PROVIDENCE, R.I. [Brown University] — For nearly two decades, the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter has captured finely detailed images of the Red Planet’s dusty surface. But in early October, NASA turned the camera’s gaze on a new subject: the interstellar comet 3I/Atlas. 

On Oct. 2, HiRISE snapped images of 3I/Atlas as it streaked past about 19 million miles from Mars. Because it’s only the third known interstellar object to pass through the solar system, scientists were eager to get as many looks at it as they could. The comet has already been imaged by powerful telescopes, including the Hubble and James Webb space telescopes, but its pass by Mars offered the closest vantage point yet. 

The federal government shutdown prevented NASA from releasing photos to the public and scientific community until Nov. 19, so the wider research community is just beginning to analyze what HiRISE was able to see. For Ingrid Daubar, an associate research professor at Brown and deputy principal investigator on HiRISE — the High-Resolution Imaging Science Experiment — it’s yet another feather in the cap of an instrument that has provided a remarkable scientific return. 

“HiRISE’s main job for the past 19 years has been to image the surface of Mars,” Daubar said. “But it’s a great camera, and we’ve got great engineers, so we’re also able to point it away from Mars and get images of other bodies with really accurate targeting. That’s what the team was able to do here.”

Not that it was easy. The comet is currently traveling at about 130,000 miles per hour, making it the fastest object ever observed in the solar system. The team had to catch the streaking object while also avoiding the wispy Martian atmosphere as well as stray light bouncing off the planet, both of which could contaminate the image. The team also needed to plan the image to avoid bright background stars, which could swamp light from the comet. 

When it images the Martian surface, HiRISE produces an incredible resolution of about 9 inches per pixel. That’s close enough to resolve boulder formations, rockslides, dust devils and other features on Mars’ dynamic surface. Given 3I/Atlas’ distance from Mars, the comet images have a resolution of about 19 miles per pixel. That’s still enough to gain some key scientific insights, according to Sam Birch, an assistant professor of Earth, environmental and planetary sciences at Brown who studies comets. 

In addition to offering clues about 3I/Atlas’ size, Birch says the images might be able to resolve some features in and around its coma, the mass of ice and dust that surrounds a comet’s nucleus. 

“As it’s illuminated by the sun and heats up, there’s going to be material coming off in the form of jets pointed toward the sun,” Birch said. “So, we might be able to resolve some of these sunward jets or other features in the near-nucleus environment.”

The HiRISE images also may provide data that scientists can use to compare the comet’s motion before and after its closest pass to the sun later this month. 

“We know comets spin up as they approach the sun,” Birch said. “If the HiRISE team can capture a rotation rate now, and then another imaging system captured rotation after the close approach to the sun, that would be really cool.”

While 3I/Atlas is only the third interstellar object ever found (hence the 3I in its name), scientists believe objects like this streak through the solar system often. As the recently commissioned Vera C. Rubin Observatory launches the Legacy Survey of Space and Time — a 10-year scan of the south sky — scientists expect to see many more of these objects. Birch says this investigation of 3I/Atlas will be good practice for the interstellar science to come. 

Daubar says the new images add to the already rich scientific return HiRISE has provided for the past two decades. 

“We used to think that Mars was a cold, dead planet,” she said. “But being able to watch seasonal changes or how the ice caps change over time has really told us a lot about the active processes on Mars, how the climate is changing, and how interactions between the surface and the atmosphere work.”

Not bad for an instrument that was originally designed to last two years. “I think you could definitely argue that most of the exciting science results have come from the extended mission,” Daubar said. 

And while no one could have foreseen that mission extending to the realm of interstellar comets, HiRISE proved to be up to the task yet again.