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In November
2017, scientists pointed NASA’s Spitzer Space Telescope toward the object known
as ‘Oumuamua
– the first known interstellar object to visit our solar system. The infrared
Spitzer was one of many telescopes pointed at ‘Oumuamua in the weeks after its
discovery that October.
‘Oumuamua was
too faint for Spitzer to detect when it looked more than two months after the
object’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 are
reported in a new study published today in the Astronomical Journal and
coauthored 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 limit
is consistent with the findings of a research paper published earlier
this year, which suggested that outgassing was responsible for the
slight 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 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.)
“‘Oumuamua
has been full of surprises from day one, so we were eager to see what Spitzer might
show,” said David Trilling, lead author on the new study and a professor
of astronomy at Northern Arizona University. “The fact that ‘Oumuamua was
too small for Spitzer to detect is actually a very valuable result.”
‘Oumuamua was
first detected by the University of Hawaii’s Pan-STARRS 1 telescope on
Haleakala, Hawaii (the object’s name is a Hawaiian word meaning “visitor
from afar arriving first”), in October 2017 while the telescope was
surveying for near-Earth asteroids.
Subsequent detailed
observations conducted by multiple ground-based telescopes and NASA’s 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 half a mile (2,600 feet, or 800 meters) in its longest
dimension.
But Spitzer
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 were
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 meters), 460 feet (140 meters) or perhaps as
little as 320 feet (100 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.
Small but Reflective
The new study
also suggests that ‘Oumuamua may be up to 10 times more reflective than the comets
that reside in our solar system – a surprising result, according to the paper’s
authors. Because infrared light is largely heat radiation produced by
“warm” objects, it can be used to determine the temperature of a
comet or asteroid; in turn, this can be used to determine the reflectivity of
the object’s surface – what scientists call albedo. Just as a dark T-shirt in sunlight
heats up more quickly than a light one, an object with low reflectivity retains
more heat than an object with high reflectivity. So a lower temperature means a
higher albedo.
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.
‘Oumuamua is
on its way out of our solar system – almost as far from the Sun as Saturn’s
orbit – 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 and
measure it again until we understand what we’re seeing,” said Davide
Farnocchia, of the Center for Near Earth Object Studies (CNEOS) at JPL and a
coauthor on both papers. “But this one is gone forever; we probably know
as much about it as we’re ever going to know.”
JPL manages the Spitzer Space Telescope mission for NASA’s
Science Mission Directorate in Washington. Science operations are conducted at
the Spitzer Science Center at Caltech in Pasadena, California. Spacecraft
operations are based at Lockheed Martin Space Systems Company in Littleton,
Colorado. Data are archived at the Infrared Science Archive housed at IPAC at
Caltech. Caltech manages JPL for NASA.
For more information about Spitzer, visit:
https://spitzer.caltech.edu
https://www.nasa.gov/spitzer
News Media Contact
Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
[email protected]
2018-262
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