The Alignment of Planets

Space ships can coast to Mars by following a Hohmann Transfer Orbit. [learn more from JPL's "Basics of Space Flight"]

NASA has been considering a human mission to Mars for years. Larry Kos, a mission planner at NASA's Marshall Space Flight Centre, notes that timing is everything. "The best time to launch a mission to Mars," he says, "is usually a few months before Earth and Mars are closest together -- a time astronomers call 'opposition'. When Mars missions take off, they head toward an apparently empty point in space. The planet isn't there yet, but it will be when the spacecraft arrives." Of course, if humans go to Mars they will need to come back, too. "For a return trip we would wait 26 months for a similar Earth-Mars alignment and once again launch a few months before opposition. That geometry would minimise the return propulsion needed."

While Earth and Mars approach each other every 26 months, their minimum separation varies over a 15 year cycle due to the elliptical nature of each planet's orbit. Indeed, it can vary by almost a factor of two. Choosing the right year to launch will have a significant impact on the propulsion power required to fling a payload from Earth to Mars, and back again.

The next best times to go to Mars will come in 2003, 2018, and 2020 -- years when Earth and Mars will be unusually close together. Humans might finally visit the Red Planet in 2018 or 2020, but alas, they won't travel there aboard vessels like the USS Enterprise or the Millennium Falcon. Our first Martian explorers will probably blast off on chemical rockets after intensive calculations of capability, aim points, and timing. In that regard, human exploration of Mars will begin as have so many other adventures in history … only when the planets are properly aligned.

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