In October 2017, a fast moving comet of extrasolar origin, 1I/2017 U1 (‘Oumuamua), was discovered close to the Earth with a ground-based telescope in Hawai’i. In November 2017, NASA astronomers pointed the agency’s Spitzer Space Telescope toward the interstellar object. ‘Oumuamua was too faint for Spitzer to detect when it looked more than two months after the comet’s closest approach to Earth. However, the ‘non-detection’ puts a new limit on how large ‘Oumuamua can be. The observations also suggest that the object may be up to 10 times more reflective than solar system comets.
An artist’s impression of ‘Oumuamua. Image credit: M. Kornmesser / ESO.
‘Oumuamua was first detected by the University of Hawaii’s Pan-STARRS 1 telescope on Haleakala, Hawaii, in October 2017 while the telescope was surveying for near-Earth asteroids.
Subsequent detailed observations conducted by multiple ground-based telescopes and the NASA/ESA Hubble Space Telescope detected the sunlight reflected off ‘Oumuamua’s surface.
Large variations in the object’s brightness suggested that ‘Oumuamua is highly elongated and probably less than 2,600 feet (800 m) in its longest dimension.
But NASA’s Spitzer Space Telescope tracks asteroids and comets using the infrared energy, or heat, that they radiate, which can provide more specific information about an object’s size than optical observations of reflected sunlight alone would.
The fact that ‘Oumuamua was too faint for Spitzer to detect sets a limit on the object’s total surface area.
However, since the non-detection can’t be used to infer shape, the size limits are presented as what ‘Oumuamua’s diameter would be if it was spherical.
Using three separate models that make slightly different assumptions about the object’s composition, Spitzer’s non-detection limited ‘Oumuamua’s spherical diameter to 1,440 feet (440 m), 460 feet (140 m) or perhaps as little as 320 feet (100 m).
The wide range of results stems from the assumptions about ‘Oumuamua’s composition, which influences how visible (or faint) it would appear to Spitzer were it a particular size.
“‘Oumuamua has been full of surprises from day one, so we were eager to see what Spitzer might show,” said Northern Arizona University’s Professor David Trilling, lead author of the new study.
“The fact that ‘Oumuamua was too small for Spitzer to detect is actually a very valuable result.”
The new size limit is consistent with the findings of a recent study, led by ESA astronomer Marco Micheli, which suggested that outgassing was responsible for the slight changes in ‘Oumuamua’s speed and direction as it was tracked last year. Dr. Micheli and co-authors concluded the expelled gas acted like a small thruster gently pushing the object.
That determination was dependent on ‘Oumuamua being relatively smaller than typical solar system comets. The conclusion that ‘Oumuamua experienced outgassing suggested that it was composed of frozen gases, similar to a comet.
“Our study also suggests that ‘Oumuamua may be up to 10 times more reflective (has a higher albedo) than the comets that reside in our Solar System — a surprising result,” Professor Trilling and colleagues said.
A comet’s albedo can change throughout its lifetime. When it passes close to the Sun, a comet’s ice warms and turns directly into a gas, sweeping dust and dirt off the comet’s surface and revealing more reflective ice.
‘Oumuamua had been traveling through interstellar space for millions of years, far from any star that could refresh its surface.
But it may have had its surface refreshed through such ‘outgassing’ when it made an extremely close approach to our Sun, a little more than five weeks before it was discovered.
In addition to sweeping away dust and dirt, some of the released gas may have covered the surface of ‘Oumuamua with a reflective coat of ice and snow — a phenomenon that’s also been observed in comets in our solar system.
The findings were published in the Astronomical Journal.
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David E. Trilling et al. 2018. Spitzer Observations of Interstellar Object 1I/’Oumuamua. AJ 156, 261; doi: 10.3847/1538-3881/aae88f
