Mars has two moons, a pair of lumpy potatoes called Phobos (about 25 km long) and Deimos (12 km). On March 11, 2023, the European Space Agency mission Mars Express took images of a pretty nifty event: Phobos passing directly in front of Deimos!

Andrea Luck lives in Scotland and likes to process space images, so he took the images from the ESA archive (they’re here if you’re curious; search for the images from March 11) and created an animation of the event, but also a composite image that I rather like:

When a moon passes in front of another moon (or a planet in front of a planet) it’s called an occultation, or more generically a mutual event. In this case Phobos appeared to move in front of Deimos, and Andrea kept Phobos centered to show the more distant moon moving behind it. Pretty cool.

And holy cow, the geometry of this! Both moons orbit very nearly over the equator of Mars, so if you were on the surface eat the right spot such occultations would be common enough. But Mars Express is in a polar orbit, nearly at a right angle to the orbits of the moons! So capturing this sequence meant knowing exactly where the spacecraft and the two moons were, and getting the shots with incredible accuracy in space and time. It’s like flying in a jet fighter and getting a shot of a car driving on an overpass just as a truck drives under it. Except everybody is moving at he speed of a rifle bullet. Timing, baby. 

Super cool. Andrea is on Flickr, Mastodon, and Blue Sky, too, if you want to follow him and see more of his work. He’s good.

Become a paying subscriber of Premium to get access to this post and other subscriber-only content.

Already a paying subscriber? Sign In.

A subscription gets you:

• • Three (3!) issues per week, not just one
• • Full access to the BAN archives
• • Leave comment on articles (ask questions, talk to other subscribers, etc.)