Comet Messengers Nearing ☿ Mercury

MESSENGER spacecraft has been orbiting the planet Mercury since March 17, 2011. The instrument payload has provided, and continues to provide, 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 150,000 images and a vast amount of other data. The entire surface has been mapped. MESSENGER is scheduled to continue orbital operations until late March 2015. Click the next image for a more detailed view.


The spacecraft’s seven scientific instruments and radio science investigation are unraveling the history and evolution of the innermost planet. In early November, the spacecraft turned its attention to the approaching new comet C/2012 S1 (ISON) and the well-known short-period comet 2P/Encke. It has returned the first images of the two comets from orbit around Mercury. This is a very different perspective on the comets than ours from Earth. There will be more images in the days to come.

Three images of Encke on November 6 – 8 are on the left. Three images of ISON are shown on November 9 – 11 are on the right. The faint comets are within the >< brackets. Both comets appear to brighten a little each day when compared top to bottom. Encke was 0.5 AU from the Sun and 0.2 AU from MESSENGER. Comet ISON was 0.75 AU and 0.5 AU, respectively. An astronomical unit AU is the distance between the Sun and Earth. More images will be obtained as the comets get brighter and closer to Mercury. Details here.

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 makes it easier to watch. Several repeat viewings helps, too. Keep your eye on the spacecraft as it is passed by each planet. You will notice it slows down with each encounter and falls into a different orbit.

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. It will remain in orbit for the duration of the mission into March 2015.

Six Important Questions

The MESSENGER mission hopes 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 %-age as for Earth. Why so different?
2. What is its geologic history? The suite of 7 instruments will map 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 will determine the size of 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 Are 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 had to be mounted where Mercury was visible to them, but the Sun was not. The nearness to the Sun causes high temperature and radiation values that would shorten the life of the instruments.


Some Image Gallery Highlights

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Rotation animation showing the south pole over a 176 day period. Craters here are deep enough to never get sunshine in the bottoms. Ice exists there. (The planet does not stop halfway and pause.)

Some Findings So Far 

Surface Details

Imaging maps are providing the first global look at the planet. They reveal 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 reveal 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, for instance, is a broad area of low elevations.

Tests for polar ice deposits preserved on the cold, permanently shadowed floors of high-latitude impact craters are being done. Evidence is strong for abundant amounts of water ice in the permanently shadowed craters at the poles. Details here.


24 thoughts on “Comet Messengers Nearing ☿ Mercury

  1. Must have been a heck of an exercise in physics and mathematics, computing that spiral path to get all those flybys. Similar to locating a tiny island in the middle of the Pacific Ocean, but orders of magnitude trickier, those graphics belie the vastness of space, how tiny the targets are.

    Those features of Mercury remind me of a sci fi short story I read long ago (can’t remember the name or author now) in which iirc an astronaut on Mercury is in a pickle on the terminator there, unable to depart, with the sun gradually “rising” over the edge of a crater. The rising sun there would be pretty horrendous, not just heat but radiation. Pockets of frozen gases vaporizing and exploding. I’ll be curious to learn how accurate that vision was, topography, composition, etc.

    • Hello. It’s nice to see you. Thanks for coming by.

      I also find the machinations fascinating to make those elaborate flyby journeys. They are great cost and fuel savers.

      The scifi story sounds interesting. Yes, it would get very hot and steamy in the direct sun. The surface must be really parched.

      Thanks for your comments. I hope you are doing well.

  2. The last photo you posted made me smile. The colors are so reminiscent of Santa Fe – adobe and turquoise.

    Mercury’s recently been of concern in other circles, too. From October 21 to November 10 it was in retrograde, which doesn’t make a bit of difference to me but which unhinges some of my astrologically inclined friends. Apparently Mercury Retrograde makes appliances break and computer programs freeze up. Plans fall apart, too. Or so they say. 😉

    I really enjoy these posts, even when I don’t understand everything. The good news is that they’re here for a second and third read. Some of it will soak in.

    • That does have some nice colors in it. Good of you to point them out.
      Your retrograde comments make me laugh. As for me, I’ve been watching Mercury climb a little higher each day in the pre-dawn sky. Comet ISON will be near it on 22nd.
      You come back and read as many times as you need. I like visitors like you. 🙂

  3. Another great post Jim! Messenger is a favorite of mine, then again – I like all orbiters! Thanks also for the explanation of the slingshot effect. I think few of us really appreciate the brilliance involved in getting these spacecraft to where they’re going and into orbit.

  4. Thank Jim. I actually didn’t know anything about this mission. I guess I need to come here more often to be informed about space accomplishments.

  5. I’m impressed by the clarity and detail of those first images. It helps not to have much of an atmosphere to deal with.

    I don’t know about Mercury being in retrograde, but sometimes my mind gets that way.

    • The global views are mosaics from thousands of images. Perhaps you clicked on the first image. Then, click on Full Size under Download. That’s even more impressive.

      Mine, too. Mercury goes a lot faster than Earth. During the first week of Nov. it was passing Earth and moved ahead of the Sun in the mornings. It rose first. The separation between them grew. Now Mercury has rounded the turn in orbit and is getting less separated from the Sun. It appears to have reversed its direction in the morning sky. Soon it will in line with the Sun and appear as an evening planet. Retrograde will happen again, every 44 days. Don’t let it bother you. 🙂

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