‘Oumuamua: new findings rejig size

A nasa animation of oumuamua, showing it "outgassing" as it moves.

A NASA animation of Oumuamua, showing it ‘outgassing’ as it moves.


Scientists using NASA’s Spitzer Space Telescope have deduced new information about ‘Oumuamua, the first known object to enter the solar system from outside, through the counterintuitive approach of not seeing it.

Spitzer, an orbiting infrared telescope launched in 2003, was one of many observatories quickly pointed towards ‘Oumuamua after it was first detected the Pan-STARRS telescope at Haleakala Observatory, Hawaii, on 19 October 2017.

The NASA craft was unable to see it – and that, reports a large team headed by David Trilling from Northern Arizona University Flagstaff in the US, is a very handy finding indeed. Not being able to detect the object provides a valuable constraint on its upper size limit.

“’Oumuamua has been full of surprises from day one, so we were eager to see what Spitzer might show,” says Trilling.

“The fact that ‘Oumuamua was too small for Spitzer to detect is actually a very valuable result.”

Earlier observations by NASA’s Hubble Telescope and several ground-based facilities had used sunlight reflected off ‘Oumuamua’s surface to estimate its size amd shape. The consensus was that it was an elongated thing, probably around 800 metres long.{%recommended 7439%}

The fact that Spitzer was unable to detect it by looking for radiated infrared energy, however, suggests that these estimates are too generous.

The telescope is unable to estimate shape, so Trilling and colleagues used its data to model ‘Oumuamua as a sphere. Three sets of differing parameters produced diameters of 440, 140 and 100 metres.

The Spitzer data also threw up another, rather surprising, result. ‘Oumuamua appears to be more than 10 times as reflective as comets resident within the solar system – and that’s weird, given that it was travelling through interstellar space for millions of years, a long way from any source of heat, before swooping in close to the sun.

Trilling’s team suggest the result might be explained by the sun’s heat prompting the object to “outgas”, leading to it being covered by a fresh sheet of highly reflective ice and snow.

The truth or otherwise of this suggestion, however, will never be established.

“Usually, if we get a measurement from a comet that’s kind of weird, we go back and measure it again until we understand what we’re seeing,” says Davide Farnocchia, of NASA’s Centre for Near Earth Object Studies (CNEOS).  

“But this one is gone forever; we probably know as much about it as we’re ever going to know.”

The research is published in The Astronomical Journal.

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