Thursday 28 September 2017

Farewell, Cassini.

The full listing of contents can be found here:

Thursday 14 September 2017

Cassini EOM

How are you feeling today?

This has been a bittersweet week, watching as my Cassini colleagues gather for the final time to watch the end of this 20-year journey.  This heroic spacecraft has done everything we’ve ever asked of it, even fighting with it’s last moments to deliver new scientific insights back to Earth.  So although I’m sad that Cassini’s exploration will now be a part of the history books,  I think we can be proud of everything that this mission has accomplished.  It just shows what wonders can be achieved when 27 nations work together.

Cassini CIRS team members at their last team meeting with a working spacecraft - June 2017

What will you be looking for in the last set of images to come down from the spacecraft - especially the final one of the spot where it will plunge into the cloud tops?

Those last final looks around the Saturn system will be heart-wrenching, as our last chance to witness new photos of these environments for a generation or more.  Combined with all our recent close encounters in 2017, the resolution of the last images of Saturn’s individual swirling clouds will be revealing the meteorological complexity underlying the usual serene appearance.  But Cassini will meet its fate high in the atmosphere, so the cloud-top meteorology might have little impact on what Cassini is able to measure in its final moments.

And CIRS is going to be on until the bitter end, right? What sorts of insights are you hoping to squeeze out of those final minutes of data?

Oh yes - along with some of the other instruments, CIRS will be fighting to deliver science until the very end of the journey.  These won’t be your typical data - CIRS is used to measuring spectra slowly and returning them to Earth hours later.  In real time, we’ll be getting housekeeping data (i.e., the instrument temperatures, voltages, etc.) and looking for any spikes in our measured interferograms, particularly as the CIRS field of view sweeps over the rings in the last 20 minutes or so.  We expect measurable heating about 25 minutes before loss of signal.  But, as we’ve never done this before, we’re waiting for (and expecting!) surprises!

At one of our team meetings, it was noted that CIRS’ scan platform has travelled 106 miles in tiny 2-cm chunks to assemble its interferograms - in all, more than 170 million interferograms have been acquired.  That legacy will keep scientists going for decades.

Friday 8 September 2017

Cassini's Final Moments

After almost twenty years in space, the Cassini spacecraft is now just seven days away from its final encounter with the giant planet, ending humankind's first detailed exploration of the ringed planet.  Cassini's Grand Finale is the ambitious culmination of a mission by a nuclear-powered robotic explorer that has travelled a total of 4.9 billion miles, completing 294 orbits of Saturn, with 360 engine burns, 2.5 million commands executed and 635GB of science data collected.  That science data, which includes half a million images, has been used to generate more than 4000 papers in scientific journals, and has started the careers of many young scientists, myself included.  But to me, the most important credential that this mission can be proud of is its international nature - 27 nations, including the UK, were involved in this fantastic mission.  Cassini-Huygens will truly be the benchmark against which all future missions are compared.

Cassini's vital statistics.

The Grand Finale started on April 22nd with the 126th close flyby of Titan, which initiated a series of 22 close polar orbits around the giant planet.  The first ring plane crossing, bringing Cassini between Saturn and its innermost rings, occurred on April 26th.  The spacecraft was travelling at roughly 70,000 mph relative to the cloud tops.  Thankfully, the dust that we suspected might be present in this unexplored region was absent, so that spacecraft constraints could be relaxed just a little.  As Cassini continued to loop around Saturn once every 6 days, the final five orbits actually dipped the spacecraft down into the tenuous upper atmosphere, allowing the mass spectrometer (INMS) to directly sample the composition of the atmosphere.  Here too, Cassini was lucky - although contingency plans had been in place should anything go wrong, no rocket firings to maintain attitude control were actually required, and the spacecraft emerged from these encounters unscathed.

