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.


Afterword

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 https://arxiv.org/abs/1708.05036

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!

Wednesday, 23 August 2017

Saturn from Cassini: Image Gallery

One of the most common requests I'm getting at the moment is to provide some of my favourite Saturn imagery from Cassini, so I've assembled my top selection over on Pinterest.  These come from various sources (APOD, NASA's Photojournal), and by clicking on the links you'll be able to go to higher-resolution versions of the images.  Here's a screenshot, but it's best to pop over and take a look!

https://www.pinterest.co.uk/leighfletcher/cassini-saturn-gallery/


Monday, 10 July 2017

Ten Facts about the Great Red Spot

In honour of Juno's close encounter with Jupiter's Great Red Spot (GRS) on July 11th 2017, here are some quick facts about the Solar System's most famous storm system:

Comparing Hubble and VLT thermal observations of the
Great Red Spot in 2006.

  1. The Great Red Spot is a very long-lived spinning vortex: hand drawings show the GRS back in the Victorian era, but it might have persisted for even longer than that - even Robert Hooke's and Giovanni Cassini's 17th century observations suggest a feature at this particular latitude, so it might have been around for almost four centuries.
  2. The Great Red Spot appears to roll like a ball-bearing between two of Jupiter's colourful cloud bands:  the brown South Equatorial Belt and the white South Tropical Zone.  The westward jet that separates these two bands is deflected northwards around the vortex making the storm rotate anticlockwise, meeting the eastward equatorial jet and causing lots of turbulent, chaotic structures to the northwest of the storm.
  3. If you were on a balloon at the edge of the swirling storm, you'd be blown around the vortex in about 3.5 days.  Of course, you'd need to find a way to be lighter than hydrogen and helium to float, but the anticlockwise winds would mean that you'd be blown around the swirling maelstrom - what a view that would be!
  4. The Great Red Spot has been shrinking:  We've known this for many years, as the size measured by Earth-based and space-based observers (including the Hubble Space Telescope) has been tracked over time.  Voyager measured a width of 25000 km in 1979, but that's now decreased to as small as 15000 km.  But the shrinking is not continuous - it went through a period of rapid shrinkage in 2012-2014, but has now appeared to stabilise at the new smaller size.  Who knows whether we'll ever witness the death of the spot?
  5. The Great Red Spot consumes smaller storms:  smaller storm plumes, vortices and storm clusters moving along Jupiter's jet streams can be seen being engulfed by the storm.  Maybe this provides the extra energy and angular momentum needed to sustain this swirling anticyclone?
  6. The Great Red Spot is cold:  At the cloud-tops the Great Red Spot is cold because air is rising within the vortex, expanding and cooling down.  This cooling causes gases to condense, creating the thick cloud cover high over the GRS.
  7. The Great Red Spot has a warm cyclonic heart:  Thermal infrared images show a warm core to the cold vortex, and this warm core coincides with the deepest red cloud colours, and possibly a stagnation of the winds.  Is this the core of the vortex?
  8. The Great Red Spot is not quite the same as a hurricane:  There's no ocean beneath to sustain the energy of the GRS, and no one truly knows how far down the GRS extends into the deep atmosphere (is it deep or shallow)?  That's one of the things that Juno could show us.
  9. We still don't know why it's red:  There's no distinct spectral signature for the aerosols causing the red colouration, but we think it is related to sunlight breaking down the chemical bonds of materials dredged up by the upwelling in the storm.  These chemically-altered species might contain sulphur and phosphorus, which could lead to the red colours.  
  10. Juno will be closer to the Great Red Spot than ever before on July 11th 2017:  Juno is due to fly about 9000 km above the centre of the Great Red Spot (GRS) on Monday night, about 12 minutes after its closest approach to the planet on July 11 at 01:55 UT. 

