Comet 3I/ATLAS, the interstellar interloper currently zipping through our solar system, cannot easily be observed from Earth right now, because it is on the other side of the solar system. The sun is not directly blocking the view, but trying to make out a dim comet within the daytime glare is nigh impossible.
But Mars is also on that side of the solar system right now, and spacecraft there have been able to photograph the comet, which is just the third object known to have come from elsewhere in the Milky Way galaxy.
On Tuesday, the European Space Agency released a short movie of images that one of its spacecraft, the ExoMars Trace Gas Orbiter, took on Saturday as the comet made its closest approach to Mars.
The images successfully captured a bright dot of the comet moving among the more distant stars, an impressive achievement for a camera that “is designed to observe Mars,” said Nicolas Thomas, a professor of experimental physics at the University of Bern in Switzerland and the principal investigator for the instrument.
Typically, the orbiter, which has studied Mars since 2016, points its camera downward at the surface 250 miles below, snapping about three pictures a second with an exposure time of about 1.5 milliseconds, Dr. Thomas said. Here, it was aimed at a dot nearly 20 million miles away, with five-second exposures, trying to make out something that was between 1/10,000th and 1/100,000th as bright as its usual observations.
Five seconds was long enough to gather enough light, but not too long for the dot to become smeared too much by the motion of the comet.
“Surprised it was that good, to be honest,” Dr. Thomas said.
The camera also took images through its red, near-infrared and blue filters, which could give clues about the properties of Comet 3I/ATLAS. If, for example, the ratio of blue to red light becomes larger farther away from the nucleus, that could point to comet particles becoming smaller as ice turns to vapor, Dr. Thomas said. (Smaller particles scatter blue light more efficiently.)
If there were little or no ice, then the particles would remain roughly the same size.
The scientists will have to analyze the images more carefully before they can come to confident conclusions. Although they can see the comet clearly, the color filters block more of the light, making it more difficult to detect a genuine scientific signal amid noise in the sensors.
Mars Express, an older ESA spacecraft in orbit around Mars for more than two decades, also tried to snap pictures, but its camera could make exposures only half a second long. So far, nothing has turned up in those images, taken between Oct. 1 and Oct. 7.
It is possible that by stacking images together, scientists might be able to spot the comet, “but there’s been no success so far,” said Colin Wilson, the project scientist for both Mars Express and the Trace Gas Orbiter.
By themselves, the new images reveal nothing new about Comet 3I/ATLAS, Dr. Wilson said. But they add to a series of observations that will help scientists understand how an interstellar comet differs from the ones that swing around our solar system.
Dr. Wilson said his first question to the team analyzing the photos was: Was the comet where it was predicted to be? The second: Was it as bright as expected?
In both cases, the answers appear to be yes.
Dr. Thomas said additional images of the comet should be coming from NASA’s Mars Reconnaissance Orbiter. “I know that they got data on the second of October,” he said, “and I think it’s quite good.”
Amateurs sifting through images taken by NASA’s Perseverance rover on Mars think they spotted the comet’s streak.
But, with most of NASA and the United States government currently closed, that information cannot officially be released.
“We are supposed to communicate through NASA, currently not possible due to the shutdown,” Alfred McEwen, a planetary geologist at the University of Arizona who is the principal investigator for the camera aboard the American orbiter, said in an email.
Kenneth Chang, a science reporter at The Times, covers NASA and the solar system, and research closer to Earth.
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