In "grand finale," NASA’s Cassini Saturn probe flies between the planet and its rings

Using its large dish antenna as a shield, NASA’s Cassini spacecraft, virtually out of gas after 13 years orbiting Saturn, streaked between the gas giant’s cloud tops and its innermost rings early Wednesday in the first of 22 somewhat risky “Grand Finale” orbits to collect a final set of priceless data before crashing into the atmosphere in September.

“This spectacular ending, the Grand Finale, going out in a blaze of glory, is a phenomenal conclusion to a chapter in humankind’s exploration of the planets,” said Earl Maize, the Cassini project manager at NASA’s Jet Propulsion Laboratory. “Just a chapter, the book is not complete, there’s more to come. But this has been a marvelous ride.”

The $3.4 billion mission’s final chapter began April 22 when Cassini made its 127th and final close flyby of Titan at a distance of 608 miles, using the big moon’s gravity to increase the probe’s velocity by about 1,925 miles per hour relative to Saturn.

The flyby changed Cassini’s trajectory, moving the low point of the spacecraft’s elliptical orbit from just outside to just inside the planet’s vast ring system.

The nuclear-powered spacecraft reached the high point of its new orbit at 11:46 p.m. Saturday before racing back down toward the new low point, just 1,800 miles above Saturn’s cloud tops, at 5 a.m. Wednesday. It crossed the plane of the rings at some 76,000 miles per hour, fast enough to fly from Los Angeles to New York in less than two minutes.

Scientists believe few large particles are present where Cassini flew through, but given the spacecraft’s enormous velocity close in to Saturn — 370 football fields per second — even tiny bits of ring debris posed a threat.

Playing it safe, the spacecraft made the crossing with its large dish antenna facing in the direction of travel, shielding the spacecraft’s critical systems from any potentially dangerous impacts.

As a result, Cassini was out of contact with Earth during the ring plane passage. Scientists will have to wait until at least 3:05 a.m. Thursday, after the spacecraft completes a series of science observations and aims itself back at Earth, to receive telemetry and science data confirming a safe passage.

“We expect a lot of impacts,” Maize told reporters earlier. “Very lightweight impacts, more like going through smoke than particles. Every now and then we’ll see a slightly higher impact, but still very small material.

“We believe the D ring, at the point where we’re going through, is diffuse enough that we should be fine. Nevertheless, you do that 22 times and we have a probability of losing the spacecraft of slightly over 1 percent.”

While that seems more than safe enough, NASA would not normally put a multi-billion-dollar “flagship” mission like Cassini on such a course through such an unexplored environment.

But Cassini is down to its final few pounds of propellant and once it runs out, the spacecraft will no longer be controllable. To make sure the probe does not someday crash into a moon like Enceladus, where a subsurface ocean may provide an abode for life, NASA managers decided to put Cassini on a trajectory that will end with a kamikaze plunge into Saturn’s atmosphere on Sept. 15.

The spacecraft will be vaporized, eliminating any possibility of contamination from any earthly microbes that might have hitched a ride to Saturn.

“Cassini’s own discoveries were its demise,” Maize said. “Enceladus has got a warm, saltwater (subsurface) ocean. We cannot risk an inadvertent contact with that pristine body. Cassini has got to be put safely away, and since we wanted to stay at Saturn the only choice was to destroy it in some controlled fashion. And that’s when the Grand Finale came in.”

Launched in October 1997, Cassini arrived at Saturn in July 2004 and dropped off a lander built by the European Space Agency that successfully completed a parachute descent to the surface of Titan the following January.

Since then, Cassini has flown through a complex set of ever-changing orbits, repeatedly using Titan for gravitational boosts to alter its course, enabling multiple flybys of Saturn, its huge ring system and many of its moons, earning two mission extensions in the process.

The spacecraft has operated in near flawless fashion, chalking up one of NASA’s most scientifically productive missions.

But the Titan flyby on April 22 marked the point of no return. The gravitational encounter put Cassini on a course that will lead to impact in Saturn’s atmosphere in September no matter what might happen during the 22 ring-plane crossings.

