Editor's note, Sept 23, 2014: want to follow Mars Orbiter Mission's Mars arrival live? Here's a timeline with links to live video. Best of luck to ISRO! --Emily Lakdawalla

Two spacecraft launched for Mars this month: Mars Orbiter Mission on November 5, and MAVEN on November 18. But MAVEN is already on an interplanetary trajectory, while Mars Orbiter Mission is still in Earth orbit and will not depart for Mars until the end of the month. A lot of people are asking me: why the difference?

I knew it was related to their different launch vehicles, but to get a more informed answer I turned to David Doody, a senior engineer at the Jet Propulsion Laboratory and author of the really awesome JPL online resource called "The Basics of Space Flight." Dave periodically teaches courses on how interplanetary navigation works; registration opens in April for his next one.

Dave confirmed that the different approaches were related to their launch vehicles and to the different spacecraft masses. For MAVEN, the trip to Mars is straightforward. That's because its Atlas V rocket was able to deliver both MAVEN (with a mass of 2500 kilograms, fully fueled) and an attached, powerful Centaur upper stage into Earth orbit. While already orbiting Earth, Centaur was able to kick MAVEN directly on to Mars. Dave explained: