‘Oumuamua: Asteroid, comet, or alien spaceship?


  • On October 19, 2017, astronomers operating the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 1 instrument in Maui, Hawaii, noticed an unusual object bursting out of the constellation Lyra about 32 million km from Earth, and hurtling across the Solar System towards an unknown destination.
  • It was too small to appear as more than just a point of light even on large telescopes, but it was evidently an unusually elongated object that was both rotating about an axis and tumbling along through space, given that its brightness varied dramatically every 7-8 hours — brightest when its full length faced the Earth, near invisible when it pointed towards the Earth. It was going at more than 87 km/second — fast enough to escape the gravity of the Sun.
  • Scientists named it ‘Oumuamua, Hawaiian for “scout” or “a messenger sent from far to reach out to us”, and concluded that the reddish-brown rock shaped like a cigar — about 800 m long and 80 m wide; extreme proportions never observed before — was the first interstellar object ever seen in the Solar System.

Was it an asteroid?

  • Scientists initially assumed it was a comet, but they also noted that it had no visible “coma” (atmosphere of dust and gas around a comet’s core) or “tail” (elongated cloud that points away from the Sun), the signature identifiers of comets as they approach the inner Solar System. In a paper in Nature in November 2017, the team that discovered ‘Oumuamua with the Pan-STARRS 1 telescope said in colour and imputed properties, ‘Oumuamua resembled known asteroids. It could have come from the star Vega in Lyra, which is known to have a debris disk; however, “the possibility that ‘Oumuamua has been orbiting the galaxy for billions of years cannot be ruled out”.

Or was it a comet?

  • In June 2018, Prof Marco Micheli of the European Space Agency’s SSA-NEO Coordination Centre in Frascati, Italy, made the case for ‘Oumuamua being a comet after all — and, in the process, proposed a new way to detect comets. In January 2018, the Hubble Space Telescope noted that ‘Oumuamua was gaining speed — it was more than 40,000 km ahead of its expected trajectory — and Dr Micheli said “there was something affecting its motion other than the gravitational forces of the Sun and planets”. While the acceleration of ‘Oumuamua was due to the jetting of volatile materials or “outgassing” (as it happens with comets), the fact that this was not visible as a tail was because the dust around the nucleus of this particular comet was probably “made of larger grains that were too heavy to be lifted off by the gas.

Maybe a spaceship?

  • In a paper accepted for publication in The Astrophysical Journal Letters, Abraham Loeb, Chair of Harvard’s Department of Astronomy, and his colleague Shmuel Bialy argue that “a more exotic scenario [that explains ‘Oumuamua’s sudden acceleration] is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization”. The authors say they “discuss the possible origins of such an object including the possibility that it might be a lightsail [a type of spacecraft propulsion device] of artificial origin”.
  • The possibility of ‘Oumuamua being an alien spacecraft had been discussed and discarded in the first paper written on it. “Our observations are entirely consistent with it being a natural object,” The New York Times quoted Dr Karen J Meech, leader of the Pan-STARRS 1 team, as saying a year ago.


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