Astronomical occultations are great. A celestial body passes in front of a distant star, sometimes for just a fraction of a second, and it can provide you with so much information about that object. More than you had before.
What’s not so easy is finding out when these kinds of events are occurring. Thankfully, the European Space Agency's Gaia satellite is preparing the most accurate map of the stars in our galaxy. Now, researchers have used its first data release to forecast a stellar occultation by Europa, the famous icy moon of Jupiter.
This is the first time that astronomers have observed Europa occulting a star. Only Io and Callisto, two other Jovian moons, have been previously observed creating these peculiar eclipses. The occultation has led to a precise measurement of the moon’s shape and size. These observations are reported in Astronomy & Astrophysics.
The team refined Europa's mean radius to 1,561.2 kilometers (970 miles). Europa, just like Earth, is not a perfect sphere, but rather an ellipsoid. It's wider at the equator, reaching 1,562 kilometers (970.6 miles), and slightly shorter at its poles, spanning 1,560.4 kilometers (969.6 miles).
“We used data from Gaia's first data release to forecast that, from our viewpoint in South America, Europa would pass in front of a bright background star in March 2017 – and to predict the best location from which to observe this occultation,” lead author Bruno Morgado, from the Brazilian National Observatory, said in a statement. “This gave us a wonderful opportunity to explore Europa, as the technique offers an accuracy comparable to that of images obtained by space probes.”
Using Gaia, the team has been able to find evidence of even more occultations. Three of them have already taken place. The team was able to observe two of them (by Europa and Callisto on June 4 and June 5, 2019, respectively), and even more are coming.
“Jupiter is passing through a patch of sky that has the galactic center in the background, making it drastically more likely that its moons will pass in front of bright background stars,” explained Dr Morgado. “This would really help us to pin down their three-dimensional shapes and positions – not only for Jupiter's four largest moons, but for smaller, more irregularly shaped ones, too.”
The four largest moons are known as the Galilean satellites, after Galileo who discovered them. The upcoming occultations will be by Europa (June 22, 2020), Callisto (June 20, 2020, and May 4, 2021), Io (September 9 and 21, 2019, and April 2, 2021), and Ganymede (April 25, 2021). Even amateur astronomers with telescopes as small as 20 centimeters (8 inches) should be able to see them (if they are in the right region).