In 1994, the Hubble Space Telescope was carrying out a detailed study of the Sun's nearest stellar neighbour, a faint dwarf star called Proxima Centauri.

Training its Fine Guidance Sensors to search for small deviations in the position of Proxima Centauri that could reveal the presence of an unseen planetary companion, it actually saw something else.

Rather than let the main telescope sit idle while this study went on, astronomers took an image of a nearby patch of sky in the constellation of Centaurus. It was hoped that such images can be used to study the evolution of stars that make up our galaxy.

Most of the stars in the resulting image lie near the centre of our galaxy, some 25,000 light-years distant. But one object - the blue curved streak in the image - is something much closer.

A mile wide

It turned out to be an uncatalogued, mile-wide bit of rocky debris orbiting our Sun.

Over the years, this and about a hundred other interlopers have been accidentally found by Hubble astronomers.

Analysis of the object's motion suggests this asteroid's orbit could cross Mars' path.

Seen briefly by the HST, it is too small and faint to track for long enough for a precise orbit to be determined.

The mysterious asteroid has now vanished into deep space. Perhaps it will be seen again in hundreds or thousands of years. Perhaps never.