INTERSTELLAR VISITORS

In 2017, astronomers identified a comet from a completely different solar system passing through our own. Discovered by the Pan-STARRS telescope at Haleakala Observatory, Hawaii, it was called ‘Oumuamua, the Hawaiian word for ‘scout’. At first, it looked like an asteroid because it did not display tails like a comet, nor a surrounding gas cloud known as a coma.

But later, it began to accelerate in a way that could not be described by the gravitational pull of the Sun or the planets. Although this led to some lurid headlines about it being an alien spacecraft, such behaviour is precisely what comets do. It occurs because the heat from the Sun releases gas from the ices below the comet’s surface, and the escaping gas acts like a small rocket motor.

In 2018, another interstellar comet was identified cruising through the Solar System. Called Borisov, it made its closest pass to the Sun in December 2019 and was estimated to be losing two kilograms of dust and 60 kilograms of water every second.

The idea of using Comet Intercepter to see something from another solar system is clearly tantalising for the science team, but they are also realistic about their chances of actually targeting one.

“It’s a pretty remote chance to be quite honest,” says Dr Colin Snodgrass, an astronomer from the University of Edinburgh. This is because we have no idea how frequently they pass by. It may simply be blind luck that we have identified two in as many years. Astronomers will know more when the Large Synoptic Survey Telescope (LSST) begins work in 2022.