Interstellar asteroid is actually just a ‘tiny, weird comet’ after all

28 Jun 2018

An artist’s impression of the comet ‘Oumuamua. Image: ESA/Hubble, NASA, ESO, M Kornmesser

An object that has captivated astronomers since its discovery in our solar system has raised many more questions than answers, at least until now.

With Asteroid Day (30 June) fast approaching – a day in which we make ourselves aware of the threat posed by interstellar objects hitting Earth – it seems fitting that a nearby asteroid has been found to not actually be an asteroid at all.

Since its discovery towards the end of last year, ‘Oumuamua was heralded as the first interstellar object to appear in our solar system, but confused many astronomers with its peculiar trajectory and the fact that it could potentially harbour some form of biological life.

During the many months since, attempts to classify it have flip-flopped, with it being changed from an asteroid to a comet and back again as time went on.

Now, new research published in Nature seems to finally determine that the object is indeed a comet and not an asteroid.

Using the Hubble Space Telescope and some of the most powerful Earth-based telescopes on the planet, an international team of astronomers revealed that the object is actually moving faster than we had predicted.

The measured gain in speed is tiny and it is actually slowing down because of the pull of the sun, but still not as slow as predicted by celestial mechanics.

More questions

To explain why it is going faster than we thought, the team led by the European Space Agency’s Marco Micheli said the likeliest explanation is that ‘Oumuamua is venting material from its surface as it gets heated by the sun, in a behaviour known as outgassing.

This small and steady boost is enough to push it to a cruising speed of 114,000kph; in doing so, this confirms it is a comet, not an asteroid.

“We think this is a tiny, weird comet,” Micheli said. “We can see in the data that its boost is getting smaller the farther away it travels from the sun, which is typical for comets.”

The mystery is by no means over, however. While a typical comet’s outgassing emits dust and gas to form a cloud of material, no such cloud was detected at ‘Oumuamua. This led the team members to believe that the comet might just vent unusually large, coarse dust grains that they can’t see.

Now, the team admits, the recalculated speed will make it even more difficult to find where it actually came from.

Colm Gorey was a senior journalist with Silicon Republic