We can't go to Mars any time we want to. At the speed of today's spacecraft, the travel time from Earth to Mars is months even when the planets are closest to each other. So when is the best time to go to Mars?
Since Mars is farther from the Sun than Earth, it takes Mars longer to complete an orbit around the Sun, so the Earth overtakes Mars about every 780 days (2 years 50 days).
When this happens, the Earth is directly between the Sun and Mars, meaning that from our perspective, we see Mars in the opposite direction from the Sun. Astronomers call this alignment "opposition." At about this time the planets are also closest, and this is when we can best make the trip.
If Mars' orbit were circular, this closest distance would stay the same, about 49 million miles. But because the orbit of Mars is not circular, the distance at closest approach varies between about 35 million and 63 million miles.
In 2003, when the planets were at opposition, Mars was near "perihelion," the point in its orbit closest to the Sun, and was only 34.6 million miles from Earth. (This was the closest approach since 57,617 BC!)
On March 5, 2012, Mars will again be at opposition but this time near "aphelion," the point in its orbit farthest from the Sun, so the distance between the planets will be about 62.6 million miles.
When do we have to leave to get to Mars in 2012? Taking into account the distance, the time of the closest approach, and the speed of the spacecraft, scientists can determine a launch window. For this opportunity the launch windows for spacecraft occur during November and December of this year.
Both Russia and the U.S. planned missions for Mars at this time. The Russian mission, Phobos-Grunt, was launched Nov. 9, but it failed to leave Earth orbit for Mars when cruise stage rockets failed to fire; communication with the craft was lost, and the window closed on Nov. 21.
The launch window for the U.S. mission, Mars Science Laboratory (MSL), opens today and continues until Dec. 18. Thus, the best time to leave to arrive at Mars in 2012 is right now. Watch for news reports on the status of this mission and read more about it at marsprogram.jpl.nasa.gov/msl/.
This closest approach is also the best time to view Mars from Earth. Currently Mars rises about midnight in the constellation Leo; by 6:30 a.m. it is due south.
To the naked eye Mars is never very large, but as the Earth approaches Mars, its apparent size will get larger and larger and it will appear brighter and brighter in the night sky until on March 5, 2012, it will be a little more than twice the size it is now.
It will then shrink in size and grow dimmer after we pass the planet.
This month is a good time to start viewing Mars with a 4- to 8-inch telescope. You will not be able to see surface detail yet, but this is the season for Mars dust storms.
By mid-December you should be able to see the North Polar Cap, and by mid-January surface detail should be well defined.
Enjoy the views; it will be two years before you get another chance this good!
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.