The wild howls of Saturn, in a strangely empty space

Cassini has just now completed its second close pass; the data isn’t back yet, but in the meantime, mission controllers have released a pleasant surprise from the first pass — although the big High Gain Antenna was used as shield during the pass, the plasma wave instrument (which peeks out from behind the antenna’s big reflector dish) detected almost no particle hits at all, and what it did encounter was no bigger than smoke particles (<1 micron).  This is happy, because it means Cassini will not need to use the dish to shield anymore, except on a couple of passes that will penetrate some ways into the D ring.

But it’s also a puzzle, which is always a fun and exciting thing to encounter in science, because this space was not expected to be so empty.  The corresponding space on the outside of the rings is definitely not so empty, and you can hear the difference in these two audio clips.  The clips were made by converting the information from the plasma wave instrument into audio.

Here’s from a ringplane crossing outside of the rings.   Each crackle and pop is a particle hit, and at the time of the ring crossing itself, there’s a very clear spike:

Now, for contrast, the inside of the rings, where the lack of pops and crackles is made all the more obvious by the fact that the impacts are no longer drowning out the whistles and howls that Saturn’s magnetosphere makes normally, allowing them to crank the gain way up but still without hearing a lot in the way of impacts.  This one sounds a lot wilder, since here you can listen to Saturn itself:

The third periapsis will be in under a week.  Things are moving fast now!

Cassini has survived the first dive!

Cassini’s first dive inside the rings has been completed, and the spacecraft has regained contact with Earth, right on schedule.  As I type this, Cassini is busy downlinking data from the close pass via the DSS 43 dish, the largest one at the Deep Space Network’s Canberra, Australia location.  (If you’re ever curious who’s talking to who in deep space, visit NASA’s DSN Now page.)  The reason they were out of contact during the pass was that Cassini was oriented so that the high gain antenna faced into the direction of travel, using the massive dish as a shield.

There’s still considerable data still to be downloaded, but JPL has posted the first images, which are in the vicinity of Saturn’s north pole.  The north pole is home to Saturn’s strange hexagon feature, now seen in greater detail than ever before.  These raw, unprocessed images are just a taste of what will be available when the whole data set is down.

Cassini’s Grand Finale Has Begun

In the wee hours of the morning today, Cassini made its 127th and final close flyby of Saturn’s giant moon Titan, the second largest satellite in the solar system.  There were many targeted observations planned on this final encounter, including radar studies of the “magic island”, a landform that has appeared and disappeared in Ligeia Mare, one of Titan shallow methane seas.  It’s presumed this disappearing trick is the work of changing levels in the sea, but more data is needed.

But there’s more to this flyby than just some great science.  Cassini has relied heavily on Titan’s mass to adjust its trajectory for very little fuel expenditure, and today was no exception.  Today, Cassini used Titan’s gravity to lower its orbit significantly, bringing the periapsis (the low point) within the rings, and shortening the entire orbit to just about a week.  Cassini will make twenty two dives into the area within Saturn’s rings, low enough to begin to directly sample some of Saturn’s tenuous upper atmosphere, gradually sinking on each closest approach until finally, next September, its predicted to impact the giant planet’s cloud decks and burn up.

It’s bittersweet, to be sure.  Cassini has functioned like a champ, long past its original design life.  But all things must come to and end, and Cassini will go out with a bang.

So as we prepare to say farewell in a few months, here’s a parting shot from Cassini: the last photograph it will be able to take of the Earth and Moon.  This was taken on April 12, taking advantage of a viewing geometry that will not occur again on the mission, where Earth peeked through the gap between the A ring and the F ring:

Space updates: Soyuz MS-02 returns, John Glenn to fly again, Chinasat 16, and Cassini’s next step

I’ve been way busy the last few days, so I regret I have not posted as often as I’d like.  But I’ll start making up for that.  😉  First off, the landing of Soyuz MS-02 from the ISS!  The imagery is great; you even get to see the capsule venting hydrogen peroxide as it descends under parachute (at which point the thrusters are no longer useful, so they dump the propellant to make it safer on the ground).  This completes the Expedition 50 mission.  On board were Sergey Ryzhikov, Andrey Borisenko and Shane Kimbrough.  Two crew will launch on the next Soyuz, due to funding constraints at Roscosmos which has forced them to make the difficult decision to reduce their crew size.  On a positive note, the commander of Expedition 51, who took command upon this crew’s departure, is Peggy Whitson, and NASA has just decided to extend her mission by three months.  She currently holds the female spaceflight endurance record, and by the end of her extended mission, will also capture the American spaceflight endurance record.

