The James Webb Space Telescope is a machine of many talents. It can peer through dense space dust, glimpse the stellar drama of distant galaxies, and even observe the universe’s first stars. But some of Webb’s most riveting work will be answering questions much closer to home, like the compelling quandary known as interstellar objects.
These objects — whether comets, asteroids, or maaaaaaaybe alien artifacts (depending on who you ask) — are born from another star, and they carry valuable data locked away in their gaseous coma and icy nucleus.
Astronomers have tracked two of these objects and gathered as much information as possible from ground- and space-based telescopes during their short-lived journey through the Solar System. But a planned study using Webb’s 6.5-meter mirror and advanced suite of infrared cameras will sketch an even clearer picture of these interstellar objects, their foreign host stars, and the uniqueness of our own Solar System among the billions of stars that make up the Milky Way.
What makes an interstellar object?
In October 19, 2017, the Pan-STARRS telescope in Maui, Hawaii, spotted a strange object streaking through the solar system. The object’s speed along with its extreme eccentricity meant it wasn’t gravitationally bound to our Sun. This object, named ‘Oumuamua (meaning “messenger from afar arriving first”), was the first interstellar object ever recorded — and what an object it was.
Described as “cigar-shaped”, ‘Oumuamua’s length was 10 times its width, an extreme oddity among space objects, and astronomers originally classified this interstellar interloper as a comet. But because it didn’t have a coma, a kind of atmosphere around a comet caused by sublimating ice, NASA reclassified it as an asteroid — but this badminton nomenclature doesn’t end there.
On its way out of the Solar System, ‘Oumuamua unexpectedly accelerated past the Sun due to a cometary trait called “outgassing.” To make matters even more confusing, some astronomers think Oumuamua is neither a comet nor an asteroid but instead a sort of “cosmic iceberg.”
Because it was only a quarter-mile long, ‘Oumuamua was no longer visible by any telescope — terrestrial or orbital — by January 2018. It’s estimated that ‘Oumuamua is now crossing the orbit of Neptune as it continues its journey toward the constellation Pegasus. Astronomers will never study this once-in-a-lifetime object ever again.
‘Oumuamua, along with the discovery of the mostly normal rogue comet Borisov in 2019, are the only two interstellar objects that have ever been recorded in our solar system. Because astronomers have no idea when the next interstellar object will streak across the sky, what it’ll look like, or how long it’ll be observable, Webb could gather both comet and asteroid experts to be ready for anything the galaxy throws at us .
And if ‘Oumuamua is any indication, they’ll need all the help they can get.
A complex Webb of interstellar science
Earlier this month, NASA released the first images captured by the $10 billion Webb Telescope. One of the astronomers eagerly waiting for those images was Michael Kelley, a cometary scientist at the University of Maryland and a co-investigator on the telescope’s interstellar object proposal. Although his first taste of Webb’s potential lived up to the hype (its snap of the Carina Nebula is now his desktop background), Kelley sees Webb as perfectly suited for his line of work.
“I like to call it a ‘cometary science machine,’ Kelley tells Reverse. That’s because Webb’s imaging and spectrograph capability will give astronomers like Kelley an extraordinary amount of detail, including whether that comet is interstellar or homegrown.
Comets are primarily composed of three main ices — water, carbon dioxide, and carbon monoxide — Webb’s Near-Infrared Spectrograph (NIRSpec) will analyze the chemical composition of these ices simultaneously as they are slowly vaporized by the Sun’s heat. This gives astronomers clues as to where in its host system an interstellar object possibly formed. A lot of water, for example, could mean the object coalesced closer to its star because other ices require cooler temperatures, according to Kelley.
Using the Mid-Infrared Instrument Instrument (MIRI), Webb will also provide some clarity on the composition of dust emanating from these bodies. The data it provides could support some of the biggest theories explaining how the oceans formed or how life took root in our solar system.
“Carbon is a great molecule because it’s the thing that enables life,” Kelley says. “The thought is that comets and asteroids brought carbon to the surface of the Earth … one of the things we’re looking for is that carbonaceous dust.”
Comets are like cosmic time-capsules. Because they usually form in the furthest reaches of a star system, comets are often encased in ice, preserving the geology of their protoplanetary formation for billions of years.
Asteroids, on the other hand, are often subjected to warmer temperatures causing them to melt or change in other ways. While scientists think comets make up a majority of interstellar objects (because comets are more frequently ejected), an interstellar asteroid isn’t out of the question.
Cristina Thomas is a planetary scientist at Northern Arizona University, a co-investigator on the Webb proposal, and an all-around asteroid expert. While Kelley is interested in the chemistry of interstellar comets, Thomas is laser-focused on what Webb could tell us about the solid surface of a possible interstellar asteroid.
“I think a lot of people think of asteroids as sort of monolithic, like Han Solo flying through an asteroid field,” Thomas tells Reverse. “They’re actually so incredibly different from each other in so many different ways.”
Webb will be able to glimpse those differences using near- and mid-infrared wavelengths and provide data about an asteroid’s silicates, mineral signatures, possible surface hydration, and the composition of its surface — whether rocky or porous (like Bennu).
Because of ‘Oumuamua’s asteroid-like features, Thomas and her team used the cigar-shaped conundrum as a test case to determine what Webb will see if another interstellar asteroid comes to our galactic neighborhood.
“With ‘Oumuamua we didn’t get a lot of in-depth observations. [With Webb], we’re going to get wide wavelength coverage — it’s going to tell us a lot about the object itself and that’s going to really enable a full comparison,” Thomas says. “We might assume that other systems have similar building blocks, but to actually see that will be really great.”
Waiting for Rubin
While Webb is ready to gaze at the next interstellar object that crosses its path, one big hurdle remains — scientists have to find one first.
While two have been found in the past seven years, Kelley thinks a more reliable frequency of discovery is spotting one interstellar object every decade. Luckily, a new ground-based telescope perched on the Cerro Pachón ridge in north-central Chile will change all that.
As part of its Legacy Survey of Space and Time (LSST), the Vera C. Rubin Observatory will map the Solar System more accurately than ever before. The Hawaiian Pan-STARRS telescope discovered ‘Oumuamua with a 1.8-meter mirror. In comparison, Rubin is equipped with a mirror more than four times that size, which scientists estimate could explode the known number of objects in our Solar System by a factor of 10 (or even more). And among their number could be a number of strange objects whose speed and eccentricity points to interstellar origins.
“We need a new discovery in order for this project to work. It all depends on what gets discovered,” Kelley says. “Borisov was easily observed for a long time. ‘Oumuamua had to be observed within a month.”
Because astronomers could spot an interstellar object at any time, this particular study is part of NASA’s Target of Opportunity program. This program allows teams studying time-sensitive phenomena, like supernovae or interstellar objects, to interrupt Webb’s regularly scheduled programming with as little lead time as three days. This particular study falls under the category of “Disruptive Target of Opportunity,” meaning that Webb could focus its gaze on an incoming interstellar object within two weeks of discovery.
In the future, data from Webb on interstellar objects could inform other, even more ambitious space missions, such as the European Space Agency’s Comet Interceptor. Launching in 2029, this spacecraft will be parked at Lagrange Point 2 waiting for an appetizing long period comet or interstellar object to chase down and study. While telescopes are great, nothing compares to the science gathered from a spacecraft mission, which high-profile science projects like Rosetta and Osiris-REx prove without a doubt.
“Everytime we send a spacecraft somewhere we learn something completely new,” Thomas says. “But even if James Webb isn’t the same as sending a spacecraft, we’re still going to see so much more…It’s going to change the paradigm about how we think about these objects.”