The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft is due to crash into the surface of Mercury on April 30, 2015 after it runs out of fuel. Impact will be at a speed of 8,750 miles per hour (3.91 kilometers per second) on the side of the planet facing away from Earth. It first arrived into orbit around Mercury March 17, 2011. The instrument payload has provided a wealth of information. It is the first spacecraft to orbit that planet. It has given us views of Mercury that mankind has never seen. The spacecraft has acquired more than 255,000 images and a vast amount of other data. The entire surface has been mapped.
Launched August 3, 2004, the accomplishments of MESSENGER are many. This graphic from 2014 celebrates many of those.
How Do You Get to Mercury?
The animation below illustrates how MESSENGER followed a path through the inner solar system, including one flyby of Earth, two flybys of Venus, and three flybys of Mercury. The fullscreen button in the lower right makes it easier to watch. Use the Settings button ❋ to slow the speed to 0.25 or 0.50. Keep your eye on the spacecraft as it is passes each planet. You will notice it slows down with each planet encounter and falls into a different orbit closer to the Sun.
Each time the craft made a flyby of a planet, its speed and direction were altered by what is known as the slingshot effect. The speed of MESSENGER changed due to the gravitational pull of the planet as it was passed by the planet. With the new speed and direction, the craft was set on a new orbit around the Sun that intersected the orbit of the next inner planet. As it fell inward toward the Sun in these stages, it gradually settled into a final orbit that intersected the orbit of Mercury. On March 17, 2011, the on-board engine was able to slow the craft into a final eccentric orbit around Mercury.
Six Important Questions
The MESSENGER mission tried to answer several questions about Mercury.
1. Why is it so dense? The metal-rich core has 60% of the planet’s mass, twice the percentage as for Earth. Why so different?
2. What is its geologic history? The suite of 7 instruments mapped the surface history of the entire planet as well as some of the interior structure.
3. What is the nature of the magnetic field? Earth and Mercury have magnetic fields. Venus and Mars do not. What accounts for the difference?
4. What is the nature of its core? Measurements examined Mercury’s core and whether it is surrounded by a liquid layer.
5. What are the highly reflective materials at the poles? Is water ice able to exist in the permanently shadowed craters?
6. What volatile gases are in its exosphere? Hydrogen, helium, oxygen, sodium, potassium, calcium, and magnesium are known. Are there others? What is their origin?
What Instruments Were Used?
Selecting the scientific instrumentation for a mission is a balance between resources for mass, power, space, schedule, and cost. In the case of MESSENGER, it was especially difficult. Mass was limited to 50 kilograms (110 pounds). It needed extra rocket fuel for final orbit insertion burns. The instruments were mounted where Mercury was visible to them, but the Sun was not. The nearness to the Sun caused high temperature and radiation values that would shorten the life of the instruments.
Some Image Gallery Highlights
Additional images from the mission are available at this NASA link.
Some Interesting Findings
Imaging maps revealed broad expanses of smooth plains near Mercury’s north pole, likely among the largest expanses of volcanic deposits on Mercury. Volcanism shaped much of Mercury’s crust.
Higher resolution observations at up to 10 meters per pixel revealed light patchy deposits of rimless, irregular pits varying in size from hundreds of meters to several kilometers associated with central peaks, peak rings, and rims of craters. These appear to be venting spots of some sub-surface gases.
Magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet’s surface show that Mercury’s surface is not dominated by feldspar-rich rocks. There are large amounts of sulfur at Mercury’s surface as well as radioactive isotopes of potassium and thorium.
“The abundance of potassium rules out some prior theories for Mercury’s composition and origin,” says Larry Nittler, a staff scientist at the Carnegie Institution of Washington. “Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin.”
The north polar region of Mercury is a broad area of low elevations.
Tests for polar ice deposits preserved on the cold, permanently shadowed floors of high-latitude impact craters was done. Evidence is strong for abundant amounts of water ice in the permanently shadowed craters at the poles. Details here.
This following animation shows the south pole over a 176 day period. It shows the changing direction of sunlight as Mercury rotates. The crater at the pole is deep enough to never get sunshine in the bottom. Ice exists there in the subsoil of the deepest shadows. (The planet does not stop halfway and pause.)