Meanwhile, in Florida, crews are preparing the next Cygnus vehicle, named for astronaut John Glenn, to be launched aboard an Atlas V to the ISS.  This trip will carry experiments to create new targeted chemo drugs in microgravity for Oncolinx (an experiment which will consume a lot of crew time; it’s stuff that cannot be done anywhere else), a crystal growth experiment that goes beyond the basic science of previous experiments and aims to build new radiation detectors, a mini greenhouse (the most sophisticated sent to space to date) with wheat and Arabidopsis seeds, 34 Cubesats in the pressurized compartment (to be deployed later from Kibo), and 4 Cubesats to be deployed by Cygnus itself after departing the station.  Finally, there are two experiments to be carried after Cygnus has completed its primary mission — the third SAFIRE test to better understand fire in microgravity, and three small reentry bodies that will be ejected prior to Cygnus’ reentry, a process which they are expected to survive.  They will splash down in the ocean and sink, however, so they aren’t expected to be recovered.  Instead, they will be continuously transmitting temperature data via the Iridium constellation, allowing testing of new heat shield materials under real-world circumstances.  Note: launch was delayed from March to April 18 due to a launch vehicle technical issue which has been resolved.

And although Falcon 9 has taken a lot of business away from Chinese launch vehicles, they still have a solid lock on their burgeoning government program.  A Long March 3B blasted off from Xichang with the Shijan 13 (Chinasat 16) communications satellite on board.  This is the highest-bandwidth spacecraft that China has launched, and in addition to acting as a technology demonstrator for several projects (including ion propulsion and laser communications), it will provide high-bandwidth Internet service to airline, ship, and train passengers in and near China.

And lastly, on a bittersweet note, yesterday JPL uploaded the instructions for Cassini’s next Titan flyby.  In six days the Cassini spacecraft is moving towards a major milestone — the last flyby of Titan.  This flyby will be used as a gravity assist to move the spacecraft from its current ring-grazing phase to the final phase of the mission, called the Grand Finale.  It will fly closer to Saturn that anything ever has before, completing several orbits before impacting Saturn in September.  But it will return astonishing data that could not be captured any other way, including passes through the tenuous outer atmosphere of Saturn and through the D ring itself.

Saturn’s Aviators Up Close

Saturn has aviators?  What?

Well, not really, no, but it does have propellers.  These strange structures were first spotted in Cassini imagery in 2006, but not conclusively identified until 2010, when the lighting conditions were right again.  They are now mapped well enough to confirm that they are not transient structures but persist over long timescales, long enough to justify giving them names.  Appropriately enough, they’re named for famous aviators from the early, propeller-driven days of aviation: Bleriot, Earhart, and Santos-Dumont are three that have been closely monitored.

This monitoring has allowed planetary scientists to confirm that they clumped ring particles being pulled along by moonlets embedded in Saturn’s ring (mainly in the A ring, in a region now known as “the propeller belt” because there are so many of them there).  With Cassini now moving closer to the rings than ever before, the propellers are being closely watched, as they represent a natural lab of planetary formation.

Here’s Earhart, photographed on March 22:

That black area to the right is the Encke Gap, which is shepherded by the moon Pan.  I blogged about Pan not too long ago, as Cassini recently revealed that it is shaped like a ravioli.  It’s a tiny little world, but it dwarfs the tiny moonlet in the middle of Earhart.  Pan is big enough to sweep the gap open, although it tugs and pulls as it goes, creating waves through the rings.  Earhart’s moonlet is much too small and the A-ring too dense; the moonlet displaces material, but it quickly slumps back in after the moon’s small gravity well passes.

