In November2017, scientists pointed NASA's Spitzer Space Telescope toward the object knownas 'Oumuamua- the first known interstellar object to visit our solar system. The infraredSpitzer was one of many telescopes pointed at 'Oumuamua in the weeks after itsdiscovery that October.

'Oumuamua wastoo faint for Spitzer to detect when it looked more than two months after theobject's closest aproach to Earth in early September. However, the "non-detection"puts a new limit on how large the strange object can be. The results arereported in a new study published today in the Astronomical Journal andcoauthored by scientists at NASA's Jet Propulsion Laboratory in Pasadena,California.

Scientists have concluded that vents on the surface of 'Oumuamua must have emitted jets of gases, giving the object a slight boost in speed, which researchers detected by measuring the position of the object as it passed by Earth in 2017. Credit: NASA/JPL-Caltech Larger view

The new size limitis consistent with the findings of a research paper published earlierthis year, which suggested that outgassing was responsible for theslight changes in 'Oumuamua's speed and direction as it was tracked last year:The authors of that paper conclude the expelled gas acted like a small thrustergently pushing the object. That determination was dependent on 'Oumuamua beingrelatively smaller than typical solar system comets. (The conclusion that'Oumuamua experienced outgassing suggested that it was composed of frozen gases,similar to a comet.)

"'Oumuamuahas been full of surprises from day one, so we were eager to see what Spitzer mightshow," said David Trilling, lead author on the new study and a professorof astronomy at Northern Arizona University. "The fact that 'Oumuamua wastoo small for Spitzer to detect is actually a very valuable result."

'Oumuamua wasfirst detected by the University of Hawaii's Pan-STARRS 1 telescope onHaleakala, Hawaii (the object's name is a Hawaiian word meaning "visitorfrom afar arriving first"), in October 2017 while the telescope wassurveying for near-Earth asteroids.

Subsequent detailedobservations conducted by multiple ground-based telescopes and NASA's HubbleSpace Telescope detected the sunlight reflected off 'Oumuamua's surface. Largevariations in the object's brightness suggested that 'Oumuamua is highly elongatedand probably less than half a mile (2,600 feet, or 800 meters) in its longestdimension.

But Spitzertracks asteroids and comets using the infrared energy, or heat, that they radiate,which can provide more specific information about an object's size than opticalobservations of reflected sunlight alone would.

The fact that'Oumuamua was too faint for Spitzer to detect sets a limit on the object's totalsurface area. However, since the non-detection can't be used to infer shape, thesize limits are presented as what 'Oumuamua's diameter would be if it werespherical. Using three separate models that make slightly different assumptionsabout the object's composition, Spitzer's non-detection limited 'Oumuamua's "sphericaldiameter" to 1,440 feet (440 meters), 460 feet (140 meters) or perhaps aslittle as 320 feet (100 meters). The wide range of results stems from theassumptions about 'Oumuamua's composition, which influences how visible (or faint)it would appear to Spitzer were it a particular size.

Small but Reflective

The new studyalso suggests that 'Oumuamua may be up to 10 times more reflective than the cometsthat reside in our solar system - a surprising result, according to the paper'sauthors. Because infrared light is largely heat radiation produced by"warm" objects, it can be used to determine the temperature of acomet or asteroid; in turn, this can be used to determine the reflectivity ofthe object's surface - what scientists call albedo. Just as a dark T-shirt in sunlightheats up more quickly than a light one, an object with low reflectivity retainsmore heat than an object with high reflectivity. So a lower temperature means ahigher albedo.

A comet'salbedo can change throughout its lifetime. When it passes close to the Sun, acomet's ice warms and turns directly into a gas, sweeping dust and dirt off thecomet's surface and revealing more reflective ice.

'Oumuamua hadbeen traveling through interstellar space for millions of years, far from anystar that could refresh its surface. But it may have had its surface refreshed throughsuch "outgassing" when it made an extremely close approach to our Sun,a little more than five weeks before it was discovered. In addition to sweepingaway 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 alsobeen observed in comets in our solar system.

'Oumuamua ison its way out of our solar system - almost as far from the Sun as Saturn'sorbit - and is well beyond the reach of any existing telescopes.

"Usually,if we get a measurement from a comet that's kind of weird, we go back andmeasure it again until we understand what we're seeing," said DavideFarnocchia, of the Center for Near Earth Object Studies (CNEOS) at JPL and acoauthor on both papers. "But this one is gone forever; we probably knowas much about it as we're ever going to know."

JPL manages the Spitzer Space Telescope mission for NASA'sScience Mission Directorate in Washington. Science operations are conducted atthe Spitzer Science Center at Caltech in Pasadena, California. Spacecraftoperations are based at Lockheed Martin Space Systems Company in Littleton,Colorado. Data are archived at the Infrared Science Archive housed at IPAC atCaltech. Caltech manages JPL for NASA.

For more information about Spitzer, visit:

https://spitzer.caltech.edu

https://www.nasa.gov/spitzer

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NASA Learns More About Interstellar Visitor 'Oumuamua