In the far reaches of the solar system, between the orbits of Jupiter and Neptune, a multitude of city-sized chunks of rock and ice, known as “Centaurs,” circle the sun. Occasionally Jupiter’s gravitational tug flings one of these Centaurs into a new orbit that takes it into the inner solar system. As it approaches the sun it heats up and various gases, including water vapor from ice locked in the object’s interior, are released. As solar wind pushes this material away from the object, it can form a distinctive “tail”—and a comet is born.
And while more than a dozen robotic missions have studied comets and asteroids in our solar system over the last two decades, we’ve thus far never observed a comet at its moment of birth. But if an ambitious project goes ahead, that will change. The idea, put forward by University of Chicago physicist Darryl Seligman and his colleagues in a study accepted for publication in Planetary Science Journal, would see a spacecraft “parked” near Jupiter; when a Centaur is flung toward Earth, the spacecraft would follow along, effectively hitching a ride on the inbound object.
“If you could ride along with a comet as different ices ‘turned on,’ then you could see that entire process happen, in real time. You’d see not only the comet’s start, but its evolution,” says Seligman. “Scientifically, it would be incredibly useful to watch H2O turn on for the first time, and to see what that looks like, as a function of the object’s distance to the sun.”
Seligman even has his eye on a particular target—a Centaur known as “P/2019 LD2 (ATLAS),” or LD2 for short. (The objects are likened to the mythical half-human, half-horse creatures because of their hybrid nature: they’re a bit like asteroids, which are generally inert chunks of rock; and they’re a bit like comets, which are more “active” due to the emission of gases as various frozen ices vaporize, in a process known as sublimation.) LD2 is estimated to be about eight miles across—about the size of Staten Island—and is currently orbiting the sun with a period of about 12 years, in an orbit that keeps it close to Jupiter.
Preliminary data suggests that LD2 will have a particularly close encounter with Jupiter in 2063—a brush which, according to computer simulations, will likely send the object toward the inner solar system. Once it’s on its new path, it will be a “Jupiter-family comet,” a particular class of short-period comets that pass near the sun every few years. However, because these calculations involve a certain margin of error, the exact inward trajectory that LD2 will take can’t be pinned down precisely.
Seligman’s studyexamines the characteristics and orbital dynamics of Centaurs, and predicts that many more such objects likely remain to be discovered; it also details how a spacecraft could be sent to LD2 at relatively low cost. They suggest a launch date of 2061, with the spacecraft rendezvousing with LD2 shortly after its 2063 encounter with Jupiter.
“It’s a very exciting idea,” says Laura Woodney, a planetary scientist at California State University, Santa Barbara, who was not involved with the current study, but has worked on similar investigations for possible Centaur missions. The proposed target, LD2, “is one of these pristine, outer-solar-system objects that would be really fascinating to follow, to study, to see what it’s composed of.”
When the solar system was forming—4.5 billion years ago, give or take—it was likely awash in countless rocky and icy bodies constantly smashing into one another. The largest chunks of material coalesced into the eight major planets—plus the sun, of course—while the smaller bits became the minor bodies, mostly asteroids and comets, that account for the remainder of the solar system’s mass. Because LD2 has not yet ventured close to the sun, astronomers see it as an undisturbed relic, its composition closely mirroring that of the early solar system. Studying it can shed light on the “building blocks” from which the Earth and the other planets formed.
Although the first Centaur was discovered a century ago, it was only in the 1970s that astronomers came to think of them as a distinct collection of objects. They’re believed to originate beyond the orbit of Neptune, migrating inward as a result of the gravitational effects of the giant planets. Once they arrive at Jupiter’s orbit, however, they’re in a “gravitational shooting gallery,” says Jordan Steckloff, an astronomer at the Planetary Science Institute in Tucson, Arizona. Jupiter’s powerful gravity typically flings Centaurs either outward or inward after a few million years—which sounds like a long time, but is pretty quick in astronomical terms; it means that Centaurs should be thought of as objects in transition. “Orbital changes that normally take a few billion years suddenly happen much faster,” says Steckloff. And in the case of LD2, the transition is expected to happen “on a human time scale—which is what makes this so exciting.”
Most of the ice in comets is frozen water, but they can also contain carbon monoxide or carbon dioxide, along with trace amounts of other frozen gases. Because carbon monoxide and carbon dioxide are more volatile than water, they’re the first to vaporize as a comet approaches the sun. LD2 already shows small amounts of activity, due to the sublimation of these more volatile gases.
The sublimation process is important because it can affect a comet’s trajectory: if gases spew out of one side of an object more than the other side, it will be nudged in the opposite direction. Since ice is a key component of a comet, when water starts to sublimate, the comet is also in danger of breaking apart. “Comets eventually run out of steam and disintegrate” after a handful of close approaches to the sun, Seligman says, “to the point where there’s nothing left.” (Exceptions exist to this break-up rule—some, like Halley’s Comet, have proven relatively stable; Halley, which is believed to have originated from further out than the Centaurs, returns to the inner solar system about every 75 years and has survived at least 30 orbits.)
A better grasp of the mechanics of cometary break-ups can help astronomers understand the population dynamics of the Centaurs themselves. For example, the faster that comets fall apart, the higher the rate at which new, more remote objects must be migrating into the Centaur region to replace them. Understanding these processes should also help researchers predict how many more objects like LD2 they might expect to see kicked into the inner solar system in coming decades.
When LD2 eventually becomes a short-period comet, it could display a bright tail that would make it visible to the unaided eye.
While the mission that Seligman envisions sounds complex, it could be realized with previously tested technology, he says. For example, NASA’s Juno mission reached Jupiter in just five years; and the OSIRIS-Rex mission and also Japan’s Hayabusa 2 spacecraft have shown that it’s possible to follow a moving target through space, with both craft successfully collecting samples from an asteroid’s surface. NASA’s Lucy mission, meanwhile, has just been launched on a 12-year journey that will see it rendezvousing with eight different asteroids.
Seligman stresses that the idea that he describes in the paper is just a demonstration of the feasibility of a concept, a fairly common practice for astronomers and physicists at the first stages of dreaming up a possible space mission. A full-fledged mission concept study, involving dozens of scientists and engineers examining “every possible thing that could go wrong with a mission,” may follow sometime in the future, he says; that would be followed by a pitch to a space agency and to funding agencies. (By comparison, he notes, a journal article is “relatively cheap.”)
Aside from learning about the early history of the solar system, studying these small worlds is also vital for understanding the danger presented by objects in Earth-crossing orbits. Astronomers believe that most such objects originate in the asteroid belt between Mars and Jupiter, not from the more remote realm of the Centaurs–but it’s impossible to rule out the possibility that a comet might one day pose a threat to Earth, Seligman says. That’s also a good reason to study the break-up of comets: If an object that splits into, say, a dozen pieces as it passes near the sun, and those pieces in turn break apart on subsequent orbits, that would create “a greater flux of potentially hazardous debris.” So although comets don’t top of the list of likely hazards for our planet, the more we understand about their composition and motion, the better, says Seligman.