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Do you remember hearing about
that weird, strange-shaped

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object that passed through our
inner solar system late last

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year? It was the first object
we’ve confirmed to have come

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from outside our solar system,
and was given the name

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’Oumuamua, Hawaiian for “scout
or messenger from our distant

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past.” Now, scientists have
analyzed data from many

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observatories, including NASA’s
Hubble Space Telescope, and

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found that this interstellar
object has gained an unexpected

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boost of speed. After ‘Oumuamua
was discovered with Hawaii’s

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Pan-STARRS 1 telescope as it
surveyed the skies for

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near-Earth asteroids in October
2017, observatories all around

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and above the world focused on
the approximately half-mile long

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object to learn as much as
possible about this cosmic

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visitor before it flew too far
away to see. The Hubble Space

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Telescope was used to collect
observations to determine the

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object’s trajectory over two
months in late 2017 and early

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2018. The observations were
combined with data from multiple

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ground-based observatories which
showed ‘Oumuamua to have a

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small, continuous
non-gravitational

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“acceleration”. Something
besides just the gravity from

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the Sun and planets was
affecting the trajectory of

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‘Oumuamua. A probable source of
this acceleration is jets of gas

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coming off the object. This
behavior is similar to objects

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we have already identified as
comets – when they get close to

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the Sun, ices in the comet
sublimate into gas, and

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streamers of gas can push the
object along. Usually when

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comets do this the outgassing
can be seen because it ejects

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large amounts of dust which form
a cloud around the object,

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called a coma, and a tail. But
with ‘Oumuamua, no type of coma

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or tail was seen. Maybe
‘Oumuamua doesn’t have much dust

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left after its interstellar
journey – an amount no more than

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a couple coffee cans of
material, making the dust too

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sparse for us to see. Or, maybe
the dust was much larger than

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normal, making it hard for us to
detect. I know that sounds

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counterintuitive, but it’s
easiest to detect dust when the

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particles are similar in size to
the wavelength of light we’re

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using. Observatories were
looking at ‘Oumuamua in visible

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light, where we have the most
sensitive detectors, but larger

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particles of dust would be
easier to detect with microwave

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or radio wavelengths. This is
definitely an unusual object,

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and unfortunately no more new
observations of ‘Oumuamua are

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possible because it’s already
too dim and far away for even

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Hubble to see. But when there’s
one weird object there are

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probably more. Sky survey
programs will keep an eye out

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for other interstellar visitors,
and the Hubble Space Telescope

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will be ready to contribute
observations to better

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understand any more orphaned
vagabonds traveling between the

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the stars.

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www.nasa.gov/hubble 
@NASAHubble