Skip to main content.
About Us Academics Admissions Athletics Giving to the College Graduate Studies Library Student Life
Dr. Neal Sumerlin in Belk Observatory

Astronomy News

Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.

Blogs we recommend

Planetary Society Weblog
ScienceBlogs.com

Messenger Mission to Mercury

Posted on 02/04/2008

Mercury is the smallest of the eight classical planets (sorry, Pluto fans), and the one that is nearest to the Sun. Although it is relatively close to us-all the four inner terrestrial planets (Mercury, Venus, Earth, and Mars) are huddled closer to the Sun than the more widely spaced outer giant planets (Jupiter, Saturn, Uranus, and Neptune)-it has barely been visited by spacecraft. Only one spacecraft has ever imaged it at close range. 34 years ago, Mariner 10 made three passes by Mercury, coming as close as 200 miles at one point. Because of Mercury's odd rotation cycle, less than half of Mercury's surface was seen, something like the situation with our own moon before the space age.

Why such neglect of a nearby planet? After all, we have sent dozens of spacecraft to Mars, and even distant Saturn has now been visited by three missions. Mars is a special case, of course, the stuff of wild speculation and more sober assessments of its similarity to Earth. But there is a physical reason why visiting planets that are closer to the Sun than we are is more difficult than you might think.

Think of the gravitational environment that surrounds the Sun as a depression, with the Sun at the central lowest point, and the planets perched at increasingly elevated positions "uphill" from the Sun. Now imagine all of these planets whirling around that central depression, sort of like the motorcyclists in a "Cage of Death" stunt at the circus.

When a spacecraft is sent to Mars from the Earth, it is essentially climbing uphill, albeit in a curving path that mimics the orbits of both planets. We simply have to give it enough of an impulse for it to begin climbing away from the Sun. In the simplest case, when it gets to Mars it has slowed down just enough to be easily captured into orbit with minimal fuel expenditure.

But going from the Earth to Mercury is heading "downhill". Once you slow down a bit (this takes fuel, of course), you begin dropping into a lower orbit that will take you to Mercury. Instead of losing speed as you would on your way to Mars, however, you will go faster and faster as you fall closer to the Sun. Slowing down enough to be captured into orbit at Mercury takes a considerable amount of fuel, and every bit of fuel subtracts from the scientific payload that could otherwise be sent.

But planetary scientists aided by computers have gotten very sophisticated at plotting interplanetary trajectories. Passing very near a planet allows a spacecraft to "steal" a bit of energy from the planet and redirect the spacecraft's path. The energy lost by the planet is undetectable; the same amount of energy applied to the tiny spacecraft can make a huge difference in its trajectory. By carefully directing the craft, planners can greatly reduce the fuel necessary to maneuver it into position, and thereby increase the scientific payload for the mission.

Messenger is a mission to Mercury that was launched in August 2004. It has already made one flyby of Earth and two of Venus. It will not go into orbit around Mercury until 2011! Three weeks ago it made its first pass of Mercury. The exciting part of this is that it was able to see much of the part of Mercury that was hidden in darkness 34 years ago. Only preliminary conclusions are available at this time, but as always, each new discovery raises as many questions as it answers.

The Messenger web site is here: http://messenger.jhuapl.edu/index.php

The Planetary Society has a detailed account of the news conference announcing preliminary results: http://www.planetary.org/news/2008/0130_MESSENGERs_First_Mercury_Flyby_Reveals.html

Blog and journal content is produced by an individual. All opinions are those of the individual writer and may not reflect those of Lynchburg College.