Sometimes it pays to step back and try to look at spaceflight with fresh eyes. Go out and find Saturn in the sky and consider it as the ancients did, a moving celestial ember. And as you stand there, realize all over again that we’ve built a spacecraft that has been operating around that world since 2004, feeding us a datastream as its path was tweaked to look at interesting targets. The sheer magnitude of this accomplishment — Cassini is now operating between the planet and its rings! — is cause for celebration even as the mission’s end approaches.


Image: Artist’s concept of Cassini diving between Saturn and its innermost ring. Credit: NASA/JPL-Caltech/Space Science Institute.

Today’s good news is that controllers have reacquired Cassini’s signals following its plunge through the ring/planet gap on April 26, a time during which its 4-meter high-gain antenna was re-oriented to serve as an ad hoc shield against whatever dust grains or particles might be in its path. Critical maneuvers like these always produce nail-biting moments, and the spacecraft was out of contact during the entire ring-plane crossing, which occurred at 0900 UTC on the 26th, followed by renewed contact with Earth some 22 hours later.


Image: This unprocessed image and the two that follow show features in Saturn’s atmosphere from closer than ever before. These views were captured by NASA’s Cassini spacecraft during its first Grand Finale dive past the planet on April 26, 2017. Credit: NASA/JPL-Caltech/Space Science Institute.

The next dive is scheduled for May 2 in this ‘Grand Finale’ stage of the mission, during which Cassini will make a total of 22 dives, but this first one was obviously crucial in that we have a healthy spacecraft and will obviously learn more about spacecraft protection on future ring-plane crossings. As to that gap between the tenuous upper atmosphere of Saturn and the rings, it’s about 2000 kilometers. With Cassini moving through this area at roughly 34 kilometers per second relative to the planet, any small collisions could knock the craft out of commission.


But Cassini made it through. Earl Maize is Cassini project manager at JPL:

“No spacecraft has ever been this close to Saturn before. We could only rely on predictions, based on our experience with Saturn’s other rings, of what we thought this gap between the rings and Saturn would be like. I am delighted to report that Cassini shot through the gap just as we planned and has come out the other side in excellent shape.”

Science and engineering data are now being beamed back to Earth after acquisition of the Cassini signal at the Deep Space Network’s Goldstone Complex in California at 0656 UTC on April 27, with the data flow commencing minutes later. The ring-plane plunge took the spacecraft within 3000 kilometers of the cloud tops, an area where the air pressure is 1 bar, comparable to the atmospheric pressure of Earth at sea level. According to this JPL news release, Cassini also came within 300 kilometers of the innermost visible edge of the rings.


This is a good time to monitor the Cassini raw image gallery. During these ring-plane dives, Cassini will gather data about Saturn’s gravity and magnetic fields, helping us gain a better understanding of the planet’s internal structure. We’ll begin to get our closest ever imagery of the rings and atmospheric clouds even as the spacecraft’s particle detectors sample ring particles being drawn into the atmosphere. And at the end, when Cassini makes its plunge into Saturn itself, we’ll gain measurements of the atmosphere until contact is lost.

We’ve had 13 years at Saturn, and if you wonder why we can’t just keep them going, the fact is that the spacecraft is running out of the fuel needed to adjust its course. Eventually, we’d lose the ability to keep Cassini away from interesting astrobiological targets like Enceladus and Titan, with the subsequent danger of contamination. The Grand Finale maneuvers seek to draw maximum information out of Cassini’s final days before a spectacular finish.