TCS Daily

It Goes to Extremes

By Charles Matthew Rousseaux - July 30, 2004 12:00 AM

"Darling I don't know why I go to extremes / Too high or too low their ain't no in-betweens"
-Billy Joel

Scientists don't know why the planet Mercury goes to extremes, but they are anxious to find out. On August 2, the MESSENGER probe is scheduled for launch to the first rock from the sun, for a year of survey and study.

Notwithstanding its unlikelihood of hosting the X-Games anytime soon, the planet is one of the most extreme places in the solar system. Only slightly larger than the moon, to which it bears a superficial resemblance, Mercury has a greater surface gravity thanks to a comparatively huge core of iron that makes up about 60 percent of its entire mass. That core makes it the density of the solar system -- its most dense planet (if the compressing effects of gravity are corrected for). Mercury experiences the greatest surface temperature swings of any other intra-solar planet. Its daytime highs top 840 degrees Fahrenheit, while its nighttime lows sink to minus 350 degrees Fahrenheit. Despite having a surface temperature that would melt lead (and possibly even this writer's cooking), Mercury might have ice in permanently shadowed areas in the poles. It has an extremely thin atmosphere, and is the only other planet in the inner solar system to have a magnetic field.

Boy Mercury (as the B-52s called it) shoots through every degree around the sun every 88 days -- moving faster than any other planet. However, it rotates so slowly that one of its 176-day "days" (noon to noon at the same place) lasts two years.

MESSENGER's primary mission is expected to last 1 earth year, roughly divided into surveying and scientific study. Less than half of the planet was mapped by Mariner 10 during its three flybys in the mid 1970's, and then only in poor resolution. There have been no other visits, and the planet is so close to the sun that studying it from the earth, or near-earth based telescopes is extremely difficult. The Hubble Space Telescope can't stare at Mercury lest its one eye be blinded by the sun. As a consequence, according to the mission press kit, "We know less about Mercury than any of the other planets except Pluto."

In addition to allowing scientists to again indulge in their delightful habit of making word plays wherever possible (sending MESSENGER to the planet named after the messenger of the gods), the acronym also describes what the spacecraft will do. MESSENGER stands for, "Mercury Surface Space, Environment, Geochemistry and Ranging mission. The craft carries seven scientific instruments, including a laser altimeter and an imaging system for mapping the planet's features, and a magnetometer and several types of spectrometers for determining the planet's composition.

Robert Palmer observed, "Some like it hot," but scientific instruments do not. To keep from them overheating, MESSENGER is covered by a sunshade composed of heat-resistant ceramic cloth. The craft itself will also be wrapped in a layer of the material.

Unlike its eponymic counterpart, MESSENGER will take its time in transit. After departing earth, it will swing by in a year for a gravity assist, and then do two flybys of Venus before it finally flies by Mercury in January of 2008. MESSENGER will do several flybys before finally settling into final orbit around its namesake in 2011. Even then, the probe's orbit will be extreme -- approaching as close as 124 miles to the planet but then receding by as much as 9,420 miles. That leisurely transit is necessary to slow down MESSENGER so much that Mercury will capture it when it finally flies by. Doing so will also require several breaking burns.

Many people go to the extremes to find something deep inside themselves, and there are similar motivations for sending MESSENGER to Mercury. Mercury may have been the first planet to form in the solar system, and so it may hold many clues to its beginnings. The mission's website noted that "To really understand Earth, and how it became the way it did today (rather than going the way of Mars or Venus) we need to understand how all of the terrestrial planets formed and, since it is the extreme case, Mercury is the key to that." Besides, as Sean Solomon, MESSENGER's principal investigator pointed out, Mercury is also a model for something even more extreme -- the formation of the extra-solar planets now being discovered.

About the only thing not extreme about the mission is its price tag -- about $426 million dollars from beginning to end. Even better is the fact that, by the time MESSENGER arrives at Mercury, another spacecraft should be well on its way to the other extreme edge of the solar system. In 2006, the New Horizons spacecraft will head up and out for an eventual flyby of Pluto in 2015.

Charles Rousseaux is an editorial writer for The Washington Times and a frequent contributor to Tech Central Station. E-mail:


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