This is the last image that the New Horizons spacecraft transmitted before it began its final approach to Pluto. Credit: NASA/JHUAPL/SWRI

Pluto's bright white 'heart' stars in this closest photograph yet of the dwarf planet. NASA’s New Horizons spacecraft sent it back to Earth, from 5 billion kilometres away, on the night of 13 July before slipping into radio silence for an expected 21 hours.

The US$720-million spacecraft is expected to fly approximately 12,500 kilometres above Pluto’s surface just before 7:50 a.m. US Eastern time. If all goes well, the probe will radio its controllers just before 9 p.m. tonight, with a ‘phone home’ signal containing no science data, just a quick check-in.

Last night’s download is a ‘fail-safe’ data dump on the off chance that something goes wrong and New Horizons does not survive the fly-by. Mission scientists put the odds of failure — the most likely cause would be a high-speed dust particle slamming into the spacecraft — at no more than 1 in 10,000.

New Horizons’ seven instruments are exploring the geology, atmosphere and other characteristics of Pluto and its largest moon, Charon. It will also peek at Pluto’s four other known moons: Nix, Hydra, Kerberos and Styx.