Nasa Messenger Probe set to become Mercury's first artificial satellite

NASA's Messenger spacecraft is set to become the first probe to go into orbit around Mercury.

The planned year-long mission could reveal why the sun-baked planet is the densest in the solar system and whether it harbours any ice in darkened craters at its poles.

Messenger launched in 2004 and flew past Mercury three times as it followed a looping path designed to bring it into orbit around the planet. 

Before that, only one other spacecraft had visited Mercury – NASA's Mariner 10 probe, which flew past the rocky world three times in 1974 and 1975.

"Mercury has been comparatively unexplored, considering its proximity in our solar system," principal investigator Sean Solomon of the Carnegie Institution of Washington in DC said at a new briefing on Tuesday.

That is set to change on Friday, when Messenger will burn its thrusters for about 15 minutes, beginning at 0045 GMT, to slow it down enough to be captured into orbit by the planet's gravity. The manoeuvre will use up about a third of the fuel the probe carried when it launched.

It is designed to go into an elongated, 12-hour orbit that passes within 200 kilometres of Mercury's surface at its closest point and 15,000 km at its farthest.

Safety first

Even if all goes as planned, it will not send any images back to Earth immediately, as its instruments will remain off for about a week. "It's in the interest of the spacecraft health and reduction of risk," Solomon said. 

He said that even though the probe has been as close to the sun as it will be then, it has never been in orbit around the planet and subjected to the heat radiated by its surface. The first image taken from orbit will be released on 29 March, according to the mission team.

The probe will begin regular observations of the planet on 4 April, when it will use seven instruments to study the composition of the planet's surface, measure its topology and record the planet's magnetic field.

Studies of the composition of the surface could help reveal why Mercury is denser than any other planet in the solar system, with an enormous two-thirds of its mass made up of its metal core.

Three theories compete to explain this phenomenon. Denser, metal-rich planetary building blocks may have been drawn closer to the sun before the planet formed. Or the sun might have vaporised part of the planet's rocky exterior, early on in its life. Or, as many scientists suspect, an impact with a similar-sized object blasted away the planet's outer layers.