The Hubble Space Telescope has detected a cosmic iceberg a mere 975-metres across floating through the Kuiper Belt of comets 6.76 billion kilometres away from the Sun. It’s the smallest and dimmest object ever found in the Kuiper Belt, and is evidence that cometary bodies are ground down by collisions with other objects over billions of years in the far reaches of the Solar System.

The previous smallest icy object in the Kuiper Belt to have been discovered was 48 kilometres across, about 50 times larger than this new object, so this is a huge jump in scale, especially when you consider that this chunk of ice is an incredibly faint 35th magnitude, far too faint to be seen directly by Hubble. Instead, a team led by Hilke Schlichting of the California Institute of Technology used the innovative technique of studying the measurements made by Hubble’s three Fine Guidance Sensors.

The Fine Guidance Sensors help keep Hubble pointing in the right direction, whatever it is looking at. They pinpoint stars, measuring their location and triangulating to ascertain where the telescope is looking. In doing so, they make precise measurements of a given star’s light 40 times per second. Schlichting’s team suspected that if any unseen Kuiper Belt objects passed in front of one of these stars, it would create a dip in starlight and a diffraction effect as the starlight bent around the object.

Schlichting and her colleagues had the daunting task of sifting through four and a half years of Fine Guidance Sensor measurements, incorporating 50,000 guide stars, as they searched for any millisecond blips in starlight. All their hard work paid off – in all the observations, one single 0.3-second blip was spotted, even if the body doing the occulting was too faint to be seen. From the length of the blip its distance could be estimated, and from the magnitude of the blip the objects diameter could be calculated.

“I was very thrilled to find this in the data,” says Schlichting. Buoyed by their success, the team now plan to extend their search through Fine Guidance Sensor measurements all the way back to when Hubble was launched in 1990.