Bob Moler's Ephemeris Blog

On Thursday evening, 8:45 p.m. March 17 EDT (12:45 a.m. March 18 UTC) the MESSENGER spacecraft will complete the second of NASA’s 2011 planetary trifecta when it will, if all goes well, fire its rocket engine to drop into orbit of the tiny planet Mercury.  We’ve had six quick peeks at Mercury so far.  Three by the Mariner 10 spacecraft in the 1970’s which looked at the same half of the planet due to Mercury’s unique rotational period.  And three more by MESSENGER as it used Mercury to put on the breaks, so it would be going slow enough this time, so it’s rocket engine could drop it into polar orbit of the planet.

At first blush, Mercury looks like the moon. But it’s not.  The moon is light, being made up, apparently, of mostly the crustal materials expelled by the earth and another Mars sized body.  So it has a relatively small core.  Mercury, on the other hand has a large core, and is the second densest planet at 5.43 grams per cubic centimeter.  It’s only beaten out by the earth’s 5.52 g/cm³.

We’re going to learn a lot more about Mercury in the next year or so as MESSENGER maps Mercury and the complex interaction between it and the solar wind and magnetic field coming from the sun.  Lets look back at the early history of our knowledge of Mercury.
It seems the that early Greeks noticed this elusive planet.  They saw it in the morning sometimes, and then they saw it in the evening.  At first they thought it was two separate planets that they gave the name Hermes in the evening and Apollo in the morning before they figured out that it was the same planet. The name Mercury we know the planet by today is the Roman equivalent of Hermes.

Another revelation came later.  In my youth Mercury was thought to be in tidal lock with the sun, like our moon is to the earth.  The rather poor markings found on the planet seen low in the sky at dusk and dawn seemed to bear that out an 88 day rotation to match its 88 day revolution of the sun.  It wasn’t until 1965 that radar observations proved that the rotation was 2/3 of 88 days. Every 2 orbits of the sun Mercury rotates 3 times.  It seems that the best times to spot Mercury are when it’s in the same part of its orbit, but basically every other return to that spot.  Funny thing.  The northern hemisphere’s best views of Mercury are for its eastern elongation on spring evenings and western elongations on autumn mornings.  In effect we’re viewing Mercury at the same point in its orbit, when it is near its perihelion, when it is closest to the sun.  The southern hemispheric observers get to see more favorable views of Mercury, when it’s farthest from the sun.

As we’ve found with all the planets that we’ve gotten a close look at, the generalities of our long-standing ignorance is brushed away.  Each planet is its own unique place in the sun.

Taken from my March, 2011 article in the Stellar Sentinel,the newsletter of the Grand Traverse Astronomical Society.

Here is the MESSENGER web page.  This mission is run for NASA by the Johns Hopkins Applied Physics Laboratory.

MESSENGER’s First Look at Mercury’s Previously Unseen Side

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington