They occur because about every two years Earth's orbit catches up to Mars' orbit, aligning the sun, Earth, and Mars in a straight line, so that Mars and the sun are on "opposing" sides of Earth. This phenomenon is a result of the difference in orbital periods between Earth's orbit and Mars' orbit. While Earth takes the familiar 365 days to travel once around the sun, Mars takes 687 Earth days to make its trip around our star. As a result, Earth makes almost two full orbits in the time it takes Mars to make just one, resulting in the occurrence of Martian oppositions about every 26 months.
For additional information, contact:
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
firstname.lastname@example.org / email@example.com
Arizona State University, Tempe, Arizona
Space Science Institute, Boulder, Colorado