Friday, 30 June 2017

Earth-based observations prepare Juno for the Great Red Spot Encounter

In just a few days time, on July 11th 2017, NASA's Juno spacecraft will perform the closest-ever views of the swirling maelstrom known as Jupiter's Great Red Spot.  It was always hoped that the pre-planned polar orbit and close perijove passes would take the spacecraft over the storm, but the slow and somewhat unpredictable westward motion of the gigantic vortex meant that a little luck would be required.  That luck comes in on Perijove 7, and we'll be rewarded by breathtaking views - so close that the vortex will stretch from jovian horizon to jovian horizon.

In preparation for that encounter, myself and others have been collaborating on an Earth-based support campaign, capturing multi-wavelength views of the jovian atmosphere to provide spatial, temporal and spectral context for Juno's close-in encounters.  Today, two of the telescopes we've been using released some of our imagery from May 19th 2017, acquired during Juno's last perijove 6.  These images show the Great Red Spot as it was just a few weeks ago, and prepare us for Juno's close-in views.

Thermal Emission from Jupiter

Both observatories are located on the peak of Mauna Kea on the Big Island of Hawaii.  We have been working with the Subaru Telescope (the National Observatory of Japan) and the Gemini-North telescope.  The COMICS instrument on Subaru provides mid-infrared (7-25 µm) observations that reveal the temperature structure, gaseous composition and cloud opacity within the Great Red Spot.  The Subaru telescope has released an image at 8.8 µm, which primarily senses temperatures and aerosols, showing the planet's white zones as cold and cloudy (i.e., dark) and the brown belts as warm and cloud-free (bright).  The Great Red Spot can also be seen as cold and cloudy, something that we studied at length (using both Subaru data and those from ESO's Very Large Telescope VISIR instrument) in an Icarus paper in 2010.

Jupiter's thermal emission at 8.8 µm obtained by the Subaru/COMICS instrument on May 18th 2017.  A video created from a series of observations with the same settings on January 14th 2017 is also available. Credit:  NAOJ and JPL.

Glenn Orton was the PI of Keck exchange time to use the Subaru facilities, and said this in the press release:  "During our May 2017 observations that provided real-time support for Juno's sixth perijove, we obtained images and spectra of the Great Red Spot and its surroundings.  Our observations showed that the Great Red Spot had a cold and cloudy interior increasing towards it centre, with a periphery that was warmer and clearer.  This implied that winds were upwelling more vigorously towards its centre and subsiding at the periphery.  A region to its northwest was unusually turbulent and chaotic.... this region is where air is heading east towards the GRS and flows around it to the north, where it encounters a stream of air flowing over it from the east."  Crucially, observations of this kind, from the VLT and from Subaru, are capable of resolving 1000-km length scales on Jupiter that are comparable to Juno's microwave experiment, which will sound the deep atmospheric processes underlying those that we can see in this thermal image.

Jupiter in Reflected Sunlight

At shorter wavelengths, the NIRI instrument on the Gemini-North observatory captured the reflected sunlight from the Great Red Spot and its surroundings, as explained in their press release.  These observations required Adaptive Optics, using observations of a nearby satellite to observe and correct for the distortions caused by our own atmosphere.  Orton explains: “Back in May, Gemini zoomed in on intriguing features in and around Jupiter’s Great Red Spot: including a swirling structure on the inside of the spot, a curious hook-like cloud feature on its western side and a lengthy, and a fine-structured wave extending off from its eastern side.”  By observing Jupiter in a variety of different near-infrared wavelengths, which sense differing amounts of methane absorption, we're able to reconstruct the three-dimensional cloud structure within Jupiter's upper troposphere.

A composite colour infrared image of Jupiter reveals haze particles over a range of altitudes, as seen in reflected sunlight. The image was taken using the Gemini North telescope with the Near-InfraRed Imager (NIRI) on May 18, 2017, one day before the Juno mission’s sixth close passage (“perijove”) of the planet.  Credit: Gemini Observatory/AURA/NSF/JPL-Caltech/NASA


Version of the image above labelled by Dr. John Rogers of the British Astronomical Association.

