MESSENGER's mission at Mercury
Sen—The MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) spacecraft was launched on Aug. 3 2004 and was inserted into Mercury’s orbit on March 18 2011. The mission was initially intended to last one Earth year, but two mission extensions meant that the craft stayed working until it crashed into Mercury on April 30 2015.
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.
Power and propellant
As Mercury was much closer to the Sun than the Earth, MESSENGER needed to be protected from the intense heat. When Mercury is at its closest to the Sun, the craft experienced temperatures of up to 370 degrees Celsius, however a reflective sunshade protected the sensitive equipment and kept them operating at much lower temperatures. The orbit was also designed to be 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 was utilised by the craft’s solar panels. The solar panels only took in what MESSENGER actually needed, as too much power could have affected the electronics. An automated system turned the solar panels towards the Sun when power was needed.
The elliptical orbit around Mercury also had a negative effect. "Because the spacecraft is in a highly eccentric orbit, the gravitational pull of the Sun drives regular changes in MESSENGER's orbital parameters," explains Solomon.
MESSENGER was gradually spiralling towards the surface of Mercury since 2013. The altitude of the craft was raised several times until the propellant ran out. After this, the engineers cleverly managed to use helium pressurant as a propellant, which gave the craft another month of life.
The low orbits were put to good use before the craft's demise. MESSENGER was able to take some spectacular high resolution images of the planet's surface, and take important close up images of geological features.
MESSENGER finally impacted the surface of Mercury on April 30 2015 at a speed of around 14,000 kilometres per hour.
Mercury close up. The left-to-right field of view in this image is 21 km (13 miles) across. Imaged on July 31 2014. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Instruments on board
MESSENGER was garnished with eight instruments that were performing all those investigations. The Mercury Dual Imaging System (MDIS) contained wide and narrow angle cameras and MDIS could pivot to point to an area on the surface without adjusting the craft's position. The wide angle imager could take multi-wavelength images.
The Gamma-Ray and Neutron Spectrometer (GRNS) detected 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 could also be inferred from documenting neutrons that escape from the surface.
The X-ray Spectrometer (XRS) utilised 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) measured Mercury’s magnetic field, but it was located at the end of a 3.6 metre boom to stop interference from the magnetic field produced by the craft. The magnetometer was also equipped with its own sunshade.
The Mercury Laser Altimeter (MLA) used lasers to map the surface features, by measuring the amount of time it took the lasers to reflect off the planets surface. It could also measure the wobble of Mercury around its spin axis, known as libration, which gave researchers more information about the planet’s core.
An ultraviolet and an infrared spectrometer were combined in the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument. This measured the abundances of elements in the planet’s weak atmosphere, along with minerals on the surface.
The Energetic Particle and Plasma Spectrometer (EPPS) measured charged particles as they are accelerated by Mercury’s magnetosphere.
The Radio Science Experiment measured the craft distance from Earth, as well as its speed, in order to further understand the planet’s gravitational field.
MESSENGER was the first spacecraft to visit Mercury since Mariner 10 flew by 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 per cent of Mercury’s surface, but MESSENGER imaged the entire surface of the planet.
Mercury is the densest terrestrial planet and contains much more metals than the other three, which means that it is harbouring a massive iron core. This core is so large that it accounts for 85 per cent of the planet’s radius.
One theory to explain this unusual core size was that the outer layers of the planet could have been blown away in a giant impact. This would have stripped away volatile elements on the surface, however MESSENGER found that volatiles do exist on Mercury's surface, ruling out the impact theory.
MESSENGER investigated 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.
Old observations suggested that the planet contracted between 0.8 and 3 kilometres, but this measurement disagreed with predictions based on the planet's thermal history. Observations by MESSENGER of almost 6,000 geological markers have revealed that that the planet actually contracted by around 7 kilometres. These new results agree with the thermal models, solving a problem that had been around for decades.
It was once thought that Mercury's interior should be solid, as it should have cooled and solidified long ago due to the small size of the planet. Measurements from MESSENGER showed that a solid silicate crust and mantle sit atop a solid iron outer core. Beneath this there is a liquid layer, and possibly a solid inner core.
The liquid layer of the iron core drives Mercury's magnetic field. MESSENGER's studies of the magnetic field have revealed it be offset, so that a magnetic field line emanating from the north pole does not reach the south pole. This leaves the south pole more vulnerable to high energy particles from the solar wind.
MESSENGER has deciphered the mystery of the poles. Radar observations from Earth discovered that the poles reflected more radio waves than the rest of the planet, which suggested that water ice was hiding in permanently dark craters. MESSENGER confirmed that there is abundant water ice and other frozen volatiles at the poles. Ice is also present beneath the surface in slightly warmer areas. Some of this ice is likely to have been delivered to Mercury by comets in the past.
In November 2013, MESSENGER turned its attention to two comets, capturing images of 2P/Encke and ISON as they neared the Sun. MESSENGER was not designed to study comets, but was able to use the MACS and XRS instruments to study the comets. Ultraviolet images of comets are rare, making the MACS data extremely valuable.
"All of the scientific objectives of MESSENGER's primary mission, on the basis of which the mission was originally selected for flight by NASA, have been fully met," says Solomon. "MESSENGER's second extended mission has a further set of new scientific objectives. The MESSENGER team is tracking progress against those objectives on a regular basis, and we expect that all objectives will be met by the end of the mission."