but it’s too late for the Interceptor to check out ‘Oumuamua — we’ll need to wait for another opportunity, instead.
Meet Comet Interceptor
Comet Interceptor will carry two small probes, including one built by the Japanese space agency, JAXA. Each probe will be about 0.5 meters in diameter. When the right kind of comet comes within range, the Interceptor will unleash the probes and all three spacecraft will surround the comet as they fly past it as a single swarm. As they go by, they will gather images and data about the comet from several angles at once.
Their mission is to map the comet’s surface, study the composition of the comet’s icy nucleus, its surrounding coma of gas and dust, and its twin tails (a comet actually has two tails: one made of dust and another made of charged gas). Comet Interceptor and its probes will also measure the comet’s magnetic field and study how it changes as it draws closer to the Sun. To do the job, the main spacecraft and its two probes will carry a suite of cameras, mass spectrometers, infrared sensors, and other instruments.
Rather unusually for a space science mission, the Comet Interceptor will launch without a target in mind. Instead of aiming for a specific comet, the spacecraft will wait in a “parking orbit” around L2 — a spot 1.5 million miles away from Earth – for the right comet to come along.
Why wait so long you might ask? The reason is rather practical: Telescopes here on Earth can spot new, distant comets about a year before they make their closest approach to the Sun. A year is not enough time to build, prepare, and launch a science mission, but it is plenty of time to navigate a spacecraft from L2 to intercept the incoming comet in a cosmic meet-cute for the ages.
Already, our spacecraft have had close encounters with several comets. NASA’s Deep Impact mission crashed a 370-kilogram probe into Comet Tempel 1 in 2005. A few years later, ESA’s Rosetta mission orbited Comet 67P/Churyumov-Gerasimenko and even sent a lander to its surface in 2014.
But here’s the thing: Every comet we’ve visited so far has been what astronomers call a short-period comet. This is a comet with an orbit that makes it a frequent visitor to the inner Solar System. Every pass through the relatively cozy region of space we call home changes the comet as its ice melts and evaporates, freeing the dust trapped inside. That makes it hard for planetary scientists to tell what the comet originally looked like and what mix of elements it originally contained.
“It is sometimes difficult to say which of the features we see are from the formation period and which are evolutionary,” project manager Nicola Rando and study scientist Michael Kueppers tell Inverse. A pristine comet or a lone wandered like ‘Oumuamua, by contrast, would likely retain these evolutionary hallmarks.
The odds that Comet Interceptor will get to visit something like 'Oumuamua aren't very high, however, since astronomers have only found two so far: ‘Oumuamua and a rogue comet called 2I/Borisov (discovered in 2019). A meteor that landed in the ocean in 2014 may also have come from outside our Solar System. It could take years for another interstellar object to drift through our Solar System.
On the other hand, ground-based telescopes usually spot an average of 14 long-period comets each year. That number may increase when the Vera Rubin Observatory starts scanning the skies in 2024, according to Rando and Kueppers.
Interstellar objects and long-period comets both contain something astronomers desperately want: Unadulterated cosmic material.
“A comet on its first orbit around the Sun would contain unprocessed material from the dawn of the Solar System,” sid Kueppers in a recent statement.
Studying that material and how it changes as the Sun heats it for the first time in 4.5 billion years or so could shed light on what the Solar System was like in its infancy. That’s especially true when it comes to water and other volatile elements — many of which are important in the origin of life, and may have been delivered to the early Earth at least in part by impacts with comets and asteroids.
“We may also learn more about the original composition of comets by detecting highly volatile species in the coma that may be gone in more evolved comets,” Rando and Kueppers tell Inverse.
By comparing a pristine comet to more seasoned travelers like Tempel 1 or 67P/Churyumov-Gerasimenko, scientists can learn how comets change over time.
“The presence or absence of features seen in other comets, like layers, pits, and terraces on 67P/Churyumov-Gerasimenko observed by Rosetta, will tell us if those are from the formation period or evolutionary, formed during the many passages close to the Sun,” the astronomers say.
That knowledge may even help keep us safer in the long run.
“To be able to deflect a comet or asteroid threatening to impact Earth, we need to know the properties of those objects,” the astronomers explain.
“Therefore any mission investigating the nature of those small bodies also contributes to planetary defense.”
If everything goes according to plan, Comet Interceptor will launch in 2029. Comet Interceptor is one of ESA’s F-class or “fast” missions, which typically take about 8 years from selection to launch and involve less than 1,000 kilograms of mass. Its relatively small mass and size (about 10 cubic meters) mean Comet Interceptor can hitch a ride to space, and then to L2, with another ESA spacecraft called Ariel (which will study exoplanet atmospheres).
Once there, then the waiting game will really begin.