From the Gemini press release:  The colour filters cover wavelengths between 1.69 to 2.275 microns and are sensitive to pressures of 10 millibars to 2 bars. The Great Red Spot (GRS) appears as the brightest (white) region at these wavelengths, which are primarily sensitive to high-altitude clouds and hazes near and above the top of Jupiter’s convective region – revealing that the GRS is one of the highest-altitude features in Jupiter’s atmosphere. The features that appear yellow/orange at Jupiter’s poles arise from the reflection of sunlight from high-altitude hazes that are the products of auroral-related chemistry in the planet’s upper stratosphere.

Narrow spiral streaks that appear to lead into it or out of it from surrounding regions probably represent atmospheric features being stretched by the intense winds within the GRS, such as the hook-like structure on its western edge (left side). Some are being swept off its eastern edge (right side) and into an extensive wave-like flow pattern; and there is even a trace of flow from its north. Other features near the GRS include the dark block and dark oval to the south and the north of the eastern flow pattern, respectively, indicating a lower density of cloud and haze particles in those locations. Both are long-lived cyclonic circulations, rotating clockwise - in the opposite direction as the counterclockwise rotation of the GRS. A prominent wave pattern is evident north of the equator, along with two bright ovals; these are anticyclones that appeared in January. Both the wave pattern and the ovals may be associated with an impressive upsurge in stormy activity that has been observed in these latitudes this year. Another bright anticyclonic oval is seen further north. Juno may pass over these ovals during its July 11 closest approach. High hazes are evident over both polar regions with much spatial structure that has never been seen quite so clearly in ground-based images, with substantial variability in their spatial structure. The central wavelengths and colors assigned to the filters are:1.69 microns (blue), 2.045 microns (cyan), 2.169 microns (green), 2.124 microns (yellow), and 2.275 microns (red).

Jupiter's Deep Glow

Supplementing the two investigations above, a ground-based programme is also under way to observe the deep thermal emission of Jupiter near 5 µm.  We released images from ESO's Very Large Telescope last year, and this programme has continued for each of Juno's perijoves. A parallel Gemini programme headed by Michael Wong of the University of California, Berkeley, used an approach commonly called “lucky imaging” to obtain sharp images of Jupiter at 5 µm. Images obtained with this filter are mainly sensitive to cloud opacity (blocks light) in the pressure range of 0.5 to 3 bar. “These observations trace vertical flows that cannot be measured any other way, illuminating the weather, climate and general circulation in Jupiter’s atmosphere,” notes Wong.

Jupiter glows with thermal (heat) emission at 5 µm, thick clouds block the emission from the deeper atmosphere. The Great Red Spot is visible just below centre. This image, obtained with the Gemini North telescope’s Near-InfraRed Imager (NIRI), was obtained on January 11, 2017, so the relative positions of discrete features have changed with respect to the near-infrared image above.  Credit: Gemini Observatory/AURA/NSF/UC Berkeley 

For more information about the National Astronomical Observatory of Japan's Subaru Telescope, visit:  https://subarutelescope.org/  For more information about the Gemini Observatory, a partnership of the United States, Canada, Brazil, Argentina and Chile, visit:  https://www.gemini.edu/


For the Subaru Image:  Orton (Jet Propulsion Laboratory) and Yasumasa Kasaba (Yohuku University, Japan) led the team, with Takuya Gujiyoshi (Subaru Telescope astronomer) operating the telescope.  Other team members included James Sinclair, Anna Payne (JPL), Joshua Fernandes (California State University, Long Beach), Leigh Fletcher (University of Leicester), Patrick Irwin (University of Oxford), Padma Yanamandra Fisher (Space Science Institute), Takao Sato (JAXA), Davide Grassi (IAPS/INAF), Shohei Aoki (IASB, Belgium), Tomoki Kimura (RIKEN), Chihiro Tao, Takeshi Kuroda (NICT)l Takeshi Sakanoi, Hajime Kita, Hiromu Nakagawa (Tohuku University), Hideo Sagawa (Kyoto Sangyo University) and Joana Bulger (Subaru Telescope).

For the NIRI Image:  Orton leads the observing team for the adaptive-optics imaging and Wong heads the observing team for the thermal imaging. Additional team members include Andrew Stephens (Gemini Observatory); Thomas Momary, James Sinclair (JPL); Kevin Baines (JPL, University of Wisconsin), Michael Wong, Imke de Pater (University of California, Berkeley); Patrick Irwin (University of Oxford); Leigh Fletcher (University of Leicester); Gordon Bjoraker (NASA Goddard Space Flight Center); and John Rogers (British Astronomical Association).




