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An intriguing interstellar object known as 3I/ATLAS is, as of now, on the opposite side of the sun from Earth – making it impossible to observe from the ground.
But spacecraft orbiting around our celestial neighbors Mars and Jupiter are in a prime position to get a look at the object as it hurtles toward the sun. And over the course of the last few days, two Martian orbiters did exactly that – imaging the massive object as it passed relatively close to the Red Planet.
As 3I/ATLAS warps and grows the closer it gets to our solar system's star, public fascination with the cosmic outsider only seems to become stronger.
After all, it's incredibly rare for any kind of space object originating from a star that's not our sun to be spotted visiting Earth's neighborhood. Contributing to the object's fame (or infamy) is also a wild viral theory it could be an alien spaceship – though most astronomers are in agreement that it's almost certainly a comet.
Here's everything to know about what the two Mars-orbiting spacecraft found, and what's next as scientists race to observe this fascinating interstellar interloper.
What is 3I/ATLAS?
An object known as 3I/ATLAS made news in July when it was confirmed to have originated outside Earth's solar system.
When it was discovered, the interstellar visitor was whizzing at about 137,000 miles per hour, though it's expected to only pick up speed as it continues its journey toward the sun.
The observation was reported to the Minor Planet Center, the official authority for observing and reporting new asteroids, comets and other small bodies in the solar system. The object, eventually confirmed to almost certainly be a comet and named 3I/ATLAS, was later confirmed to have interstellar origins after follow-up observations.
How did 3I/ATLAS get its name?
The "3I" in the object's name is due to its status as one of three interstellar interlopers ever discovered in our cosmic neighborhood. Comet Oumuamua, Hawaiian for “scout” or “messenger,” became the first interstellar object ever detected in the solar system in 2017, followed by the comet Borisov in 2019.
As for "ATLAS," that portion of the name is derived from the telescope credited with the find. The telescope in Chile – part of the NASA-funded Asteroid Terrestrial-impact Last Alert System, or ATLAS – was the first to spot what initially looked like an unknown asteroid on a path approaching Earth’s orbit.
What's the difference between a comet and an asteroid?
Asteroids are made up of rocky material long left over from our solar system's formation. Most reside in the main asteroid belt between the orbits of Mars and Jupiter.
Telescope observations, though, helped astronomers determine that 3I/ATLAS displays all the telltale signs of a comet. That's because the object is still active, meaning it's composed of not just rock, but also has an icy nucleus and a bright cloud of gas and dust, known as a coma, surrounding it.
That material begins to heat up and spew out the closer a comet gets to the sun.
Mars orbiters observe comet
Two Mars orbiters – Mars Express and ExoMars Trace Gas Orbiter – were able to observe 3I/ATLAS from Oct. 1 until Tuesday, Oct. 7. The comet, though, came closest to the Red Planet on Oct. 3, when it was about 18.6 million miles from the two spacecraft as they orbited Mars, according to the European Space Agency.
Both spacecraft have a fleet of cameras designed to photograph the bright surface of Mars from just a few hundred to a few thousand miles below. For that reason, the orbiters struggled to capture accurate measurements from a comet so far away, the ESA said.
In images captured by the ExoMars, 3I/ATLAS appears as a fuzzy white dot moving downward near the center of the images. The dot is the comet's center, comprised of its icy-rocky nucleus and the surrounding coma.
Scientists continue to analyze the images captured by the Mars Express to see if they can spot the faint comet, according to the agency.
“Though our Mars orbiters continue to make impressive contributions to Mars science, it’s always extra exciting to see them responding to unexpected situations like this one," Colin Wilson, project manager for both missions at the ESA, said in a statement. "I look forward to seeing what the data reveals following further analysis.”
Jupiter spacecraft next to get look of 3I/ATLAS
The ESA’s Jupiter Icy Moons Explorer (Juice) will observe the comet between Nov. 2 and Nov. 25 when the object is expected to be in a "very active state" following its closest approach to the sun. But because the spacecraft is so far from Earth on the other side of the sun, scientists don't expect to receive its observations until February, according to the ESA.
The object will reappear in December on the other side of the sun, making it visible once again from Earth.
How close will 3I/ATLAS get to Earth?
The closest 3I/ATLAS will approach Earth is about 170 million miles, according to NASA.
The object will instead pass a little closer to the sun, coming within 130 million miles on Oct. 30. For reference, the sun is about 93 million miles away from Earth.
Webb, Hubble among telescopes to study interstellar visitor
Unlike comets bound to the sun's gravity, 3I/ATLAS is traveling on a hyperbolic orbit that will eventually carry it out of the solar system and back into interstellar space. That's why, even though the comet poses no threat to Earth, the world's astronomers and space agencies are racing to study planetary material that formed from another star.
A fleet of NASA space telescopes have already been returning plenty of images and data back to Earth gathered from glimpsing the comet.
NASA's iconic Hubble Space Telescope previously got a look at the comet in July, collecting data that allowed astronomers to estimate the size of the comet’s solid, icy nucleus as anywhere from 1,000 feet to 3.5 miles wide.
The James Webb Space Telescope then observed the interstellar object Aug. 6 in near-infrared light, followed by the newer SPHEREx telescope from Aug. 7-Aug. 15, to get a better idea of its physical properties and chemical makeup.
More recently, a ground telescope in Chile glimpsed the comet's glowing tail, which observations reveal has steadily been growing as it cruises toward the sun. The Gemini South telescope in Chile, operated by the National Space Foundation’s NoirLab, collected observations showing that 3I/ATLAS displays prominent tail and a broad coma, or a cloud of gas and dust that forms around the comet’s icy nucleus as it gets closer to the sun.
Eric Lagatta is the Space Connect reporter for the USA TODAY Network. Reach him at elagatta@gannett.com
This article originally appeared on USA TODAY: Mars orbiters get look at interstellar 3I/ATLAS. What they found, what's next
Reporting by Eric Lagatta, USA TODAY / USA TODAY
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