Do you remember hearing about that weird, strange-shaped
object that passed through our inner solar system late last
year? It was the first object we’ve confirmed to have come
from outside our solar system, and was given the name
’Oumuamua, Hawaiian for “scout or messenger from our distant
past.” Now, scientists have analyzed data from many
observatories, including NASA’s Hubble Space Telescope, and
found that this interstellar object has gained an unexpected
boost of speed. After ‘Oumuamua was discovered with Hawaii’s
Pan-STARRS 1 telescope as it surveyed the skies for
near-Earth asteroids in October 2017, observatories all around
and above the world focused on the approximately half-mile long
object to learn as much as possible about this cosmic
visitor before it flew too far away to see. The Hubble Space
Telescope was used to collect observations to determine the
object’s trajectory over two months in late 2017 and early
2018. The observations were combined with data from multiple
ground-based observatories which showed ‘Oumuamua to have a
small, continuous non-gravitational
“acceleration”. Something besides just the gravity from
the Sun and planets was affecting the trajectory of
‘Oumuamua. A probable source of this acceleration is jets of gas
coming off the object. This behavior is similar to objects
we have already identified as comets – when they get close to
the Sun, ices in the comet sublimate into gas, and
streamers of gas can push the object along. Usually when
comets do this the outgassing can be seen because it ejects
large amounts of dust which form a cloud around the object,
called a coma, and a tail. But with ‘Oumuamua, no type of coma
or tail was seen. Maybe ‘Oumuamua doesn’t have much dust
left after its interstellar journey – an amount no more than
a couple coffee cans of material, making the dust too
sparse for us to see. Or, maybe the dust was much larger than
normal, making it hard for us to detect. I know that sounds
counterintuitive, but it’s easiest to detect dust when the
particles are similar in size to the wavelength of light we’re
using. Observatories were looking at ‘Oumuamua in visible
light, where we have the most sensitive detectors, but larger
particles of dust would be easier to detect with microwave
or radio wavelengths. This is definitely an unusual object,
and unfortunately no more new observations of ‘Oumuamua are
possible because it’s already too dim and far away for even
Hubble to see. But when there’s one weird object there are
probably more. Sky survey programs will keep an eye out
for other interstellar visitors, and the Hubble Space Telescope
will be ready to contribute observations to better
understand any more orphaned vagabonds traveling between the
the stars.
www.nasa.gov/hubble @NASAHubble