Here’s Bleriot, which looks a bit different, although scientists aren’t sure whether that’s a real difference or just a difference in viewing conditions.

And here’s Santos-Dumas, in two images taken Feb 21.  This took advantage of Cassini’s motion and grabbed shots before and after periapsis, so the shot on top is from the sunlight side of the rings, and the bottom shot is from the backlit side:

The propellers are fascinating structures, and hopefully we’ll get more looks at them over the next few months, before Cassini’s mission ends.  If not, well, they’ll definitely be a hot target for any future Saturn missions, to see how they’ve evolved over time.

Pan, the Space Ravioli

Seriously, that’s what Saturn’s moon Pan looks 

Cassini has just made the closest ever pass to Pan, one of Saturn’s shepherd moons.  It orbits within (and basically creates) the Encke Gap in Saturn’s rings.  The wide flat band of material around its equator is material accreted from the rings themselves.  The best part, though, is that the Cassini imaging team at JPL put together this neat animated GIF collecting all of the raw images from this particular imaging sequence, and its glorious:

 

It’s bittersweet, knowing that these close orbits are dooming Cassini to eventually fall into the giant planet, but we’d never get these incredible images otherwise, images of the greatest natural lab we’ve ever found for studying the way a planetary system can form.

Daphnis, the ring-sculptor

Strictly speaking, Daphnis is one of many ring-sculptors — the shepherd moons that maintain some of the notable visual features of the rings of Saturn.  In the case of Daphnis, that’s the Keeler Gap, a narrow gap carved out the A ring by the gentle gravitational tug of war between Saturn and the tiny moon.

Daphnis was actually discovered by the Cassini probe in 2005, but its existence was suspected long before, when the Keeler Gap was itself discovered in images taken by the twin Voyager probes.  Voyager had already discovered the moons Prometheus and Pandora, which the Voyager team dubbed “shepherd moons” for the way their push and tug confined a group of particles to the very narrow F-ring.  So it was surmised that the other gaps would turn out to have moons creating them as well. Mimas seems to be responsible, at least in part, for both the Cassini Division and the Huygens Gap, and of course Prometheus and Pandora constrain the F ring.  Pan, discovered in 1990 from old Voyager data, is likely responsible for the Encke Gap.  And Daphnis is the sculptor of the Keeler Gap.

Or, at least, the main sculptor.  Saturn’s rings are very complex, and serve as a fascinating natural laboratory for studying gravitational interactions, and particularly the sort hypothesized to have created the solar system as we know it.  And Daphnis, like other shepherd moons, does not orbit perfectly neatly.  Its orbit is slightly inclined relative to the ringplane, and slightly elliptical as well.  Thus, it doesn’t produce a nice tidy circle, but carves out waves as it passes — waves both ahead and behind, and, as this recent Cassini image shows, sometimes it pulls off delicate tendrils of ring particles (look verrrrry closely, or just click to enlarge – you’ll see a thin wisp of material echoing the shape of the nearby wave in the ring):

That’s the closest image ever taken of Daphnis, a tiny moon roughly the same size as Mount Everest.  It appears to have striations running down its length, probably the result of accumulated ring particles — sometimes, even a tiny moon like this will manage to capture something and pull it down.  But if you want a more dramatic image of this effect, you will have to look at Daphnis near Saturn’s equinox, when the shadows are at their longest.  Then you can see what is hidden in this image: the waves aren’t just flat features.  They stand surprisingly tall.

By studying this process, scientists hope to better understand planetary formation.  Indeed, they’ve even found a few spots in Saturn’s rings where it appears that moonlets may be in the process of forming, clumping together at random until eventually one clump reaches a critical mass and begins to dominate the particles around it, gradually growing until it exhausts its immediate surroundings, carving out another gap.  Daphis itself shows signs that it may be accumulating material.  Saturn’s rings are an astonishingly and fascinatingly dynamic place.