By Jonathan Amos
Science reporter, BBC News
Messenger is imaging the surface of Mercury at high resolution
Nasa's Messenger probe is about to make its third and final fly-by of Mercury.
The pass, just 228km from the surface, will make use of the planet's gravity to help slow the spacecraft enough to enable it to enter into orbit in 2011.
Messenger will train its instruments on scientifically interesting craters and study the stream of charged particles blasted from its surface by the Sun.
By the time the fly-by is complete, the probe will have mapped 95% of Mercury's surface at high resolution.
Only some polar regions will remain unknown and un-surveyed by the time Messenger sets up station above the planet in 18 months' time.
"This third and last fly-by is the final time Messenger can use the power of Mercury to meet the demands of the cruise trajectory," explained Eric Finnegan, a mission systems engineer from the Johns Hopkins University Applied Physics Laboratory (JHUAPL), which is managing the mission for the US space agency (Nasa).
"Any shortfalls in this gravity assist will need to be corrected by the spacecraft using its limited supply of onboard propellant.
"Slowing the spacecraft by [9,500km/h], Messenger's orbital period around the Sun will be decreased by 13 days, closely matching the 88-day orbital period of the innermost planet."
Tail of Mercury
All the terrestrial planets are believed to have formed at the same time by common processes - but Mercury itself is a bit of an oddball. Messenger, the first spacecraft to target the innermost planet since Mariner 10 in the 1970s, is an attempt to solve some of Mercury's mysteries.
The planet is so dense that more than two-thirds of it has to be of an iron-metal composition.
Closest planet to the Sun; smallest in Solar System
Visited by Mariner 10 in 1970s; by Messenger currently
Diameter: 4,880km, about one-third the size of Earth
Second densest planet in Solar System; 5.3x that of water
Caloris basin is largest known feature (1,300km in diameter)
Possibility of water-ice in permanently shadowed craters
Huge iron core takes up more than 60% of the planet's mass
Surface temperatures swing between 425C and -180C
Has an extremely thin atmosphere (exosphere)
Only inner planet besides Earth with global magnetic field
A lot of this iron is at the surface, and Messenger will be using this pass to try to establish its mineralogical setting and the chemical forms in which it is bound up.
The spacecraft will also undertake manoeuvres not tried on the previous fly-bys.
On this occasion, Messenger will rock back and forth as it sweeps across the surface, holding its gaze on specific craters for a few seconds as it makes detailed observations.
"Our first target is an unnamed, [150km] across crater which is actually fairly old and has some interesting materials near the centre," said JHUAPL instrument scientist Noam Izenberg.
"We're going to be pausing on that, taking a series of observations; and then the pointing of the spacecraft is going to alter slightly, moving to a younger crater.
"We're going to look at the continuous ejecta of this crater, examining the younger, fresher material that was blown out after impact."
The probe will also get another chance to examine Mercury's "atmosphere", or exosphere as the scientists call the tenuous cloud of charged atoms which encircle the planet.
These ions bleed from Mercury's surface as it is blasted by the full force of the solar wind, a stream of particles from our star which buffets the entire Solar System.
Many of the atoms thrown up from the surface - sodium, potassium, calcium, and probably even iron and aluminium - are then pushed away by sunlight into a gigantic gas cloud tail that extends millions of km behind Mercury.
"All the materials in the tail and in Mercury's atmosphere, all the atoms heavier than hydrogen and helium, are derived from Mercury's surface," said Messenger's principal investigator Sean Solomon.
Messenger is due to go into orbit around Mercury in March 2011
"So measurement of Mercury's atmosphere and tail and its dynamics is telling us about the processes that maintain that envelope of volatile species around the planet and something about the composition of Mercury's surface materials."
Messenger carries seven scientific instruments - a camera, a magnetometer, an altimeter and four spectrometers - and one radio science experiment.
The craft is equipped with a large sunshade made from a heat-resistant ceramic fabric to protect it from the Sun.
"Advances in materials science allow spacecraft electronics and sensitive instruments to run at room temperatures behind the shade while the front surface temperature rises to over [315C]," explained Eric Finnegan.
Time of closest approach is 2155 GMT on Tuesday. The spacecraft and its antenna are turned away from Earth during the observations, and so the first data is not expected to be returned for several hours after the fly-by.