Section 2.5.1.
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Window On Mars

James Oberg

Real-time picture of positions of Earth and Mars
Image courtesy of NASA Ames Research Center
Even if the rockets, spacecraft and astronauts are ready, a journey to Mars cannot begin until the planets are in alignment. The logic sounds astrological, but in fact it is mathematical.

Relative to the sun, a space craft before launch is already moving at Earth's orbital speed. To take the most economical path through space, the craft's engines must add to the orbital speed, throwing the vehicle onto an elongated orbit that reaches farther from the sun that the Earth does. If (and only if) the farthest point on the elongated path intersects Mars's orbit at just the right place and time, the craft will be able to reach the planet.

As Earth and Mars circle the Sun like lapping race cars, they repeat the proper relative "starting positions" for such a rendezvous about every 26 months. Then, during a "launch window" of only a few weeks, travelers can follow the most efficient path from Earth and arrive at Mars about six to ten months later. Timing is similarly tricky for the return journey; to take the best path home, Mars explorers will have to wait on the planet for 15 to 20 months until the next window opens.

As a result, using so-called "minimum energy" trajectories requires a mission time of more than two years, far longer than humans have yet endured in space. Shorter missions are possible if craft carries more fuel (an added expense), or if special sling-shot short-cuts past Venus can be built into the trajectory. But even under the most optimistic designs, mission durations are still 18 months or more.

This article was first published in Technology Review, Jan-Feb 1999, pp. 54-59. Republished here with the permission of the author.

Related articles by James Oberg in this section of the Artemis Data Book:
Missionaries to Mars
Cheap Seats?

James Oberg's Home Page

Use AltaVistaTM to search the Artemis Project web for articles about Mars.


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