On 18th October 2017 a strange object was seen by
astronomers that could only have come from outside the Solar System. It was
given the name `Oumuamua, which means “scout” in
the Hawaiian language.
The cigar-shaped object, no more than a kilometre in length and
about 140 metres in diameter, was clearly moving at high speed and it soon passed
out of the range of Earth-based telescopes. Efforts were then made to find it
by using the Spitzer Space Telescope (which operates in the infrared frequency
range) but these were unsuccessful. The Hubble Space Telescope was also used in
the search, and had more success, but not a lot more.
However, there were various factors that limited the
observations that could be made. One was `Oumuamua`s small size, another was
its dimness. When a comet traverses the inner Solar System it emits a trail of gases
and particles that make it relatively easy to see, but this was not the case with
`Oumuamua, which emitted almost no carbon dioxide or monoxide.
Another reason for the lack of direct observation was that `Oumuamua
was not moving at the speed that had been predicted for it. This might have
been due to ice on its surface turning into gas as it passed the Sun and speeding
it up. When the Spitzer telescope was pointed at its expected position it had
already moved beyond it.
Another oddity about `Oumuamua it that it is tumbling end
over end but still moving at great speed. This speed is such that there is no
possibility of any probe being able to launch and move towards it, because any
such probe would never be able to catch up.
As to what `Oumuamua is and where it came from, these are
questions that have yet to be answered. One suggestion - probably the least likely - is that it could be part
of an alien spaceship that broke up in interstellar space, possibly millions of
years ago.
It has been speculated that `Oumuamua is not alone in visiting
the Solar System from somewhere else. Some of these objects might well be
captured by the Sun’s gravity and end up in orbit as asteroids or comets.
Several asteroids have been identified that are on eccentric or tilted orbits
that suggest an origin outside the Solar System. These will clearly be much
easier to investigate than a fleeting visitor like `Oumuamua.
© John Welford
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