Artist concept of MESSENGER arriving at Mercury.
Credit: NASA

History has been made yet again: NASA’s MESSENGER spacecraft is now in orbit around the solar system’s innermost planet!

This is exciting to me. When I was a child, Mercury was a contender in my mind for “favorite planet”—right up there with Pluto, being so far away and mysterious, and Uranus, being such a pretty shade of blue. Mind you, that was back before spacecraft had visited any of them….

Being closest to the Sun gave Mercury a claim to fame—just as Venus being the hottest planet or Earth being the home planet or Mars being the red planet or Saturn having rings…okay, they’re all special.

At first glance, Mercury presents the appearance of Earth’s Moon, but doing just a little math tells you there’s more to it. Just divide its mass by its volume and you get 5.43 grams per cubic centimeter—Mercury’s average density. That’s only a shade less than Earth’s average density of 5.51 g/cc, which indicates that Mercury contains more heavy elements than the Moon—elements like iron, for example. (Personally, I suspect there’s gold on that thar planet, too….) In fact, after Earth, Mercury is the densest known planet in the solar system. What does it hide under that cratered and radiation-baked surface?

An interesting tidbit about Mercury: because of its high density—because of the amount of material packed within its smallish confines—the surface gravity on Mercury is about the same as on Mars, even though Mars is larger. (Mercury has a diameter of just over 3000 miles, while Mars is about 4200 miles across.) On both worlds, you’d weight about 38% what you weigh on Earth.

And now, Mercury has joined a new club: planets that have been orbited by spacecraft—in this case, NASA’s MESSENGER. MESSENGER has been en route to its final orbit around Mercury for over six and a half years. Since its launch on August 3, 2004, MESSENGER has been winding its way through the inner solar system, swinging back past Earth about a year after launch, making a couple of close passes by Venus, then three fly-bys of Mercury itself, before settling into orbit on March 17th.

This long and meandering path was a great fuel saver: by making flybys of planets, MESSENGER’s trajectory and speed were altered to make the transition from Earth orbital velocity to a stable orbit around Mercury without burning a lot of fuel to do so. Other spacecraft have used gravitational “slingshot” maneuvers to change direction or gain speed for free, including the Voyagers and New Horizons (on its way to Pluto).

At Mercury, sunlight is up to ten times more intense than at Earth—and since Mercury has no atmosphere to filter out ultraviolet and X-rays, let’s just say the sunburn you’d get standing on its surface would be nothing short of lethal. Never mind the lack of air….

To keep cool in the intense solar radiation environment around Mercury, MESSENGER takes advantage of the same physical principle that keeps the dark side of Mercury itself cool (as cold as negative 185 degrees F): insulation. Down on Mercury, the night-time surface is protected from the Sun’s intense rays by an entire planet of material. With no atmosphere to trap heat, at night the temperature plummets as heat radiates from rocks and soil directly into space.

MESSENGER possesses a shield: a “sun screen” made of heat-resistant ceramic cloth, situated between the Sun and spacecraft much like someone holding a parasol to stay in the shade.

Poking out from behind its shield, MESSENGER wields two solar panels that provide the spacecraft all of its power—one distinct advantage of the otherwise troublesome intensity of Mercurial sunlight.

Now that MESSENGER is in orbit, what do we hope to discover in the days and months ahead?

Is there ice at the bottom of polar crater floors? How is Mercury’s magnetic field generated? Does Mercury have, or did it have, plate tectonics? What’s it made of? How does Mercury interact with plasma flowing from the Sun—the solar wind? Are there any strange obelisks on its surface? …to make just a partial list…. Answers to follow (for most of them, anyway).

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  • john

    Why did it take so long to reach Mercury? The moon would take several days. Why did it take many years?

  • Ben Burress

    It was a fuel related thing, and so also a cost-effectiveness issue. In short, the mission was far less expensive by maneuvering the spacecraft to Mercury orbit using Earth, Venus, and Mercury–their gravity–rather than equipping the spacecraft with the engines and fuel necessary for a quick flight. The main problems of a flight to a planet like Mercury are first crossing the distance too the planet and second slowing the spacecraft sufficiently for it to enter orbit (and not just fly on by). If you blast the probe to Mercury on a quick flight, when it gets there it’s going very fast and must be slowed, and that requires more fuel, which adds weight to the spacecraft, which in turn increases the need for fuel (because you also have to slow down the weight of all that fuel).

    It’s a little different with a mission like New Horizons, heading to Pluto. That spacecraft was brought to high speed from the start, and even speeded up by a maneuver past Jupiter, so that it could cross the great distance to Pluto in a “reasonable” amount of time (in this case, 10 years!)–but there is no requirement to slow it down when it gets to Pluto because it’s not going into an orbit, but rather flying by Pluto at high speed.


Ben Burress

Benjamin Burress has been a staff astronomer at Chabot Space & Science Center since July 1999. He graduated from Sonoma State University in 1985 with a bachelor’s degree in physics (and minor in astronomy), after which he signed on for a two-year stint in the Peace Corps, where he taught physics and mathematics in the African nation of Cameroon. From 1989-96 he served on the crew of NASA’s Kuiper Airborne Observatory at Ames Research Center in Mountain View, CA. From 1996-99, he was Head Observer at the Naval Prototype Optical Interferometer program at Lowell Observatory in Flagstaff, AZ. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

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