The Final Week

And so we arrive at Cassini's final week before its plunge into the Saturnian clouds on September 15th.  On Saturday September 9th, at 01:09BST (subtract one from all my times to get them in UT),  Cassini will execute its final dive between Saturn and the rings, skimming just 1680 km above the clouds.  Then on Monday September 11th at 20:04BST, the final distant encounter with Titan (the 127th flyby at 120,000 km distance, known as the "goodbye kiss") will slow the spacecraft sufficiently that Isaac Newton and the force of gravity will never allow Cassini to escape its final orbit.

On Wednesday and Thursday September 13th-14th, Cassini will be assembling its final picture show, including colour mosaics of Saturn and its rings, a movie of Enceladus setting behind the northern limb of Saturn, and observations of tiny moonlets within the ring system.  Linda Spilker, Cassini's project scientist, described these bittersweet images beautifully as "like taking one last look around your home before you move out."  The final image will be taken at 20:58BST on Thursday September 14th, before the spacecraft is reconfigured for atmospheric entry on Thursday night.

The Final Moments

Instruments working during Cassini's final moments at Saturn.
On Friday September 15th, at 06:08BST, Cassini will cross the orbit of Enceladus one final time.  Two hours later, the spacecraft will roll to ensure that the mass spectrometer is pointed in the direction of travel, able to access the oncoming gases during the plunge.  Rather than storing science data onboard, Cassini will begin transmitting in real time (3.4 kbps) for the last 3.5 hours, with its high-gain antenna pointed directly at Earth.  Eight of Cassini's 12 instruments will be on and taking data - the mass spectrometer (INMS), magnetosphere and plasma science instruments, the radio science subsystem, and the infrared and ultraviolet spectrometers (CIRS included).

At 11:31BST the fight begins, as the spacecraft begins to enter the atmosphere at about 1190 miles (1920 km) above the cloud tops and needs to use its thruster to battle against the torques on the spacecraft.  The thrusters will ramp up to 100% of their capacity in a matter of seconds as the probe falls through 250 miles (to around 940 miles above the clouds), where the torques are expected to overcome Cassini's ability to correct its attitude.  At this point, we expect Cassini to begin to tumble around several axes, such that the high gain antenna is no longer locked on Earth.  The final photons will have been transmitted back to the Deep Space Network of radio telescopes on Earth at 11:32BST.  Cassini will continue to fight, its fault-protection systems trying in vain to stabilise the spacecraft, but within seconds the high loads on the spacecraft will start to destroy structural components.  The spacecraft will break apart, burning up like a meteor and melting, the individual materials dissociating so that the debris forever becomes a part of Saturn.

Meanwhile, those last photons from a ghost spacecraft will take 83 minutes to cross the 1.5 billion kilometres to Earth, where the final loss of signal is expected at around 12:55BST.   At that moment, I expect the silence at JPL, and all the other laboratories that have been part of this grand mission, to be deafening.


At a Cassini CIRS team meeting in June, I learned that the instrument operations team had uplinked a very special sequence of four tables to the spacecraft, to be uploaded to the instrument memory during that final plunge.  The four tables contained a message from all of the 282 scientists and engineers that had been involved in CIRS between 1990 and 2017.  It's an incredible privilege to be on this list, and to know that Cassini will be thinking of home as it completes its final journey!

The names in Cassini/CIRS memory in the final seconds.

Finally, Ralph Lorenz has a nice article on arxiv about whether we'll be able to see this event from Earth

My Cassini Experience

In June 2017 I was interviewed about my feelings on the demise of the Cassini spacecraft - here are some personal reflections on my time with Cassini.  The quotes were picked up in JoAnna Wendel's excellent piece on EOS.

How did you get into planetary science?

I’d been interested in space exploration since childhood, watching with awe as the space shuttle completed its missions and Hubble delivered stunning views of the universe.  But when I went off to university in 2000, solar system studies weren’t really on my radar:  I was set on studying physics and maths, and all my physicist friends were getting excited about cosmology and particle physics.  But the more I looked into those fields, I realised I wanted to study something more tangible - something closer to home, something we could see with our own eyes, something that we might one day be able to reach out and touch.  After a course in geophysics at Cambridge, I started to look into planetary science opportunities at PhD in 2003.

