Study Suggests ‘Oumuamua Is Much Smaller Than Previously Thought

Christopher Davidson
November 16, 2018

One of the tools astronomers trained on 'Oumuamua's fleeing form was NASA's Spitzer Space Telescope, which is optimized to view the cosmos in infrared light - in other words, heat.

An artist's concept of interstellar asteroid 1I/2017 U1 ('Oumuamua) as it passed through the solar system after its discovery in October 2017. Image credit: M. Kornmesser / ESO.

Officials with NASA's Jet Propulsion Laboratory (JPL) came to their conclusion after pointing the United States space agency's Spitzer Space Telescope toward 'Oumuamua in November of 2017 after it was initially detected in Hawaii by the Pan-STARRS 1 telescope the month prior.

Subsequent detailed observations conducted by multiple ground-based telescopes and the NASA/ESA Hubble Space Telescope detected the sunlight reflected off 'Oumuamua's surface.

Previous observations, which focused on determining 'Oumuamua's size by studying its brightness, suggested that the mysterious object was roughly 2,600 feet long and shaped like a cigar. Now, NASA says it has a better idea of its size.

The fact that 'Oumuamua was too faint for Spitzer to detect sets a limit on the object's total surface area.

The non-detection can't be used to infer shape (believed to be needle-esque), but it does set limits on the odd item's size. Based upon different assumptions about the object's composition and the amount of light it reflects, the upper limit on 'Oumuamua's "spherical diameter" is 100 to 440 meters. 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.

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"The fact that 'Oumuamua was too small for Spitzer to detect is actually a very valuable result", said David Trilling, a professor of astronomy at the Northern Arizona University and the lead author of the study in a recently issued statement.

As the study report surfaced online, adamant conspiracy theorists have started claiming that the top-notch cloaking technology used by aliens traveling inside Oumuamua could be the result of its undetected journey.

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 previous year.

Astronomers also noted a display of "outgassing", in which expelled gas acts like a thruster, possibly changing the object's speed and direction. The research showed the forces created by the sublimation of frozen gases could explain slight shifts in the object's trajectory, but the theory required 'Oumuamua to be relatively small. Oumuamua, the elongated chunk of space stuff that might be an asteroid or might be a comet (or might be something else entirely), slipped around our Sun and back out into space so fast scientists barely had time to spot it.

The study also found that the object "may be up to 10 times more reflective than the comets that reside in our solar system", NASA wrote, because outgassing may have swept accumulated filth off of it and revealed underlying ice and snow.

A comet's albedo can change throughout its lifetime.

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