The distance from Earth to Mars changes on a minute to minute basis. The two travel along elliptical orbits that are not synchronized, so they are constantly alternating between getting closer together and farther apart.
Theoretically, Mars and Earth are closest together when Earth is at farthest point from the Sun(aphelion) and Mars is at its closest to the Sun(perihelion). At that point the two would be within 54.6 million km of each other. That point is said to be theoretical because it has not been observed during recorded history. The closest known approach was in 2003, when Earth and Mars were separated by only 56 million km. On the opposite end of the scale, Mars and Earth can be 401 million km apart when they are in opposition and both are at aphelion. The average distance between the two is 225 million km.
Times of close approach are called ”launch windows” in the spaceflight vernacular. Even though Mars doesn’t always come close enough for clear visual observation, navigators have an opportunity to launch a spacecraft to Mars every 26 months. Spaceships don’t just take off in any direction. Escaping Earth’s gravitational pull requires a tremendous amount of energy. Even with that energy a spacecraft would never reach the orbit of Mars without a little help from its friends. To get a spacecraft to Mars, navigators time the launch so the spacecraft reaches Mars when it is on the opposite side of the Sun from Earth in order to take advantage of solar gravity. The Sun’s gravitational pull will cause a spacecraft to “fall” toward the Sun. This motion follows predictable physical laws. These trajectories and orbits are collectively referred to as Hohmann transfer orbits. ”What you want to do is launch the spacecraft so it goes around the Sun to meet Mars,” says Moriba Jah at NASA’s Jet Propulsion Laboratory, a navigator on the Mars Reconnaissance Orbiter, We give it the least amount of energy possible and let the Sun’s gravity do as much as it can for us. As a result the spacecraft comes up on Mars from behind, but instead of sailing past, a short blast from the boosters allow it to slow down and be captured into orbit by Mars’ gravity. The trip takes about 214 days.
The distance from Earth to Mars is the main reason that there has never been a manned flight to the Red Planet. Scientists around the world are working on ways to shorten the trip with the goal of sending a human into Martian orbit within the next decade.
This website lists every Mars opposition time, from recent past all the way in the far future. You can also use NASA’s Solar System Simulator to see the current position of any object in the Solar System.
Have you heard the hoax that Mars is going to be as large as the Moon in the sky? Yeah, that’s not ever going to happen.
Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about Mars at Astronomy Cast. Episode 52: Mars. We have also done an episode explaining distances, Episode 10: Measuring Distance in the Universe.
Sources:
http://www.nasa.gov/vision/universe/watchtheskies/18jun_approachingmars.html
http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky022005/
http://mars.jpl.nasa.gov/allaboutmars/nightsky/nightsky03/
Jerry Coffey is a Registered Nurse and father of 5. He enjoys skydiving, astronomy, and time with his children.
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