Messenger to Mercury

Tonight at 12.45am [GMT] Nasa’s Messenger spacecraft will begin a 15 minute engine burn timed to take it into orbit around Mercury – the first spacecraft to do so.  (All images credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Launched on 3 August 2004, Messenger’s taken a circuitous route to the innermost planet.

It’s second, and final, look at the Earth was in August 2005.

Twice past Venus, the second time in June 2007

Before not one, not two fly-bys, but three fly-bys of its final destination.

As the BBC’s Jonathan Amos says

Mercury’s proximity to the Sun means exposed equator surfaces can reach more than 600C; and yet there may be water-ice at the poles in craters that are in permanent shadow.

It is so dense for its size that more than two-thirds of the body has to be made of an iron-metal composition.

Mercury also retains a magnetic field, something which is absent on Venus and Mars.

In addition, the planet is deeply scarred, not just by impact craters and volcanic activity but through shrinkage; the whole body has reduced in size through Solar System history.

And Mercury fascinates because it may be our best guide to what some of the new planets might be like that are now being discovered around distant suns.

Many of these worlds also orbit very close in to their host stars.

“We’ll be looking at the composition of the planet and how it ended up so dense, and what planetary formation processes gave rise to the high fraction of core,” said Dr Solomon

“The answer to that question lies in the composition of the surface that we can sense remotely from orbit, but we need time in orbit to do that.

“We’ll also be taking more images, but images at higher resolution and in optimum lighting compared with the conditions we had during the flybys.”

Key to the success of the whole endeavour will be maintaining the health of Messenger in the harsh conditions it will experience.

“The sunshade is made of a ceramic material that keeps the heat on the outside of the spacecraft from getting on the inside,” explained Eric Finnegan, the Messenger mission systems engineer at the Johns Hopkins University Applied Physics Laboratory (APL).

“We also had to develop thermal protection for the solar arrays. We still need to generate power but we had to make sure the solar arrays themselves wouldn’t melt. So, we built a solar panel that’s only populated with one-third solar cells. The other two-thirds of the panel are basically mirrors to reflect the sunlight off of the panels.”

When Messenger enters orbit, it will be some 46 million km (29 million miles) from the Sun, and about 155 million km (96 million miles) from Earth.

The spacecraft is scheduled to remain in orbit for a year, allowing the probe to fly around Mercury 730 times.

If Messenger stays in good health and the funding allows, a one-year mission extension is likely to be granted.