Hold on to your hats and keep a pair of binoculars handy: After a 26-month sprint around the track of the solar system, we are about to lap Mars again.

Today, the red planet is in "opposition," an event that puts Earth between Mars and the Sun. On June 21, Mars will be at its closest distance from Earth since 1988, a mere 67.3 million kilometers (approximately 42 million miles). All summer long, Mars will be brighter than usual, particularly for sky-watchers in the southern United States and those in the Southern Hemisphere.

On average, Mars is 50 percent farther from the Sun than Earth is. Because of its tighter orbit, Earth passes Mars every couple of years. The reduced distance between the two planets and better solar illumination angle give Earthlings the best Mars-viewing opportunity. Through October, Mars will be easy to spot looking south, especially around midnight. The better view will be reserved for those living in the Southern Hemisphere, with Mars high in the sky. In the Northern Hemisphere, Mars will be low on the horizon. The more south the observer is, the higher the red planet will appear in the sky.

For centuries these favorable observing conditions have excited human imagination, providing closer views and new details and features. Now with better tools and the same hunger for discovery, NASA's Jet Propulsion Laboratory is still paying close attention to Mars' position to launch spacecraft at the most favorable opportunity to save fuel and time. Taking advantage of the upcoming alignment will be the 2001 Mars Odyssey orbiter, the most recent mission to the red planet, which was launched April 7 and will arrive October 23, 2001.

To understand the mechanics of timing launches to Mars, Dr. E. Myles Standish, a JPL astronomer who specializes in studies of planetary positions, compares Earth and Mars to two cars on different nearly circular tracks. The car on the shorter inside track is going faster, getting ahead of the one on the outside, and eventually catching up with it and overtaking it.

"If you were in the car on the inside track and wanted to throw a ball to someone in the car on the outside, you can do it only at certain times: you have to throw the ball outward at a specific time before your car has caught up with the outside one, and you have to aim at a spot ahead of the outside car. It is very similar with spacecraft," Standish said.

While Martian opposition occurs every other year, the minimum distance between the two planets is not always the same because of the elliptical orbits of the two bodies, particularly Mars. However, this is not a major concern for mission planning.

"The distance between the two planets is important, but it doesn't matter nearly as much as the timing, which is crucial," Standish said.

The next Mars opposition will be in August 2003, when the two planets will be the closest they ever been in at least 5,000 years, approximately 55.7 million kilometers (34.6 million miles). At that time, NASA will send two JPL-built rovers to Mars, each capable of exploring distances greater than the Sojourner rover of Mars Pathfinder fame.

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New pictures illustrating Mars opposition are available at

JPL manages the Mars Exploration Program for NASA's Office of Space Science, Washington D.C. JPL is a division of the California Institute of Technology in Pasadena.

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