How did you get involved in Cassini research? When was that?

It turned out to be the perfect moment to start looking at PhD opportunities in planetary science in the UK.  Cassini was just months away from Saturn Orbit Insertion, and the UK teams involved in the mission were advertising a number of opportunities.  I was interviewed for positions to study magnetic fields and plasma science, but I ultimately settled on a PhD in planetary atmospheres at the University of Oxford.  Oxford had provided hardware (focal plane assemblies and a cooler) to the Composite Infrared Spectrometer (CIRS) instrument, led by Goddard Spaceflight Center.  CIRS measures thermal radiation from planetary atmospheres, rings and satellite surfaces, and the Oxford team were looking for PhD candidates to analyse Saturn and Titan observations from those preliminary Cassini orbits.  I was offered the place in 2004, completed my degree in the summer and waited with baited breath to see if Cassini would survive that first ring-plane crossing.  I then arrived in Oxford in October 2004, ready to roll up my sleeves and start analysing CIRS data - that formed the basis of my doctoral thesis in 2007, Saturn’s Atmosphere:  Structure and Composition from Cassini/CIRS.  It’s safe to say that I wouldn’t be where I am today if not for that opportunity to work with the Cassini team from the start of the Saturn mission.

Are you still working with Cassini data, and if so, what mysteries still exist about Saturn, its moons, etc? 

Absolutely!  CIRS has now been observing Saturn for 13 years, almost spanning from solstice to solstice, providing us with the best chance of characterising a seasonal giant planet.  I’m still tracking the evolution of temperature, cloud and compositional changes arising from both slow seasonal variations and short-term outbreaks of storm activity on the gas giant.  Cassini has revealed Saturn’s atmosphere to be deeply interconnected, with activity in vastly separated regions having substantial consequences elsewhere - for example, the deep roiling tropospheric storm of 2011 had substantial side-effects in the stratosphere and possibly even the ionosphere.  We’re still trying to understand what connects different regions of Saturn’s atmosphere, and what deep processes, hidden well below the clouds, are responsible for the timescales and violence of the massive outbreaks that we see.

Will you be able to continue using Cassini data after the mission ends?

We’ll be trying to provide a complete 13-year temperature, composition and aerosol dataset that spans the entire mission, as a resource for future researchers studying atmospheric processes on giant planets.  So I’m sure I’ll be delving into the CIRS dataset for many years to come.

How has Cassini changed the way that scientists understand your particular research/field?

I think I answered this one above, when I spoke about Saturn’s atmosphere being deeply interconnected.  But beyond that, I think we’ve started to show that ideas inherited from the study of terrestrial meteorology and climatology (jet streams, Hadley circulations, moist convective storms and lightning, polar vortices, equatorial oscillations) can be applied to gas giant atmospheres, despite the vastly different environmental conditions.  It’s showing that a number of atmospheric processes are commonplace across vastly different worlds.

What kinds of feelings do you have now that Cassini is ending?

Pride in what we’ve accomplished; gratitude that I was offered a chance to become involved; and sadness that a team I’ve worked with for 13 years will now be moving on to pastures new.  Whenever I watch the CGI movie of Cassini’s final demise, it’s hard not to feel moved.  If you forget all the exciting science, Cassini is an incredible testament to spacecraft engineering and operations, having worked so well for so long.  Cassini is the best example of US-European collaboration that i know of, and it’s hard to imagine that we’ll ever have another spacecraft like it.  That said, just think what we might accomplish with Cassini-style exploration of the next two great outposts in our solar system:  Uranus and Neptune?  I hope that I’ll be able to offer new PhD candidates the opportunity to study data from robotic explorers of the ice giants, some day!