A PASSION FOR SPACE
Dr Michele Bannister recalls the scramble to study the first confirmed interstellar object.
`Oumuamua’s brief visit instigated a frantic scramble to gather information on this interstellar wanderer
Back in October last year, the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) telescope in Hawaii photographed a moving dot of light. That dot turned out to be a skyscraper-sized world, later christened 1I/2017 U1 `Oumuamua. It’s the first interstellar minor planet to be seen, but our Galaxy must be full of wandering asteroids and comets like it, tossed out of untold billions of planetary systems as they were forming around stars.
Astronomy is international. At 6.30am on 25 October I saw the announcement of `Oumuamua’s discovery – emailed from the International Astronomical Union’s Minor Planet Center in Boston, which sends out notifications of new comets and asteroids. This institution is known for low-key gravitas, so I was astounded to read: “If further observations confirm the unusual nature of this orbit, this object may be the first clear case of an interstellar comet.”
I tweeted a link to the news and went for my morning run. By the time I got back, a friend who was at the Palomar Observatory in California looking for asteroids was already observing `Oumuamua.
Travelling at a speed of 26km/s, nearly twice as fast as the fastest spacecraft yet launched, `Oumuamua was only close enough to be visible to the world’s biggest telescopes for a few days. With a flurry of emails between colleagues in the UK and abroad, we asked for urgent observations to be made by telescopes big enough to study the object: the Gemini Observatory in Hawaii, the Very Large Telescope in Chile and the William Herschel Telescope in the Canary Islands.
Initial findings
Observations with these telescopes enabled us to gather a wealth of precious photons reflected from `Oumuamua’s surface. This visible and near-infrared light encodes the composition of the top few microns of `Oumuamua’s crust. After comparing this information with that collected by Col-OSSOS (COLours for the Outer Solar System Object Survey), which we’ve been using to map the surfaces of distant minor planets, it seems `Oumuamua could call these larger worlds its cousins. Like Jupiter’s Trojan asteroids and some of the small worlds on inclined, eccentric orbits beyond Neptune, `Oumuamua is slightly reddish in colour relative to sunlight.
Its dry exterior has no trace of coma like a comet, but it could hide an icy heart: `Oumuamua’s passage by the Sun, though inside the orbit of Mercury, was so fast that its surface only reached cakebaking temperatures. A scant metre of insulating organicrich material (a composition suggested by its reddish surface) would be enough to prevent `Oumuamua becoming a comet by protecting any buried ice from the heat. Watching `Oumuamua over a few days showed a dramatic change in brightness: more than 1.5 magnitudes. That implies `Oumuamua has a cucumber-like shape, slowly turning to change the face we see.
This interstellar wanderer’s home is not yet known. No star in the local neighbourhood matches its past trajectory. Like a piece of driftwood on the tide, it was scooped up by the Sun – an event that could have happened more than once at other stars. `Oumuamua has wandered the Milky Way for at least tens of millions of years, and may yet wander for billions more. S