Messenger reaches its destination

It's taken the best part of 7 years - including a dozen laps of the inner solar system and six planetary flybys - but at a little after midnight tonight NASA's Messenger spacecraft will fire its main thrusters and manoeuvre into orbit around the planet Mercury.

The "burn" should last about 15 minutes, slowing the spacecraft enough for it to be captured by Mercury's relatively weak gravitational field, and allowing the seven scientific instruments on board to begin Messenger's year long primary mission: to map and study the surface of the smallest and closest planet to the sun.

"What's wonderful about the orbital mission," according to principal investigator Dr Sean Solomon from the Carnegie Institution in Washington, "is that we will be at Mercury continuously. We will be making continuous measurements not only of the surface but of the whole environment of Mercury and how it changes in response to solar activity".

Mercury is a world of violent extremes. While daytime temperatures can reach 427degrees celsius, at night that plummets to -184 degrees. And despite the planet's proximity to the sun - a mere 36 million miles - Messenger could well discover ice at its poles.

The fleet-footed messenger of the gods in Roman mythology, Mercury was one of five planets known to ancient astronomers. A fitting description for the fastest planet in the solar system, which completes a single orbit of the sun once in every 88 earth-days. Oddly, the planet also rotates very slowly so that a single blast-furnace day on Mercury lasts for nearly two of its years.

Partly because of this brutal physical environment, Mercury has often been dismissed as little more than a barren, scorched rock.

Messenger could be about to change all that. Unlike any of the other inner solar system planets Mercury has a huge metal core. Understanding how the planet was created could provide fresh insights into the processes of planetary formation.

The suite of instruments on board will photograph and map the surface of the planet, study its atmosphere and its geological structure and history.

The sun-scorched messenger of the Gods could yet deliver a more profound understanding of our own solar system, and the planetary systems orbiting distant stars.