The event that the Hubble space telescope captured this week only happens once or twice every ten years. What we're seeing here is three of the four Galilean satellites - moons, that is - moving around Jupiter's gaseous surface, all within the same frame at the same time. Their shadows are all in the frame at the same time, at least. Here you'll see the moons "Io", "Callisto", and our good friend "Europa." That last one we'll be visiting in the next 9 years if we're lucky.