Category Archives: MESSENGER

Mercury’s Swift MESSENGER

Exploring a Hot/Cold Little World

This WAC image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of ~450 km (280 miles) above the planet during the spacecraft’s first orbit with the camera in operation. The area is covered in secondary craters made by an impact outside of the field of view. Some of the secondary craters are oriented in chain-like formations. Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Yesterday was a banner day for the folks who run the MESSENGER mission to Mercury.  The first images that the spacecraft has taken since it achieved orbit around the planet began streaming back to Earth.  Now, there have been a good many flyby images of Mercury, not all of them high-res, since we began exploring the solar system in the mid-1960s with spacecraft. But, these are the first taken from a spacecraft orbiting the planet.  This means that we’re going to (finally) get to see every bit of this world’s ancient, cracked and cratered surface.

The first images (which you can browse at the MESSENGER Web Site) are pretty detailed.  For example, this Wide Angle Camera view shows an area near the north pole of the planet.  It was taken as the spacecraft made its closest approach to the sunlit side of the surface.  The long shadows are caused because the Sun is at an oblique angle to the spacecraft, which was just about to cross over into darkness.

The polar region of the planet is of great interest to astronomers because some radar studies of the area taken from Earth seem to imply that there might be ice on the walls of those craters. Sunlight never penetrates into the craters, so that would be an exciting cache to study someday.  The water could be from impacts of comets, and studying THAT ice would tell us about the compositions of the comets that left their water behind.

The full imaging mission, which is planned to last for at least a year, begins on April 4; currently scientists and engineers are testing the spacecrafts systems and instruments to see how it is responding to its permanent home in orbit around Mercury — and enduring the harsh environment so close to the Sun.

Mercury might seem the last place you’d want to visit — and it probably won’t be the subject of human exploration for a long time. It’s a pretty extreme place, experiencing the widest surface temperature swings of any place in the solar system. It has little to no atmosphere, and its surface takes a radiation pounding from the Sun. Still, if you’re going to completely understand the solar system you inhabit, it pays to study the extreme places as well as the temperate (relatively) ones. In Mercury’s case, astronomers have a very specific set of questions to ask to help understand this place:

Why is Mercury so dense?  What is its geologic history? What is the nature of its magnetic field? What is the structure of the planet’s core? What are the unusual materials seen at the poles?  What volatiles (gases) are important at Mercury?

Some of these questions are those you’d ask at any planet to help you understand its composition and history.  So, with Mercury, we’ll finally get some closure on the last of the major planets of the solar system.  The only remaining world to visit (and there’s a spacecraft on the way) is dwarf planet Pluto. New Horizons will give it a swing-by in 2015, on its way out to explore the outer solar system. Another extreme, to be sure, but now we’ve got precedent with Mercury.

Standard Mercury Orbit, Mr. Sulu

Mercury Has a Long-Term Visitor

MESSENGER at Mercury (artist's conception). Courtesy MESSENGER Mission.

Last night I went down to the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado for a briefing and “wait-it-out” event for the MESSENGER spacecraft’s orbital insertion. The lab built one of the instruments onboard the spacecraft — the Mercury Atmospheric and Surface Composition Spectrometer (MASCS). As I was sitting there watching the folks in the control room at the Johns Hopkins Applied Physics Laboratory (by remote link), I thought about all the times we’ve seen “standard parking” orbit commands applied in shows like Star Trek. They make it sound so easy. The captain just says “Standard orbit” to the helm, the officer punches a few buttons or slides a fader or waves a hand over the console, and the ship slides into the correct spot.  What we don’t see are the ship’s thrusters firing to nudge the ship — a massive behemoth compared to a small planetary probe like MESSENGER —  from its previous course into the parking orbit around the world it’s visiting.

We didn’t actually see MESSENGER’s thruster fire either last night. What we heard was a stream of announcements that told us what the Doppler readings were from the spacecraft as its fuel tanks fed the thruster that nudged it into a parking orbit (highly elliptical at that!) around Mercury.  You can see an animation of how it might have looked here.

To see the kind of orbital trip MESSENGER has taken since its launch on August 3, 2004 , go here for an animation showing its long journey. As of today, the spacecraft has traveled 4,902,668,000 miles. That’s 7,890,000,000 kilometers, or 52.7 astronomical units. If Messenger had traveled in a straight line for that distance, it would be well beyond the most distant point of Pluto’s orbit!

MESSENGER's orbit around Mercury is very elliptical, meaning its lowest point is only about 124 miles (200 km) above the surface and gets as far asway as 9,300 miles (15,000 km) at its most distant. The closest approach of the orbit will take it high over the north pole, where there is some radar evidence of something -- possibly water ice -- hidden on crater walls. Courtesy NASA/JHUAPL.

MESSENGER is small, only about the size of a minivan, so it wouldn’t take nearly as much power to put it into “standard orbit” as a giant starship would need.  But, the principles are the same no matter what size of object you’re trying to put in orbit around another one.  The incoming object has a certain path it’s following, and it’s going at a certain speed. If you want the spacecraft to merely fly by, you wouldn’t need to deviate the probe from its path.  But, if you want it to go into orbit, you have to slow it down at the right place, the right time, and at the right rate. Once you do that, you’ve got your spacecraft (regardless of its size) at a point where the gravitational pull of the planet it’s orbiting is EXACTLY matched by the spacecraft’s speed around the planet. It’s a constant tug of war that must be balanced correctly.

Now that MESSENGER is safely orbiting Mercury, scientists are testing its systems to make sure they’re doing okay in the hot, harsh environment around the planet.  The spacecraft is subject to solar heating eleven times hotter than we experience here on Earth, and it is protected with a sunshade to protect the delicate instruments inside. As soon as they’re satisfied that all systems are working on the nominal, they’ll turn the instruments on and commence the next phase of mission science.  The first images should start streaming to Earth in the first part of April, so stay tuned!