An object that came from outside our solar system is hurtling towards the sun at roughly 61km (38 miles) a second.
First detected in July, this visitor from outer space – known as 3I/Atlas – is only the third known interstellar object to have been observed, after ‘Oumuamua in 2017 and 2I/Borisov in 2019.
Excitement has been growing since the Hubble space telescope captured an image of the interstellar interloper, offering scientists a rare opportunity to analyse its size and chemical makeup.
“We’ve never had an object like this to study before,” says Dr Rebecca Allen, co-director of Swinburne’s Space Technology and Industry Institute.
What exactly is 3I/Atlas?
“We don’t know exactly where it came from, but it definitely came from outside the solar system,” says Dr Laura Driessen, a radio astronomer at the University of Sydney.
The name comes from “i” for interstellar, “3” as the third such object detected, and Atlas for the network of telescopes that identified it, called the Asteroid Terrestrial-impact Last Alert System.
While some have speculated about an alien incursion, scientists say 3I/Atlas is probably a comet. You may remember the speculation over ’Oumuamua, which astronomers checked for radio signals in case it was an alien craft. It wasn’t.
“There’s no evidence to point towards [3I/Atlas] being anything other than a really fun space rock that came from outside the solar system,” Driessen says. “Which, to be honest, is cool enough.”
Why are scientists excited about the interstellar object?
In the Hubble image, 3I/Atlas looks a bit like a “fuzzy snowball”, Allen says. Its hazy appearance is due to its “coma”, which is formed as the comet’s surface is heated by the sun, releasing dust and gas, and there are signs of a comet tail.
Those chemical signatures – of elements and molecules from outside our solar system – will become more visible the closer it gets to the sun, Allen says.
“We can use a tool called spectroscopy, and we can use light to actually understand exactly what that material is. Does this comet represent a solar system that had very similar conditions to our own, or is it something completely different?”
Researchers have already used the Hubble image to estimate the diameter of 3I/Atlas to be less than 5.6km, and potentially as small as 320 metres.
Where is 3I/Atlas heading?
The space object is travelling at more than 200,000km/h, Allen says.
“It’s going to kind of cruise through the inner solar system – in between Mars and Earth’s orbit – and then it will fly past the sun.
“This one is on what we call a hyperbolic orbit. So it’s not gravitationally bound to the sun. It’s travelling much too fast, and so it will just fly out the other side of the solar system.”
Its interstellar origins make 3I/Atlas different from other comets that orbit within the solar system, such as Halley’s comet, says Con Stoitsis, the comet and meteor director at the Astronomical Society of Victoria.
It is expected to make its closest approach to the sun between October and December – coming within about 210m kilometres – and then continue its journey into space.
According to Nasa, the closest 3I/Atlas will come to Earth is about 270m kilometres.
“We’re not really sure where it’s going to end up until it comes around from the other side of the sun, and starts heading out to the outer solar system again,” Stoitsis says.
Can we see the comet?
While faint at the moment, the comet is expected to brighten as it approaches the sun, eventually bringing it “within reach of most large amateur telescopes”, Stoitsis says.
Even then, 3I/Atlas will probably appear like a smudge, he says.
“We get bright-ish comets, three or four times a year, which are visible with small telescopes and binoculars. This is definitely not one of those,” he says. “Unless you’ve got a decent-sized telescope, you won’t see it.”