In October 2017, astronomers were surprised by an asteroid that came racing into our Solar System from interstellar space. Now, they have determined that this object, named ’Oumuamua, is unlike anything normally found in the Solar System. It appears to be a dark red, highly-elongated metallic or rocky object, about 1,312 feet (400 m) long.
This artist’s impression shows the first interstellar asteroid — 1I/2017 U1 (’Oumuamua). Image credit: M. Kornmesser / ESO.
On October 19, 2017, the Pan-STARRS 1 telescope in Hawai’i picked up a faint point of light moving across the sky.
It initially looked like a typical small asteroid, but additional observations over the next couple of days allowed its orbit to be computed fairly accurately. The orbit calculations revealed beyond any doubt that this body did not originate from inside the Solar System, like all other asteroids or comets ever observed, but instead had come from interstellar space.
Although originally classified as a comet, additional observations revealed no signs of cometary activity after it passed closest to the Sun in September 2017. The object was reclassified as an interstellar asteroid and named 1I/2017 U1 (’Oumuamua).
Since its discovery, the object has faded from view. Its rapidly increasing distance from the Earth and Sun now makes it too faint to be studied by even the largest telescopes.
“Needless to say, we dropped everything so we could quickly point the Gemini telescopes at ’Oumuamua immediately after its discovery,” said Dr. Laura Ferrarese, Director of the Gemini Observatory.
“We had to act quickly. ’Oumuamua had already passed its closest point to the Sun and was heading back into interstellar space,” added ESO astronomer Dr. Olivier Hainaut.
This very deep combined image shows ’Oumuamua. It is surrounded by the trails of faint stars that are smeared as the telescopes tracked the moving asteroid. This image was created by combining multiple images from ESO’s Very Large Telescope as well as the Gemini South Telescope. The object is marked with a blue circle and appears to be a point source, with no surrounding dust. Image credit: ESO / K. Meech et al.
The astronomers found that ’Oumuamua varies dramatically in brightness by a factor of 10 as it spins on its axis every 7.3 hours.
“What we found was a rapidly rotating object, at least the size of a football field, that changed in brightness quite dramatically,” said Dr. Karen Meech, of the Institute for Astronomy, Hawai’i.
“This change in brightness hints that ’Oumuamua could be more than 10 times longer than it is wide — something which has never been seen in our own Solar System.”
The scientists also found that the asteroid is dense, possibly rocky or with high metal content, lacks significant amounts of water or ice, and that its surface is now dark and reddened due to the effects of irradiation from cosmic rays over millions of years. They also confirmed that it is completely inert, without the faintest hint of dust around it.
’Oumuamua is estimated to be at least 1,312 feet (400 m) long and approximately 335 feet (102 m) in diameter.
“While study of ’Oumuamua’s colors shows that this body shares characteristics with both Kuiper Belt objects and organic-rich comets and trojan asteroids, its orbital path says it comes from far beyond,” Dr. Meech said.
Preliminary orbital calculations suggested that the object had come from the approximate direction of the bright star Vega, in the northern constellation of Lyra.
However, even traveling at a breakneck speed of about 59,030 mph (95,000 km/hour), it took so long for the interstellar object to make the journey to our Solar System that Vega was not near that position when the asteroid was there about 300,000 years ago.
’Oumuamua may well have been wandering through the Milky Way, unattached to any star system, for hundreds of millions of years before its chance encounter with the Solar System.
Astronomers estimate that an interstellar asteroid similar to ’Oumuamua passes through the inner Solar System about once per year, but they are faint and hard to spot so have been missed until now.
“We are continuing to observe this unique object and we hope to more accurately pin down where it came from and where it is going next on its tour of the galaxy. And now that we have found the first interstellar rock, we are getting ready for the next ones,” Dr. Hainaut said.
The team’s findings are published in the journal Nature.
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Karen J. Meech et al. A brief visit from a red and extremely elongated interstellar asteroid. Nature, published online November 20, 2017; doi: 10.1038/nature25020
