NASA - National Aeronautics and Space Administration

On Nov. 8, 2006, telescopes in North America and other parts of the world captured the image of a little dot crossing the face of the sun. That dot was Mercury, and its passing between the Earth and sun is known as a transit.

Image to right: Mercury appeared to be 194 times smaller than the sun during its November 2006 transit, captured by the Japan Aerospace Exploration Agency's HINODE satellite. Credit: JAXA

Transits of Mercury take place about 13 times each century. The event highlights how small the planet is compared to the sun -- during the transit, Mercury appeared to be 194 times smaller than the sun. The transit is also a reminder of how little is known about what is now the smallest planet in the solar system, after tiny Pluto was recently downgraded from planetary status.

Because of its orbit, Mercury is hidden from Earth-based optical telescopes except for brief periods just after sunset or just before sunrise. Even then, the haze of Earth's atmosphere obscures the view. In fact, ground-based observations have raised more questions than answers -- questions such as whether the bright areas that show up at Mercury's poles in radar images are ice, and how ice could exist on a planet where daytime temperatures climb as high as 450 degrees Celsius (840 F).

Moving at an average speed of 38 kilometers per second (84,500 miles per hour), NASA's MESSENGER spacecraft is on course to take some of the mystery out of Mercury.

Launched in August 2004, MESSENGER is scheduled to fly near Mercury three times between January 2008 and September 2009. During the flybys, two imaging cameras will map nearly the entire surface of the planet. NASA's Mariner 10 -- the only other spacecraft to fly near Mercury -- was able to image only about 45 percent of the planet during flybys in 1974 and 1975.

MESSENGER will then enter into orbit around Mercury in March 2011, the start of a yearlong, in-depth study of the planet. The spacecraft will orbit Mercury twice every 24 hours, flying as close as 200 kilometers (about 125 miles) from the planet's surface. A total of seven instruments will collect data aimed at answering the following questions:

-- Why is Mercury so dense?
-- What is the geologic history of Mercury?
-- What is the structure of Mercury's core?
-- What is the nature of Mercury's magnetic field?
-- What are the unusual materials at Mercury's poles?
-- What volatile elements are important on Mercury?

Scientists also hope that by learning more about the elements, minerals and processes that helped form Mercury, they will gain a better understanding of the early evolution of the solar system and why Earth is so different from nearby planets.

One of the most important features of MESSENGER is not an instrument. Rather, it's a protective cloak of heat-resistant fabric. This "sunshade" will keep the instruments and entire spacecraft from overheating as MESSENGER travels to, approaches and orbits Mercury, the closest planet to the sun.

Image to left: Neal Bachtell of the Johns Hopkins University Applied Physics Laboratory attaches the middle section of the fabric "sunshade" to MESSENGER. Credit: NASA/JHUAPL/Carnegie Institution of Washington

To reach Mercury, MESSENGER must fly by Venus in June of this year. The flyby will serve as practice for the three Mercury flybys. MESSENGER will also make scientific observations during its closest approach to Venus, in cooperation with the team operating the European Space Agency's Venus Express spacecraft, now in orbit around Venus.

"Staying Cool" is an education module that simulates the challenges faced by the engineers who designed MESSENGER. A series of activities helps students understand the power of the sun as they figure out how to keep certain items cool. Lessons are organized by grade level -- ranging from pre-kindergarten through grade 12 -- and address various national science education standards.

Related Resources + NASA MESSENGER + MESSENGER Education and Public Outreach

In a lesson for pre-kindergarten through first grade, for example, students think about how to keep a lunchbox cool during a trip to the beach. Meanwhile, students in grades 9-12 design a container that will keep a pat of butter from melting even when placed in boiling water.

Through MESSENGER Education and Public Outreach, NASA continues its tradition of investing in the nation's education. It is directly tied to the agency's major education goal of attracting and retaining students in science, technology, engineering and mathematics disciplines. To compete effectively for the minds, imaginations and career ambitions of America's young people, NASA is focused on engaging and retaining students in education efforts that encourage their pursuit of disciplines critical to NASA's future engineering, scientific and technical missions.

Dan Stillman, Institute for Global Environmental Strategies