Wednesday, 28 June 2017

Saving Cassini - ESA and NASA in 1994

In June 1994, as a result of threatened cuts during Dan Goldin's tenure as NASA administrator, our epic mission to the Saturn system was under extreme threat of cancellation.  The background to these decisions is covered in Michael Meltzer's excellent book, but I'd always heard of the striking letter sent directly to Vice President Al Gore (i.e., bypassing Goldin) from ESA's Director General, Jean-Marie Luton.  I managed to track this letter down in an appendix to a 1998 book from the National Academic Press on U.S.-European Collaboration In Space Science, and it's reproduced here.  Further background can be found in the NASA in the World book.

Letter from the European Space Agency to the Vice President of the United States, June 13, 1994

european space agency

agence spatiale européenne

D.SCI/RMB/db/3948

Paris, 13 JUNE 1994

Jean-Marie Luton
Director General

The Honorable Albert Gore, Jr.
Vice President of the United States
Old Executive Office Building
Washington, DC 20501
USA

Dear Mr. Vice President,

I have recently received a number of disturbing reports that suggest that the continuation of the joint U.S./European CASSINI mission could be threatened by ongoing Congressional deliberations on NASA's FY95 Appropriations Bill.

I am aware that the House version of the Bill, as marked up by the House VA-HUD and Independent Agencies Subcommittee on June 9, retains the necessary funding for NASA's portion of the mission. However, I am also aware that the House Subcommittee's Senate counterpart is faced with a more stringent budget allocation. I am told that the Subcommittee Chair, Senator Mikuiski, has indicated that without an increase in said allocation, termination of a major NASA programme would have to be contemplated, with specific reference being made to the CASSINI mission.

In the field of space science, CASSINI is the most significant planetary mission presently being undertaken by either the European Space Agency (ESA) or NASA, involving the exploration of Saturn, the most complex planet in the solar system and of its Moon, Titan. It is expected to provide at least a ten-fold increase in our knowledge of both bodies as compared to NASA's highly successful Voyager mission.

In making the commitment to participate with the U.S. in 1989, ESA oriented its overall space science programme in order to select this cooperative project, rather than opt for one of a number of purely European alternatives that were proposed at the same time. This decision was taken on the basis of scientific merit and in the belief that the cooperation would be of major benefit to both the U.S. and European scientific communities as well as the international science community in general. Over the past five years, while ESA's Long-Term Space Plan has been forced to undergo a series of significant revisions, driven primarily by our own budget limitations, the Member States have maintained a full commitment to the space science portion of the plan, of which CASSINI is an essential component.

To date, the Member State governments of ESA have committed around $300 Million to our portion of the mission (the Huygens Probe that will descend into the atmosphere of Saturn's Moon Titan, and several elements of the Saturn Orbiter Payload), of which two-thirds have already been spent, and have committed to a further expenditure of around $100 Million to see the mission through to completion. These figures do not include the approximately $100 Million contribution of Italy via a NASA/Italian Space Agency bilateral agreement.

The HUYGENS programme has been in the hardware phase for the past four years, with probe delivery to NASA due to take place in two years time. The hardware integration and testing phase started in early May this year.

The CASSINI mission has generated intense interest in Europe, both within the scientific and engineering community and from the public at large. Approximately 900 European scientists and engineers are working on the programme with more than 30 European institutes and universities involved in the preparation of CASSINI/HUYGENS science.

Europe therefore views any prospect of a unilateral withdrawal from the cooperation on the part of the United States as totally unacceptable. Such an action would call into question the reliability of the U.S. as a partner in any future major scientific and technological cooperation.

I urge the Administration to take all necessary steps to ensure that the U.S. commitment to this important cooperative programme is maintained so that we shall be able to look forward to many more years of fruitful cooperation in the field of space science.

Respectfully,

J.M. Luton