In Roman mythology, Mercury was the fleet-footed messenger to the gods.
It’s therefore fitting that NASA went to great pains to name the first spacecraft to orbit the planet Mercury MESSENGER. That’s an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. (Personally, I would have tried to find a way to name the orbiter FREDDIE … )
The MESSENGER mission launched from Florida in 2004, sending up a craft built by the Johns Hopkins University (JHU) Applied Physics Lab, which now manages spacecraft operations.
MESSENGER lifting off on top of a Boeing Delta II rocket.
—Photograph courtesy NASA
This week MESSENGER will finally settle into orbit around Mercury, in a maneuver planned for when most people in the U.S. will be in bars drinking green beer: March 17, at 8:45 p.m. ET.
So what took us so long to get a craft in orbit around Mercury? The only other planets never to have long-term guests are Uranus and Neptune, and the incredible distance from Earth to those outer worlds has a lot to do with that.
Mercury is one of our inner solar system neighbors, a mere 48 million miles (77 million kilometers) from Earth at its closest point.
There have been Earth-based observations of Mercury, but they haven’t revealed as much as scientists would like to know. For starters, Mercury is hard to see from Earth, because it’s the smallest and fastest planet—and it’s the closest to the sun.
Space-based optical telescopes such as Hubble are so sensitive to light that it’d be dangerous to point them toward Mercury and risk “blinding” the instrument.
In 1962 Russian scientists found a way to bounce a radar signal off Mercury, leading to decades of radar studies, many by the Arecibo Observatory in Puerto Rico.
It wasn’t until 1974 that the Mariner 10 spacecraft offered the first close-up optical images of the planet. The probe flew past Mercury in 1974 and ’75, but because of its flyby path, the craft imaged just 45 percent of the planet’s surface before moving on.
What we learned from Mariner 10—aside from some new details about Mercury’s surface—was how to conduct gravity assists, maneuvers that allow engineers to change a craft’s speed and direction without burning fuel.
In fact, the MESSENGER mission is really only possible because of gravity assists, according to JHU.
Still, the orbital dance is more of a slow waltz than a jitterbug. It took six years for MESSENGER to get from Earth and be ready to orbit around Mercury because the craft had to make six gravity-assist flybys: one past Earth, two past Venus, and three past Mercury itself.
—Video still courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
The flybys helped MESSENGER speed up so that it could spiral in from Earth, which is going around the sun at 18.5 miles (29.7 kilometers) a second, and snuggle close to Mercury, which zips around the sun at 29.8 miles (47.9 kilometers) a second.
Perhaps unsurprisingly, MESSENGER is over budget, costing $446 million U.S. When you’re spending that many tax dollars, mission managers tend to have a “waste not, want not” approach.
For MESSENGER, the three Mercury flybys were used to start collecting data: The craft snapped the first pictures of the “missing” side of Mercury in 2008, and it collected data on Mercury’s volcanic history, crater weathering, and magnetic environment on subsequent sweeps.
Once in orbit, MESSENGER will go around Mercury in an egg-shaped path roughly above the poles. It’ll pass closest to the planet’s north pole, coming as little as 124 miles (200 kilometers) above the surface before zooming out to a maximum distance of 9,420 miles (15,160 kilometers) above the southern face.
—Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington
Cozy in its new home, MESSENGER will then have a year to collect data for six main scientific goals:
1) What is the geologic history of Mercury?
Mercury is full of unusual geologic features: huge volcanic plains, spider-shaped rays, long rounded cliffs. With the first global picture of these surface features, it’ll be possible to piece together the events that shaped Mercury from when it first formed 4.5 billion years ago to now. This tells us more about the overall formation of the solar system, which informs our theories for how planets are born across the universe.
2) Why is Mercury so dense?
One mystery of Mercury is that radar studies showed the planet is incredibly dense—so dense that its iron-rich core must make up 60 percent of the planet, twice the size of the metal cores in Earth, Venus, and Mars. Three main theories have been competing to explain this oddity. By studying the planet’s surface composition, MESSENGER can put each theory to the test.
3) What is the structure of Mercury’s core?
Density tells us how much core there should be. But is it solid all the way through, or is part of it liquid, like Earth’s? MESSENGER can get better readings of how much Mercury wobbles on its rotational axis, which in turn reveals whether the mantle is floating on a liquid outer core or is fixed to a solid one. (Read about radio studies that suggest Mercury has a liquid core.)
4) What is the nature of Mercury’s magnetic field?
Like Earth, but unlike Mars and Venus, Mercury has a magnetic field, and it’s unclear if that field is being driven by a core dynamo like the one we think powers Earth’s magnetism. Studying how Mercury’s magnetic field interacts with charged particles from the sun can tell us more about the field’s shape and properties, which in turn adds to our knowledge of how magnetic fields work in general.
5) What chemicals are important in Mercury’s thin atmosphere?
Technically, Mercury has only an exosphere, the uppermost layer of an atmosphere, in which molecules are held to the planet tenuously by gravity. The layer of gas is so thin on Mercury that molecules don’t often collide, and instead move around by bouncing off the planet’s surface like rubber balls. MESSENGER will look at the exact composition of the exosphere to trace how each type of element got there and how the layer changes over time.
6) What are the unusual materials at Mercury’s poles?
When the sun is shining, surface temperatures on Mercury can reach 800 degrees F (427 degrees C). But Mercury isn’t always the hottest place to be: Because it has no real atmosphere, Mercury doesn’t trap any of the sun’s heat, and the night side of the planet can plummet to -290 degrees F (-179 degrees C). At the poles, Mercury has craters that are deep enough to be perpetually cold, and radar observations show that something in a few of those craters is oddly bright. One theory is that the craters are filled with water ice—a very unexpected possibility for the planet closest to the sun. Since MESSENGER will swoop very close to the planet’s north pole, the craft may be able to find out for sure.
—Image courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
MESSENGER will try to accomplish all this using a full suite of instruments, carefully protected from the sun’s heat and radiation.
The probe carries a dual-imaging camera, which can take color, black-and-white, and stereo pictures in wide and narrow fields of view. There’s also a couple different kinds of spectrometers to map Mercury’s surface elements, gases, and plasma environment and a laser altimeter to map topography.