By NASA // November 17, 2018
In 2017, the NASA Spitzer Space Telescope discovered the object known as & quot; Oumuamua
(NASA) – In November 2017, scientists pointed the NASA Spitzer Space Telescope toward the object known as "Oumuamua – the first interstellar object known to visit our solar system." The infrared Spitzer was one of the many telescopes pointed out in & # 39; ; Oumuamua in the weeks after its discovery in October.
'Oumuamua was too weak for Spitzer to detect when it seemed more than two months after the object's closest approach to Earth in early September. However, "no detection" puts a new limit on how large the foreign object can be.
The results are reported in a new study published today in the Astronomical Journal and co-authored by scientists at the NASA Jet Propulsion Laboratory in Pasadena, California.
The new size limit is consistent with the results of a research paper published earlier this year which suggested that gas output was responsible for the slight changes in speed and direction of Oumuamua last year: The authors of this article conclude the gas expelled acted like a small propeller gently pushing the object.
This determination depended on & # 39; Oumuamua being relatively smaller than typical comets in the solar system. (The conclusion that "Oumuamua experienced the release of gas suggested that it was composed of frozen gases, similar to a comet").
"Oumuamua has been full of surprises since day one, so we were eager to see what Spitzer could show," said David Trilling, lead author of the new study and professor of astronomy at Northern Arizona University. "The fact that 'Oumuamua is too small for Spitzer to detect is, in fact, a very valuable result.'
"Oumuamua was first detected by the Pan-STARRS 1 telescope at the University of Hawaii in Haleakala, Hawaii (the object's name is a Hawaiian word meaning" long-distance visitor first ") in October 2017 while the telescope was surveying asteroids near Earth. .
Subsequent detailed observations by several ground-based telescopes and the NASA Hubble Space Telescope have detected sunlight reflected from the surface of & quot; Oumuamua & quot ;. Large variations in object brightness suggest that "Oumuamua is highly elongated and is probably less than 800 meters in greatest dimension."
But Spitzer tracks asteroids and comets using the infrared energy, or heat, they radiate, which can provide more specific information about the size of an object than the optical observations of reflected sunlight.
The fact that "Oumuamua is too weak for Spitzer to detect limits the total surface area of the object. However, since no detection can not be used to infer shape, size limits are presented as the diameter of Oumuamua if it were spherical.
Using three separate models that make slightly different assumptions about object composition, Spitzer's non-detection limited the Oumuamua "spherical diameter" to 1,440 feet (440 meters), 140 meters, or perhaps up to 100 meters. . The wide range of results derives from assumptions about the composition of Oumuamua, which influences Spitzer's visibility (or fainting) if it were a specific size.
Small but thoughtful
The new study also suggests that "Oumuamua can be up to 10 times more reflective than the comets that reside in our solar system – a surprising result, according to the authors of the article.
Because infrared light is largely the heat radiation produced by "hot" objects, it can be used to determine the temperature of a comet or asteroid; in turn, this can be used to determine the surface reflectivity of the object – what scientists call albedo. Just as a dark T-shirt in the sunlight heats faster than a light, an object with low reflectivity retains more heat than an object with high reflectivity. So, a lower temperature means a larger albedo.
The albedo of a comet can change throughout its life. As it passes close to the sun, ice from a comet heats up and turns directly to gas, sweeping the dust and dirt from the comet's surface and revealing more reflective ice.
"Oumuamua traveled through interstellar space for millions of years, far from any star that could refresh its surface, but may have had its surface refreshed through such a" gas scrubbing "when it made an extremely close approximation to our Sun, a little more than five weeks before it was discovered.In addition to sweeping dust and dirt, some of the released gas may have covered Oumuamua's surface with a reflective layer of ice and snow – a phenomenon that has also been observed in comets in our solar system.
"Oumuamua is on its way to our solar system – almost as far from the Sun as Saturn's orbit – and is well beyond the reach of any existing telescope.
"Normally, if we get a measure of a comet that's kind of weird, we go back and measure again until we understand what we're seeing," said Davide Farnocchia of the Center for the Study of Near-Earth Objects at CNNOS at JPL. and co-author of both works. "But this one is gone for ever; we probably know as much about it as we'll know. "
JPL manages the Spitzer Space Telescope mission to the NASA Science Mission Directory in Washington. Scientific operations are conducted at the Spitzer Science Center, Caltech, in Pasadena, California.
The spacecraft's operations are based at the Lockheed Martin Space Systems Company in Littleton, Colorado. The data are archived in the Infrared Science Archive, located at Caltech's IPAC. Caltech manages JPL for NASA.
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