After a 6.5 year journey around the inner Solar System, the MErcury Surface, Space ENvironment, GEochemistry and Ranger spacecraft, MESSENGER, has become the first to ever enter orbit around innermost planet Mercury

After a period of systems checking, on 4 April its suite of instruments, including imaging cameras, spectrometers, a magnetometer and an altimeter, will be turned on and the science phase of the mission will commence. Throughout its one year nominal science mission, MESSENGER will orbit the planet once every 12 hours in order to glean information on Mercury's surface features, its magnetic field and exosphere.

The spacecraft will have a highly elliptical orbit, passing between 200 kilometres and over 15,000 kilometres from the planet's surface. This extremely elliptical orbit allows MESSENGER's temperature to be better regulated – at an altitude of 200 kilometres, the re-radiated heat from the planet alone is four times the solar intensity at Earth. As Mercury orbits the Sun, the spacecraft will stay in a fixed orientation to keep its sunshield facing the Sun. Once a day the spacecraft will downlink its data via the Deep Space Network.

The mission aims to answer six primary science questions:

Why is Mercury so dense?
Earth, Mars and Venus are all known to posses dense iron-rich cores, but despite Mercury being the smallest of the four innermost planets, it has the highest density – so high that it implies that its core could dominate as much as 60 percent of the planet's volume. Is Mercury's density a result of the planet forming close to the Sun, its lighter rocky particles lost to the Sun? Could heat in the early phase of planet formation have vaporized Mercury's outer rocky layer? Or did a giant impact strip Mercury of its crust and upper mantle? MESSENGER will measure the composition of the planet's surface using X-ray, gamma-ray and neutron spectrometers, to determine which one of these theories is correct.

What is the geological history of Mercury?
Before MESSENGER made its first flyby of Mercury, over half of the planet had never been seen before. Now, including Mariner 10 observations and MESSENGER's three flybys, 98 percent of the planet has been observed, confirming that its cratered, ancient surface is dominant across the whole globe. Volcanic features also suggest that volcanism persisted in Mercury's early history, and scarps, or cliffs, are believed to have been formed when the entire planet contracted – shrunk – as it cooled, buckling up the surface. Topography measurements, mapping, and compositional analysis will help piece together the geological history of the planet.

What is the nature of Mercury's magnetic field?
It is thought that Mercury has a dynamic magnetosphere much like the Earth's, albeit on a smaller scale. How does Mercury's magnetosphere change with solar activity? How is it driven – from fluid motions in the planet's core? MESSENGER's magnetometer will characterize the planet's magnetic field to determine precisely its strength at different positions and altitudes, and how it interacts with the solar wind.

What is the structure of Mercury's core?
We know that Mercury has a large iron-rich core and a global magnetic field, but what is the structure of that core? A core of pure iron would be completely solid today, but if other elements such as sulphur, were present, portions of the core might remain molten. Determining the composition of the core will provide constraints on the planet's thermal history and evolution. MESSENGER's laser altimeter will be used to measure Mercury's libration – it's 'wobble' on its rotational axis; the libration of the rocky outer part of the planet will be twice as large if it is floating on a liquid outer core than if it is solid. Improved gravity field measurements will also help constrain the size and structure of the core.

What are the unusual materials at Mercury's poles?
Despite Mercury's proximity to the Sun, some polar craters remain in permanent shadow. Earth-based radar imaging shows a highly reflective substance on the floors of these large craters. Could water-ice be present on Mercury, or is the feature a result of low temperature silicates? MESSENGER's neutron spectrometer will search of the signatures of hydrogen in the polar deposits, and the ultraviolet spectrometer and energetic particle and plasma spectrometer will look for signatures from hydroxide or sulphur in the vapour over these deposits. The laser altimeter will also provide information about the topography of the craters.

What volatiles are important at Mercury?
Mercury is surrounded by a thin envelope of gas – an exosphere – containing hydrogen, helium, oxygen, sodium, potassium, calcium and magnesium. These components are not stable on long timescales so each element must have a source. Hydrogen and helium are supplied by the solar wind, while other elements may come from comet or micrometeoroid impacts. Processes such as vaporization of surface rocks during impacts, evaporation of elements from the surface when exposed to sunlight, sputtering from solar wind, or diffusion from the planet's interior, may all play a role. MESSENGER's spectrometers will determine the composition of the exosphere and compare it with details of the planet's surface. Variations in the composition will also be monitored to determine the processes that contribute to it.

MESSENGER has already completed three flybys, which not only revealed new images and details of the planet, but also provided the vital gravity assists to bring the spacecraft into orbit around the planet on 18 March.

For more information on the MESSENGER mission see our dedicated Focus in the March issue of Astronomy Now magazine.