Tag Archives: Cassini

Threading the needle with Cassini at Saturn

The hugely successful Cassini mission to Saturn will come to a fiery end in September, and you can hardly blame NASA for going a little Star Trek on us.

Ron Hobbs

Ron Hobbs. Photo: Greg Scheiderer.

“We’re going somewhere where no spacecraft has ever gone before, into this region between the glorious rings of Saturn and the cloud tops of the planet,” said Ron Hobbs, a NASA Solar System Ambassador, at this month’s meeting of the Seattle Astronomical Society. After 22 orbits through the eye of that needle—a 2,500-kilometer-wide gap—they’ll splat Cassini into the planet and burn it up.

“Now that we’ve discovered that there’s at least one moon, and maybe several, that could have the conditions for life, it’s very important to not leave a derelict spacecraft orbiting around Saturn,” Hobbs noted. “One of the important things at the end of the solstice mission will be to dispose of the spacecraft.”

The second extended mission of Cassini was named solstice because it is almost the beginning of summer in Saturn’s northern hemisphere.

Let’s do science

Before they crash Cassini, they figured there was some time to do some great science in that place where no spacecraft has ever gone. Most importantly, they will get a better picture of the internal structure of Saturn and examine its ionosphere, inner radiation belts, and auroral region.

“This would have been worth sending a spacecraft to Saturn for just that measurement,” Hobbs said, noting that it is essentially what Juno is doing at Jupiter. They’ll also check out the particles of Saturn’s D ring at close range, and be able to better gauge the mass of the ring system, which will help pin down its age.

“I can’t wait for the pictures,” Hobbs added. “The pictures that come out of this mission are just going to be spectacular.”

Shooting the gap

Hobbs said NASA has been using interactions between Cassini and Saturn’s moon Titan to nudge the spacecraft’s orbit to where they want it to be.

“Titan is really the only object in Saturn orbit that has enough mass to allow it to do gravitational assists and re-direct its orbit,” he said. “That allows [Cassini] to change its orbit and change the plane of its orbit.”

Cassini orbits

This graphic shows the closest approaches, or periapses, of Cassini’s final two orbital phases.The ring-grazing orbits are shown in gray; grand finale orbits are shown in blue. The orange line shows the spacecraft’s final plunge into Saturn. Credit: NASA / Jet Propulsion Laboratory – Caltech

In late November a brush with Titan dropped Cassini’s perichron—the point closest to Saturn in its orbit around the planet—down to just outside the F ring. In April, another Titan flyby will drop that perichron down to between the D ring and Saturn’s cloud tops.

“That’s when it’s going to get really exciting,” Hobbs said. Cassini will do 22 “grand finale” orbits through the eye of this needle, each lasting six days, collecting science data until one final encounter with Titan puts the spacecraft on a trajectory to splat into the planet on September 15.

It’s amazing how much planning and politics went into all of this. Hobbs said the actual trajectories of the orbits for this grand finale were determined a little over three years ago. Ever since then there’s been a spirited discussion between scientists, engineers, and mission leaders about what science to do to get as much data as possible out of the final mission. That determination was just completed last month.

“The spacecraft drivers are now writing the code for these orbits,” Hobbs said. That will tell Cassini where to go and where to point its instruments to make the observations as planned.

A good ride

Hobbs noted that Cassini was launched in October 1997, and so will end its mission just shy of twenty years in space.

“Without a doubt it has been one of the most successful and audacious missions NASA and the international community have operated,” he said. “This is going to be one of the highlights of space exploration in the last couple of decades.”

Celebrating 10 years at Saturn

The Cassini spacecraft went into orbit around Saturn ten years ago, on July 1, 2004 in universal time. Ron Hobbs, a solar system ambassador of the NASA Jet Propulsion Lab, says some of the mission’s most exciting science has occurred quite recently.

10 years at SaturnHobbs spoke at the most recent meeting of the Seattle Astronomical Society about Cassini’s decade at Saturn, and notes that recent measurements of the gravitational field of the moon Enceladus have yielded some interesting findings.

“We are very confident that there is body of liquid water at the south pole that extends at least to 50 degrees south latitude on Enceladus,” Hobbs says. “There’s a body of water that’s in contact with rock. We know that some of the ice particles that get shot out into the E-ring have salt and organics in them. This has become on a very short list of places in our own solar system where we might find life.”

