... To help answer these questions, the largest and most mysterious moon of Saturn got a quick first look from the Cassini robot spacecraft soon after entering orbit around the giant planet last week. Cassini Images Density Waves in Saturns Rings . ... Cassini to Venus . ... Yesterday, Cassini became the first spacecraft to enter orbit around the gas giant Saturn, rocketing through a 25,000 kilometer wide gap in the distant planet's magnificent system of icy rings at about 15 kilometers per second. ...