The MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched on 3 August 2004 and was inserted into Mercury’s orbit on 18 March 2011.
The mission was initially intended to last one Earth year, but it has since been extended until 2013.
MESSENGER’s trip to Mercury was a complex one involving multiple flybys of Earth, Venus and Mercury itself before being placed into orbit. These flybys enabled MESSENGER's speed to be lowered enough to enter Mercury’s orbit.
“A direct trajectory from Earth to Mercury would result in an encounter velocity much too high for MESSENGER's propulsion system to be able to slow the spacecraft sufficiently to be captured into orbit about Mercury,” explains Sean Solomon, Principal Investigator of the MESSENGER mission. “Those flybys, however, meant that MESSENGER had to complete more than 15 revolutions about the Sun over an interval more than six and a half years in duration between launch and orbit insertion”
The flybys of Mercury allowed MESSENGER to map the planets surface as it whizzed past, essentially giving a “preview” of the planet and allowing mission scientists to refine their plans for Mercury’s orbital mission.
As Mercury is much closer to the Sun than the Earth, MESSENGER needs to be protected from the intense heat. When Mercury is at its closest to the Sun, the craft experiences temperatures of up to 370 degrees Celsius. A reflective sunshade is therefore used to protect the sensitive equipment and keeps them operating at much lower temperatures. The orbit is also designed to be quite elliptical to limit the spacecraft’s exposure to heat reflected from the planet’s surface.
However there is an advantage to being in close proximity to the Sun in that there is an endless supply of power, which is utilised by the craft’s solar panels. The solar panels only take in what MESSENGER actually needs, as too much power could affect the electronics. An automated system turns the solar panels towards the Sun when power is needed.
MESSENGER is the first spacecraft to visit Mercury since Mariner 10 over 30 years ago, and as such there are many mysteries yet to be solved about the innermost planet. Mariner 10 was only able to observe 45% of Mercury’s surface, but MESSENGER has already surpassed this and now only 2% of Mercury’s surface now remains unseen.
Mercury as imaged by MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is the densest terrestrial planet and contains more metals than the other three, which implies that 60% of the planet is made of an iron-rich core. This core is so large that it accounts for 75% of the planet’s radius. Several theories exist as to why this is so.
For example, the outer layers of the planet could have been stripped off by a giant impact or else by extreme heat when the planet was still forming. Scientists hope that MESSENGER will solve this riddle by examining the abundances of material on the surface to see if volatile elements (those that evaporate easily) are depleted, as well as estimating the crust thickness.
MESSENGER will also investigate the geological history of Mercury. So far it has been discerned that volcanism persisted on Mercury for the first half of the planet’s lifetime. There are also towering cliffs which stretch for hundreds of kilometres that are spread across the planet’s surface, known as scarps, and it is thought that these were formed when the entire planet contracted early in its history. MESSENGER hopes to gain a better insight into how these were formed, and thus the thermal history of Mercury.
A scarp is seen on the left side of the picture. Credit:NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Preliminary observations by MESSENGER have shown that the outer core might be liquid, and thus drive the planet’s magnetic field. MESSENGER will fully characterise this magnetic field in order to determine the strength and source.
Another of MESSENGER’s objectives is to explain why some of the core is liquid, as it should have cooled and solidified long ago due to the small size of the planet. It is possible that if sulphur is mixed in with the iron, then the cooling time of the core is increased allowing it to remain a liquid for longer.
MESSENGER will also decipher the mystery of the poles. The presence of ice in permanently dark craters at the poles has been inferred from radar observations on Earth. However it is unknown if this is water ice or something else, so MESSENGER’s spectrometer will see if any hydrogen is present and thus if it is actually water ice.
There are seven elements so far detected in Mercury’s exosphere, including magnesium which was first detected by MESSENGER. The composition of this fragile exosphere can be detected by MESSENGER, and therefore the source of the exosphere can be determined. As Mercury is such a small planet, it cannot easily retain any sort of atmosphere and so there must be something replenishing it, such as impacts by micrometeoroids.
MESSENGER is garnished with eight scientific instruments that are performing all these investigations. The Mercury Dual Imaging System (MDIS) contains wide and narrow angle cameras and MDIS can pivot to point to an area on the surface without adjusting the crafts position. The wide angle imager can take multi-wavelength images.
The Gamma-Ray and Neutron Spectrometer (GRNS) detects elements on Mercury’s surface via the gamma rays emitted after the surface is struck with cosmic rays. The amount of hydrogen contained in water molecules can also be inferred from documenting neutrons that escape from the surface.
The X-ray Spectrometer (XRS) utilises solar X-rays bombarding the planet to measure X-ray emission from elements within the top millimetre of the planet’s surface.
MESSENGER’s magnetometer (MAG) measures Mercury’s magnetic field, but it is located at the end of a 3.6 metre boom to stop interference from the magnetic field produced by the craft. The magnetometer is also equipped with its own sunshade.
The Mercury Laser Altimeter (MLA) uses lasers to map the surface features by measuring the amount of time it takes the lasers to reflect off the planet's surface. It can also measure the wobble of Mercury around its spin axis, known as libration, which will give researchers more information about the planet’s core.
An ultraviolet and an infrared spectrometer are combined in the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument. This measures the abundances of elements in the planet’s weak atmosphere, along with minerals on the surface.
The Energetic Particle and Plasma Spectrometer (EPPS) measures charged particles as they are accelerated by Mercury’s magnetosphere.
The Radio Science Experiment measures the craft distance from Earth, as well as its speed, in order to further understand the planet’s gravitational field.
The extension of the mission will allow a new science phase to begin. “The spacecraft will be placed in an 8-hour orbit, which will enable MESSENGER to spend a greater fraction of its time close to Mercury’s surface than during the primary mission with its 12-hour orbit,” Solomon told Sen.
“Moreover, the accomplishment of the global mapping carried out during the primary mission will free many of MESSENGER’s instruments for a new mix of measurements to address a fresh set of scientific objectives that address questions raised by the findings from orbital observations to date”
The new scientific objectives include discovering exactly how long volcanism persisted on Mercury, how the solar cycle affects Mercury’s exosphere and discovering the source of Mercury’s energetic electrons.