“If we really get surprised, if indeed we’ve modeled so badly there are BB-size materials out there, Cassini will still finish up exactly where we planned, but we may end up with a little bit less science than we had hoped for,” Maize said. “But we’ve got every confidence that our models are correct.”

And if Cassini survives to the end, the science payoff will more than justify the increased risk.

“The Grand Finale mission is absolutely incredible,” said project scientist Linda Spilker. “Flying in a region that no spacecraft has flown before and getting this close to the rings and the planet, that’s a once-in-a-lifetime experience for a scientist like me. We’ve wanted to do this for a long time, and we had to wait for just the right time and just the right mission design to actually fly in this particular region. I can’t wait for those new discoveries.”

One of the primary objectives of the Grand Finale is to measure the mass of the rings by studying how Cassini’s trajectory is affected by Saturn’s gravity when the spacecraft was flying just outside the rings and then again when it was inside. Scientists should be able to isolate Saturn’s contribution and thus compute how much mass must be present in the rings to account for the difference.

“If the rings are a lot more massive than we expect, perhaps they’re old, as old as Saturn itself, and they’ve been massive enough to survive the micrometeoroid bombardment and erosion and leave us with the rings we see today,” Spilker said.

“Now, on the other hand, if the rings are less massive, perhaps they’re very young, maybe forming as little as 100 million years ago. Maybe a comet or a moon got too close, got torn apart by Saturn’s gravity and we have the rings that we see today.”

The rings are made up of 99 percent water ice, but scientists are not sure about the remainder.

“What is it made of? Could it be tiny grains of iron, silicates, organics, a mix of all three, something else we haven’t even thought of?” Spilker asked. “When our cosmic dust analyzer goes through the ring plane, we’ll scoop up ring particles and directly taste and measure the composition of those particles.”

Cassini also will get an unprecedented close-up look at Saturn’s auroral displays and its turbulent atmosphere, including the strange six-sided formation at the north pole that looks like a huge hexagon.

“As we’re skimming close to the planet, we’ll have the best views ever of the poles,” Spilker said. “We’ll see the giant hurricanes at the north and south poles, we’ll also see the giant hexagon, six-sided jet stream that’s two Earth diameters across. What keeps those six sides in place? It’s been there for decades. Perhaps getting close with Cassini we can answer the question, what keeps the hexagon there in this particular shape.”

Scientists expect to bring down the best pictures yet of the atmosphere. “Then, of course, in the last five orbits, we’re actually going to dip our toe in the atmosphere of Saturn, with our ion and neutral mass spectrometer, measure the composition of the atmosphere. It’s mostly hydrogen and helium, but what else is there? And in what abundance?”

By precisely measuring Saturn’s magnetic field and how Saturn’s gravity affects Cassini’s trajectory, scientists hope to indirectly peer beneath the clouds to “measure the size of the rocky core on Saturn,” Spilker said. “Is it one Earth diameter in size? Two Earths? Smaller? We’ll find out for the first time, we’ll actually be peeling back the atmosphere and looking inside the planet.”

But it will all come to an end at 8:07 a.m. on Sept. 15.

“Cassini will enter Saturn’s atmosphere, valiantly fighting against the ever-increasing torques from the atmosphere, pointing its high-gain antenna to Earth to send down every last precious bit (of data),” Maize said. “We will be sampling the atmosphere of Saturn, every bit will be transferred back as quickly as we can.

“Inevitably, Cassini is going to lose the battle. It’s got small thrusters, it wasn’t built for the atmosphere and eventually it will lose point and control. Shortly after that, it will fall apart, break apart, it will melt, it will vaporize and will become part of the very planet it left Earth 20 years ago to explore.

“This spectacular ending, the Grand Finale, going out in a blaze of glory, is a phenomenal conclusion to a chapter in humankind’s exploration of the planets,” he said. “Just a chapter, the book is not complete. There’s more to come. But this has been a marvelous ride.”

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• NASA