Category Archives: Mercury

Mercury, MESSENGER, NASA

Standard Mercury Orbit, Mr. Sulu

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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!

astronomy news, Mercury, MESSENGER

Another Side of Mercury

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More Cool Planetary Science

A wide-angle camera view of Mercury
A wide-angle camera view of Mercury

Yesterday the MESSENGER mission did a close flyby of Mercury and began returning high-resolution images.  I’ll start with the last one first — it was was taken about 90 minutes after the spacecraft’s closest approach to the planet, while Messenger was on the way “out”.

The bright crater in the center is called “Kuiper” — named after Gerard Kuiper, a well-known planetary scientist.  This crater was first spotted in Mariner 10 images in the 1970s, but the real news is that this image shows terrain we haven’t seen before. Everything east of Kuiper (toward the limb (right edge) is new to our eyes. Notice the rays extending out across the whole right side of the image, emanating from a relatively fresh crater that had only been seen at low resolution from Earth using radar scanning techniques. This gives planetary scientists a whole new side of Mercury to interpret as they work to understand Mercury’s formation and cratering history.

A close-up of Mercury a few minutes after close approach.
A close-up of Mercury a few minutes after close approach.

Only a few minutes after closest approach, the spacecraft snapped this image of Mercury’s cratered, pitted surface. The last time anybody saw an image of this part of Mercury was from Mariner 10 in the 1970s. The largest impact feature at the top of the image is about 133 kilometers (83 miles) across and is named Polygnotus (who was a Greek painter who lived in the 5th century B.C.). It has a central peak ring and is surrounded by smooth plains material (which probably was molten rock that flowed and hardened after the impact). Another large crater at the top left of the image is called Boethius (after a 6th century Roman philosopher).  It also appears to be almost filled with smooth plains (probably formed the same way).  Sometime well after the impact which created the crater, the whole area was deformed during the formation of a prominent scarp (a cliff).

Never-before-seen terrain on Mercury
Never-before-seen terrain on Mercury

Finally, from a time about 58 minutes before closest approach, Messenger snapped this dynamic image, which I think gives a nifty 3D “feel” for those of us watching on flat screens. The features in the foreground, near the right side of the image, are close to the terminator, the line between the sunlit dayside and dark night side of the planet, so shadows are long and prominent.  You can make out two very long scarps that appear to cut across each other. One occurred first, and then sometime later, tectonic forces created the other one. The easternmost scarp also cuts through a crater, which means that it formed after the impact that created the crater. Other neighboring impact craters, such as in the upper left of this image, appear to be filled with smooth plains material (again, probably from molten rock that flowed after the impact and then cooled and hardened).

There will undoubtedly be more images coming from the MESSENGER mission. If you want to follow the action and get larger versions of all the images, point your browser to the MESSENGER website gallery. Mission scientists are examining images and putting them up as they do — so check frequently!