Objects in deep space measure up

Scientists internationally have successfully measured the diameter and brightness of an object 6 billion kilometres from Earth, uncovering another major mystery in the process.

Writing in Nature, MIT-based space scientist James Elliot and his colleagues explain how they pulled off the telescopic-equivalent of looking at a one dollar coin from 500 kilometres away.

The object they were interested in is the rather uninspiringly-named KBO 55636, which stands for Kuiper Belt Object, one of a group of many thousands of bodies circling beyond the orbit of Neptune in the outer reaches of the solar system, up to 50 times farther from the Sun than the Earth. For obvious reasons these objects are hard to study, but gaining a handle on their structure, composition and origins is critical if we're to understand correctly how the solar system formed in the first place.

Collecting samples from these Kuiper Belt Objects is clearly out of the question, so scientists have instead turned to using tricks of the light to gather important clues about their make-up. The coup this group pulled off was to accurately predict the path of KBO 55636 across the sky and work out when it would pass in front of - and most importantly - block out the light from a distant star behind it.

By measuring how long this "occultation" takes it's possible to calculate the diameter of the object itself; armed with this information, it's then possible to work out how reflective the object is and therefore what its surface is composed of. To do this the team assembled a collection of 21 telescopes along a 6000km-long path across the Earth's surface, corresponding to the shadow path that the distant object would produce.

The object's shadow passed across the Earth's surface at over 25 kilometres per second, taking just 5 minutes to cross the entire Pacific, but two of telescopes - based in Hawaii - caught the event, which lasted just 10 seconds, and made the crucial measurements.

They reveal that KBO 55636 is about 143 kilometres across but, strangely, is about 10 times brighter than it should be and must be covered by water ice. The object is thought to have been formed about a billion years ago when when a precursor planet, notionally known as Haumea, broke up during a collision. This means that the surface of KBO 55636 should be dull and tarnished by now, but it's not.

This suggests, say the scientists, that it is either being regularly re-surfaced by the formation of fresh ice, or something else is going on. Exactly what, though, remains a dark mystery, 6 billion kilometres away...