Here's what astronomers know so far about the 3rd interstellar visitor ever found

On July 1, the Asteroid Terrestrial-impact Last Alert System (ATLAS) detected what was first believed to be an asteroid. As calculations for its orbit progressed, it was found to be from outside our solar system, only the third interstellar object ever detected.

Since its discovery, astronomers have been frantically trying to gather as much information as they can about the newest interstellar interloper dubbed 3I/ATLAS. It's been challenging due to its dusty cloud of gases, known as a coma, hiding the nucleus, but they've gotten some interesting data so far.

Here's what we know, and what questions remain.

One of the first things that was discovered about 3I/ATLAS was that it was in fact something from beyond our own solar system that was kicked out from another stellar system as it was forming planets.

"We know that it is definitely interstellar, and we know that because it's travelling really, really fast, and it's actually moving faster than the escape speed from the solar system," said David Jewitt, astronomer and professor of astronomy at UCLA. "So basically, it's going so fast that the gravity of the sun cannot hold it back."

Jewitt also noted that it is the fastest thing that they've ever seen coming through the solar system, travelling at roughly 60 kilometres per second.

This also means it could be the oldest thing to ever pass through our solar system.

Another thing that astronomers discovered early on was that, rather than being an asteroid, it was a comet.

Asteroids are bodies of mainly rock, while comets are composed of frozen gases, rock, and dust. Once they get closer to the sun, the solar radiation heats up the frozen gases, which produces a coma around the nucleus, which is the main body. The coma creates the fuzziness around the nucleus that you see in photographs. It also creates a tail that is most associated with comets.

And while there was some speculation as to whether or not it could be alien tech, "It's not a spaceship," Jewitt said.

"It is doing things that we expect comets to do. It's producing the types of gasses that we see comets produce. It's got a coma and a tail now pointed in the expected direction," said James Wray, a professor at the Georgia Institute of Technology's school of Earth and atmospheric sciences.

"I would say the short summary is it looks generally like a comet. But in detail, there are some intriguing differences from from solar system comets."

One of those intriguing differences is its polarization, which simply put, is the way light comes off of it, which includes its electric and magnetic field, is very different than what has been observed with other comets, Wray said.

Instead of the light going in different directions, the light appears in one direction.

"Probably what what we're seeing … is that the the grains that have popped off the nucleus, for one reason or another and are floating around it now, have some unusual size or shape, distribution or composition, or combination of all those things, compared to the particles that tend to drift off of the nuclei of of solar system comets," Wray said.

Another interesting observation, both Jewitt and Wray noted, is its composition.

Both the James Webb Space Telescope and SphereX, another space telescope, found that there was an abundance of carbon dioxide compared to what is seen in solar system comets.

"We don't actually know for sure if it's unusual for interstellar objects, because this is the first one we've been able to measure it for," Wray said.

But it also contains things like carbon monoxide and, as is most typical, water ice.

When it comes to its size, initially it was believed that it could be tens of kilometres wide, but now that there have been more observations, its nucleus is believed to be no more than roughly 2.8 metres across, Jewitt said.

But he'd like to know more about its shape, but unfortunately, it is hidden from telescopes so far due to its dusty coma.

The first interstellar visitor was 1I/Oumuamua in 2017, discovered by Canadian Robert Weryk using the Pan-STARRS telescope at Haleakalā Observatory, Hawaii.

The second — Comet 2I/Borisov — came just two years later in 2019.

(The comets are designated with a number and the letter "I" to indicate which number the interstellar object is.)

It took another six years for the next interstellar visitor. But Jewitt said that recent calculations show that the solar system could have a lot more visitors from beyond. A lot more.

"The best estimate of the density is one of those guys every 10 cubic astronomical units of space ... AU is the distance between the Earth and the Sun. So that means that at any one time there should be about 10,000 closer to the sun than Neptune," he said.

INTERACTIVE | Paths of all three interstellar visitors

This animation illustrates the orbits of interstellar visitors 1I/Oumuamua, 2I/Borisov and the most recent one, 3I/ATLAS.

"They take about 10 years to cross the solar system from one side to the other. So that means there are 1,000 coming in per year and 1,000 going out per year in steady state."

But he adds, that calculation could still be off by a factor of 10.

What's exciting, and what could put that estimate to the test, is the recent addition of the Vera C. Rubin telescope in Chile. It scans the entire sky every few days, and it could lead to many more of these interlopers being discovered, he said. This would also help detect them earlier so that they can be studied longer.

As for 3I/ATLAS, it will disappear into the sun's glare in the coming weeks, and will reappear at night some time in December. Astronomers should have enough time to study it as it leaves the solar system, as it will be visible for roughly a year, Jewitt said.

That is, if it survives its orbit around the sun.

Fortunately, it won't come too close to the sun compared to other comets, "but you never know," Wray said.

"If it does survive [its orbit around the sun], then we should get really the best data in the December time frame, when it's closest to Earth and back away from the sun in the sky."

While Wray is disappointed that 3I/ATLAS will be lost to Earth observations for a while, he's most excited by something else.

"It's going to come a lot closer to Mars than to Earth, such that one of the Mars cameras that I have been using for over 20 years since the start of grad school, will, in this case, get better resolution than Hubble next month," he said.

He said the Mars Reconnaissance Orbiter will get about three times finer resolution than Hubble when 3I/ATLAS makes its flyby of Mars on Oct. 3.