Mars and Jupiter’s moon Europa are two others on what Hobbs calls the “astrobiological short list.” Many scientists believe that life on Earth may have originated in hydrothermal ocean vents—a safe haven during the heavy bombardment era—and so it’s reasonable to suspect that life might thrive in similar environments elsewhere in the solar system.

Hobbs calls Cassini “the largest, most complex, and capable spacecraft ever built” and notes that we may owe its existence to persistent Europeans. There was some talk in the mid-’90s that Congress would scrap the mission before it got off the ground because of budget concerns. But the Europeans had already built the Huygens probe that hitched a ride on Cassini in order to do a study of the atmosphere of the moon Titan. Hobbs says word is that protests about the proposed cuts made it all the way to the vice president.

“The fact that we have Cassini, as far as I’m concerned, is in large part due to the fact that the Europeans had the guts to talk to the U.S. government and say, ‘You don’t renege on your promises,'” Hobbs says.

Like the Mars rover missions, Cassini has far exceeded the time allotted for its original scientific mission.

“The plan for Cassini when it arrived in July of 2004 was to study Saturn for four years,” Hobbs notes. “Cassini is still one of the healthiest spacecraft we have anywhere in the solar system. All of its instruments are working great, it’s got fuel.” Nonetheless, Hobbs says he occasionally hears talk that Congress again is considering pulling the plug on the mission. He says that would be a bad idea, as we still have a lot to learn.

The NASA video below gives a preview of the work they’re planning for Cassini over the next four years.

Enceladus and the platinum age of planetary exploration

Ours is a great time to be alive if you have an interest in learning about other worlds, at least according to Ron Hobbs, a NASA Solar System Ambassador and museum educator and public programs assistant at Seattle’s Museum of Flight.

“For most of us the golden age of planetary exploration was the ‘70s and early ‘80s,” Hobbs said. “The time of the Apollo Moon missions—particularly those that did a lot of science—the Viking missions to Mars, and of course Voyager.”


At least four distinct plumes of water ice spew out from the south polar region of Saturn's moon Enceladus in this dramatically illuminated image, shot on Christmas Day 2009 by Cassini. Light reflected off Saturn is illuminating the surface of the moon while the Sun, almost directly behind Enceladus, is backlighting the plumes. Photo: NASA/JPL/Space Science Institute.

After that, things slowed down a lot, but in recent years there’s been something of a renaissance in the solar system.

“The first decade of the 21st Century has been as good as, if not better than, the golden age,” Hobbs believes. “Some people have called it the platinum age of planetary exploration. And if there’s a flagship of that platinum age of planetary exploration it’s got to be the Cassini-Huygens spacecraft.”

Hobbs gave a talk about Cassini, titled “Seven Years in the Saturn System,” last month at the museum. The presentation featured lots of the spectacular photography from the mission, including the discovery of seasonal colors on the second-largest planet in the solar system. Hobbs also discussed discoveries about Saturn’s rings and the intense study of the huge moon Titan. But the most fascinating part of the talk centered on the moon Enceladus, which Hobbs says has joined the astrobiologists’ short list, along with Mars and Europa, for further study.

That’s largely because of the discovery of enormous geysers at the south pole of Enceladus, which spew out ice crystals that form Saturn’s E-ring and cover Enceladus with fresh snow, making it one of the brightest objects in the solar system. The interesting thing about the E-ring is that it contains salt.

“The salt had to come from somewhere,” Hobbs explained, “so somewhere down in Enceladus there must be water in association with hot rocks. So you’ve got an energy source, you’ve got water. We don’t know yet, but maybe we have organic chemicals. If we do, this becomes one of the likeliest places we could find life in the solar system.

“Enceladus has rapidly become one of the most important bodies for us to study,” he added.

Cassini’s work in the Saturn system is planned to continue until the summer of 2017, when it will make a handful of spectacular proximal orbits very close to Saturn’s cloud tops before it runs out of fuel and is crashed into the ringed planet.

That’s presuming Cassini keeps working. Hobbs notes that while the warranty has long since expired, the school bus-sized craft hasn’t missed a beat since it was launched in 1997.

“Space is a harsh environment, particularly when you go a billion miles from the Sun,” Hobbs noted. “It gets cold out there, and every time you go into the shadow of Saturn or one of the moons, it drops dramatically. So given our experience with technical things, it is kind of a surprise” that Cassini is still functioning, he said. “On the other hand, we in America seem to be building some really good stuff these days.”

It will be fascinating to see what wonders Cassini finds before the mission ends five years from September.