NASA's New Horizons spacecraft has sent back its first image of Pluto's moon Kerberos, revealing some surprises.

The image, captured during the spacecraft's historic flyby on July 14, also completes the spacecraft's "family portrait" of all five moons in the Pluto system.

The new image downloaded last week shows that Kerberos has a double-lobed shape, suggesting it may have been formed by two smaller objects that collided and stuck together.

The larger lobe is about eight kilometres wide, and the smaller lobe is about five kilometres wide — overall, along its longest side, Kerberos is about 10 to 12 kilometres wide, similar in size to Pluto's other smallest moon, Styx.

Pluto family portrait

This composite image shows a sliver of Pluto’s large moon, Charon, and all four of its small moons, as captured by the New Horizons spacecraft. Charon is by far the largest, with a diameter of 1,212 kilometres. Nix and Hydra are each about 40 kilometres across their longest side. Kerberos and Styx are about 10 to 12 kilometres across their longest side. ( NASA/JHUAPL/SwRI)

Kerberos is much smaller than expected, based on measurements of its gravitational influence made using the Hubble Telescope, which suggested it was probably quite massive, but faint because it had a dark surface.

The New Horizons images show the moon is actually small and very reflective, like Pluto's other small moons. The New Horizons science team thinks that's because all those moons are covered in a clean coat of regular ice — frozen water. All four moons are much longer in one dimension than the others, which scientists think is typical of objects in the Kuiper Belt, the region beyond the planets in our solar system where Pluto and its moons are found.

New Horizons captured large amounts of data and images during its flyby of Pluto and its moons on July 14 and has been slowly sending them back to Earth, a process that is expected to take about a year.