An interstellar visit both familiar and alien

The arrival and swift departure of a mysterious object from another solar system earlier this month has raised questions about what lies beyond

Oumuamua’s elongated shape is unlike that of any asteroid seen in the Milky Way (ESO)

Visit the galaxy before the galaxy visits you.

This autumn, the galaxy came calling in the form of a small, reddish, cigar-shaped object named Oumuamua by astronomers based in Hawaii. They discovered it in October, careening through the solar system at 40,000mph, an interstellar emissary from points unknown.

Oumuamua (Oh-moo-a-moo-a), Hawaiian for “scout” or “messenger,” was not here long.

It was first noticed zooming out of the constellation Lyra on 19 October, about 20 million miles from Earth. By next May, it will already be passing Jupiter on its way out of the solar system.

The asteroid brought shades of Arthur C Clarke’s novel Rendezvous With Rama, in which explorers find and board an empty alien spacecraft sailing through the solar system. Or perhaps even reminders of the monoliths that power human evolution in 2001: A Space Odyssey.

The discovery set off a worldwide scramble for telescope time to observe the object. Astronomers from the Search for Extraterrestrial Intelligence Institute even got into the act, swinging into action to look for alien radio signals just in case.

For now, however, those are just science fiction thrills. “Our observations are entirely consistent with it being a natural object,” says Karen Meech of the University of Hawaii’s Institute for Astronomy and leader of the international collaboration that discovered Oumuamua with the Pan-STARRS1 telescope on Haleakala, Maui.

Oumuamua, an interstellar asteroid, passing through our solar system (NASA)

Meech’s team has now published the first report of their observations in Nature. The paper describes the interstellar visitor as both reassuringly familiar and utterly alien.

“We don’t see anything like that in our solar system,” Meech says.

In its colour and other imputed properties, Oumuamua resembles the asteroids we already know and fear will one day smash the Earth and human civilisation to smithereens.

But the asteroid’s shape is weird. It is extremely elongated, at least 10 times as long as it is wide, perhaps 800 yards by 80 yards.

Though the mysterious object is nearly gone, thousands like it probably lurk unsuspected and undetected in our solar system, according to the scientists.

The Pan-STARRS telescope was built to patrol the sky for dangerous asteroids in our own system, not interlopers from beyond. But astronomers got a surprise.

Meech learned in a phone call one night that her colleagues had found one whose path seemed to originate beyond the solar system altogether. “Wow, this is exciting,” Meech recalls thinking.

Astronomers had long surmised that interstellar debris might invade the solar system from time to time, in the form of icy chunks spit from the rocky disks forming faraway planets.

Such wanderers would manifest themselves as comets when they get close to our sun, vaporising and lighting up; however, they have not been seen. Now astronomers know why.

Oumuamua showed no such cometary brightening. It is so dark and faint that it could only have been detected by a powerful telescope with a wide field of view, like Pan-STARRS.

Many more should be visible to the Large Synoptic Survey Telescope, with a diameter of 8m, being built in Chile. “We have to get ready for these,” Meech says.

Oumuamua brightens and dims dramatically every 7.3 hours, which suggests that it is rotating about its short axis. That is something the little asteroid could endure without flying apart only if it were made of sterner, stronger stuff than the dirty snow that characterises most comets.

Spectral measurements have revealed that Oumuamua is dark red, the colour of many moons in the outer solar system on which icy organic molecules have been stained by radiation in outer space. Iron can also contribute to that colour, Meech says.

How Oumuamua got its shape is a mystery for now. Perhaps, Meech says, it was shot away from its home star by a supernova explosion. Or perhaps it was formed by a pair of objects that collided and stuck together. Stay tuned.

Where did it come from? Meech says the astronomers were initially excited when the orbit appeared to point to the brightest star in Lyra, Vega, which is known to have a debris disk. It would have taken the object about 600,000 years to get here from there, astronomers estimated.

But further refinements in the trajectory have made it less likely that Vega actually was the source.

The fact that Oumuamua was travelling at about the same speed relative to the sun as other nearby stars suggests that this is the asteroid’s first encounter with a new star system.

Still, the authors write in Nature, “The possibility that Oumuamua has been orbiting the galaxy for billions of years cannot be ruled out.”

Where it’s going is equally in the dark. Like the Voyager spacecraft that was slingshot around Jupiter, Oumuamua will leave the sun with more energy and heading in a different direction, Meech says.

The adventures of this asteroid and its ilk paint a very different picture of the galaxy than you might imagine while gazing up at a sky, in which the stars seem separate and sovereign, beaming away in solitude.

The oxygen and iron in our blood were created in a supernova explosion long ago and far away from here, and the gold in our wedding bands was formed in the collision of neutron stars. We now know that meteorites sprung by asteroid impacts on Mars land on Earth all the time.

Otherwise respectable astronomers speculate that one of them might have seeded Earth with life that started on Mars when it was warm and wet long ago.

But we can look even further out and backward in time for our connection to the cosmos. Consider the hundreds of thousands of years that Oumuamua might have taken to get here. While that might sound like a long time, it is a blink of cosmic time.

The Milky Way galaxy is 10 billion years old, which means that over the course of our galaxy’s lifetime so far, little Oumuamua might have cruised through some 20,000 star systems – a small fraction of the 200 billion stars in the galaxy, but still a goodly number of stamps on its cosmic passport.

Oumuamua would have trailed behind bits of dust and debris, and so the stars and the worlds of the galaxy mix it up. It may be that the universe is a small place after all.

© New York Times