March 29, 2020

A Voyage to the Inner-Most Planet

The Solar System has just become a little more known. This year our view of Mercury, the closest planet to the Sun, has changed as the Messenger spacecraft completes its first flyby of the little known world. Late in the afternoon last week, I braved the January cold to peer west. There, in the dusk twilight, was a single shining point below the crescent Moon.

Mercury usually shines at magnitude -1, and for millennia, that’s about all astronomers knew about the innermost planet. Speeding around the sun once every 88 days, the planet has always been too small and close to the sun for proper observation. Stories that Nicholas Copernicus never sighted the fleeting world are probably apocryphal. The planet is a difficult target, though not impossible! Until very recently, it was thought that Mercury kept one hemisphere “tidally locked” towards the sun. Through a telescope, Mercury shows a featureless sliver disk passing through distinct phases like the Moon and Venus.
Mercury has also proved pivotal in science history; the mysterious precession of its perihelion was first attributed to a hidden inner planet named Vulcan (See “In Search of Planet Vulcan”):

Later, it was shown that Einstein’s newly minted theory of relativity could adequately account for the anomaly; it still stands as one of the great proofs of relativity today.

Transits of Mercury can also prove to be inspiring. I viewed my first and only transit of Mercury from the grounds of the Flandreau Observatory in Tucson, Arizona on November 8th, 2006; no Vulcanoids were spotted. The next transit occurs on May 9th, 2016; a full list of transits can be viewed at;

Only one space probe has ever been sent to Mercury; in 1974-5, Mariner 10 performed a close flyby, imaging about a third of the sunlit surface. One of my favorite anecdotes of the mission was an account of a “moon” of Mercury that was briefly hypothesized. This was due to a spurious fluctuation of ultraviolet radiation, the source of which is still unknown. A cool account of this can be found here;’s_moon

Very recently, astronomers using the SOAR (SOuthern Astrophysical Research) telescope managed to achieve unprecedented ground based images using a technique known as image stacking. (See Astroguyz “Imaging with an off the shelf web cam” ;

This enabled them to get images of awesome resolution.

Now comes Messenger to fill in the gaps. Already, its sent back some stunning, never before seen images. Messenger is scheduled for three fly-bys of Mercury before final orbital insertion on March 18th, 2011. While the first impressions reveal a world remarkably like the Moon, closer inspections show peculiarites. First, their is a total lack of maria, or flat plains, that dominate the lunar surface. Obviously, Mercury has undergone a severe bombardment early in its history. Secondly, its known to be the second densest object in the solar system, next to the Earth. This high “metallicity” rate has been suggested as a boon for carbon based life as we know it; we couldn’t have evolved in a hydrogen rich, heavy element poor environment. While I’m not suggesting that life on Mercury would be at all probable, (although an Arthur C. Clarke short story from “Islands in the Sky” suggests just that!) but it may guide our search for life farther a field. Interestingly, Mercury also pops in Kim Stanley Robinson’s “Blue Mars”… for a list of Mercury in fiction go to:

But back to the science. Messenger will doubtlessly rewrite the story on Mercury, or at least render many of these old Sci-Fi tales obsolete. Messenger is armed with a science payload of six instruments. The list includes the Mercury Dual Imaging System, Gamma-Ray and Neutron Spectrometer, X-ray Spectrometer, Magnetometer, Laser Altimeter, and the Atmospheric and Composition Spectrometer. Messenger will bring an unmatched level of scrutiny to the tiny world. Look for additional updates on this site during subsequent flybys and eventual orbital insertion (yes, I do expect to be blogging that long!) Also, keep an eye out for the total lunar eclipse next month; I intend to do a two parter on measuring longitude via the Eclipse method next month! Messenger’s mission site can be found at:

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