For the first time that we know, an interstellar visitor has zoomed through our solar system. The small space rock, tentatively called A/2017 U1, is about a quarter of a mile long and astronomers across the world are racing to study it before it departs just as quickly as it arrived.
“We’ve never seen anything like this before,” said Rob Weryk, an astronomer at the University of Hawaii Institute for Astronomy.
On October 19, Weryk was reviewing images captured by the university’s Pan-STARRS 1 telescope on the island of Maui when he came across the object. At first he thought it was a type of space rock known as a near-Earth object, but he realised its motion did not make sense. It was much faster than any asteroid or comet he had seen before. He quickly realised that it was not of this solar system.
“It’s moving so fast that the sun can’t capture it into an orbit,” Weryk said.
After contacting a colleague at the European Space Agency to discuss the find, he submitted it to the Minor Planet Center, which tracks objects in the solar system, to share with other astronomers.
“I was not expecting to see anything like this during my career, even though we knew it was possible and that these objects exist,” said Davide Farnocchia, a navigational engineer with Nasa’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in Pasadena, California.
Astronomers had predicted such an occurrence, but this is the first time that it has been recorded.
For the past few days Farnocchia has been calculating the strange object’s path.
“It was obvious that the object has a hyperbolic orbit,” he said, meaning that its trajectory is open-ended rather than elliptical like the objects in our solar system. That shows that it came from outside the solar system and will leave the solar system.
The object came closest to the sun on September 9, at a distance of about 37 million kilometres. With a boost from the star’s gravity, it zoomed by at about 88km/s with respect to the sun, Farnocchia said. Then on October 14, the object came within about 24 million kilometres of Earth, zipping by at about 59.5km/s, with respect to the Earth. That’s more than three times as much velocity as the escape trajectory for the New Horizons spacecraft, which completed a flyby of Pluto in 2015, he said.
Now it’s moving away at about 40km/s, he said, and will exit the solar system at about 25km/s. That is faster than the current velocity of the Voyager 1 spacecraft, which became the first spacecraft from Earth to enter interstellar space in August 2012.
Scientists around the world are watching its journey, hoping to glean as much information as they can before it gets too far away.
“We are just scrambling right now to secure big telescope time, prepare our observations and download the data,” said Karen Meech, an astronomer at the University of Hawaii Institute for Astronomy. Because the object came from outside our solar system, it may be made up of completely different material than the asteroids and comets that we have studied. She and other astronomers think that in the next few weeks they will have more insight into the composition and size of A/2017 U1, and in time, where exactly it came from.
Meech noted that scientists did not have much warning about this object when it came into the solar system because it was blocked by the brightness of the sun. It very much came without warning, she said.
But there is no need to panic, said Lindley Johnson, Nasa’s planetary defence officer. In the realm of things that could hit Earth and obliterate our existences, an interstellar Armageddon is pretty low on the list.
“The near-Earth asteroids are many times, hundreds of thousands of times, more likely to occur, and even those are extremely rare events,” Johnson said. “It’s really nothing that people should worry about. I certainly don’t lie awake worrying about it.”