A timelapse of Jupiter’s moons Europa and Io orbiting the gas giant has been widely shared on Reddit in recent days, owing to its sheer awesomeness (in the true sense of the word, not the “well that was an awesome sandwich” way that the word has been used in recent years). Many commenters have stated that the timelapse isn’t real because it’s so good.
While it’s important to be suspicious of photographs shared online, especially when false images are disseminated as real for hits, we’re delighted to report that the lovely short film is 100% genuine. Kevin M.Gill, a NASA-JPL engineer and experienced planetary image processor, created the timelapse, as he did so many other stunning views of the planets. He used data from the Cassini space probe’s 2001 flyby of Jupiter (to acquire a gravitational assist while also photographing the gas giant and performing other studies) before it moved on to Saturn.
Io and Europa are seen orbiting their planet in this video. One of the reasons for the clip’s skepticism is because Io, which is closer to Jupiter, looks to orbit the planet at a slower rate than Europa, which is further away. This, however, has a simple explanation. “The motion isn’t quite right,” Gill clarified on Twitter, “since I made it look prettier than it was correct.” “It’s designed to represent the motion seen from a spaceship moving faster than the moons’ orbital velocity. As a result, Io would travel quicker than Europa from a stationary position.”
Because you’re looking at Europa through the eyes of a spacecraft traveling to the left, such as Cassini, it appears to be orbiting faster than Io. Europa appears to be travelling quicker now that it is closer. This is a frequent illusion: things appear to move slower as they get further away because they appear to be smaller and hence take longer to pass through our vision. Consider what happens when you’re driving down the highway and look over at the automobiles heading the other way. Behind the autos, an airplane can be seen flying in the same direction as the cars. The cars appear to be traveling quicker than the airplane from your perspective, but they are not.