Dipping Our Toes

John SpencerJohn Spencer, Cassini Scientist on the Composite Infrared Spectrometer (bio)

Today’s Enceladus flyby is a bit more adventurous than most satellite flybys by Cassini. 
We are dipping into the jet of water vapor and ice spewing from Enceladus’ south pole, because by doing so we can take full advantage of the amazing opportunity to study
fresh samples from inside this strange world.  The mass spectrometer, dust, and plasma
instruments will be running flat-out, gathering priceless information on the composition
of the gases and the ice particles for the sixty seconds or so that Cassini will be in
the dense part of the plume.

graphic showing Enceladus flybyPlume particles are wonderful things to study, but it’s possible to have too much of a
good thing – at the speed that Cassini is going, particles as small as a millimeter in
size could cause serious damage to the spacecraft if we ran into one.  So the decision to
enter the plume was not taken lightly.  The project convened an “Enceladus plume working group”, led by fellow-blogger Amanda Hendrix, which held a series of meetings to determine whether there was any hazard to the spacecraft from flying as close as we plan to go.  Though we fly 50 kilometers (30 miles) above Enceladus’ surface, this happens near the equator and away from the plumes–the closest approach to the source of the plumes is more like 200 kilometers (120 miles)–see the graphic on the left by David Seal (E3 is the official name of today’s Enceladus flyby).

We know from the Cassini images, and from observations from the dust detector during the July 2005 flyby, that there are many ice particles a few microns (a few 1000ths of a
millimeter) in diameter in the parts of the plume we’ll be traversing, and these provide
juicy samples without posing any hazard.  But was there any way that much bigger
particles could be lofted into Cassini’s path?  We reviewed the observational evidence,
and theories about how the plumes might work (for instance, figuring out how much gas it would take to accelerate an ice grain of a dangerous size to a speed high enough to reach the spacecraft) and we decided that the danger was very small.  Still, it will be good to hear from the spacecraft when it turns its antenna back to Earth and sends home word that the flyby was safely negotiated.  That will be at 7:05 p.m. Pacific time this evening, not that I’m counting…

Future flyby plans could probe even deeper into the plume, coming as close as 100 kilometers (62 miles) from the source on November 2, 2009 (though this depends on official approval of the proposed extended mission).  But we’ll review the results from this flyby before committing to anything that close.  If Cassini reports “come on in, the water’s fine!”, as we expect, we’ll dip our toes in a little further next time.


11 thoughts on “Dipping Our Toes”

  1. Am I mistaken, or does EF1 qualify as the closest planetary flyby to date? The Apollo missions around the moon went down to 10 miles, but those were orbits, by manned missions. If there’s been a planetary mission that came closer, I’d like to know what it was.

    Matt Gibbons
    Bellingham, WA

  2. I am looking forward to hearing the results. Hopefully, what you learn will be worth the risk.

    “Good luck and thanks for all the fish!”

  3. I know you folks will be busy looking for the broadcast from Cassini at 7:05 PM. Hopefully, you can post a short note on the blog at 7:06 confirming that the data is being received. Best wishes to Cassini & the team.

  4. re: On Mar 12, 2008 12:08:19 AM thiemo wrote:

    at what ground-speed (Enceladus) is cassini to pass the moon?

    greetz from europe / germany:-)

    After looking at the official flyby page, I believe the ground speed at closest approach was 14.4 km/sec (32,234 mph). I assume that is ground speed, since it doesn’t specifically state that.


  5. You’ve said that, “it will be good to hear from the spacecraft when it turns its antenna back to Earth and sends home word that the flyby was safely negotiated. That will be at 7:05 p.m. Pacific time”.

    What is the time lag to get the data stream to earth and how long will it take to download, how much data?

    Also counting. Jim; in Salt Lake City, UT

  6. Can’t wait to hear what the results of the plumes we’re and the inbound and out bound images

  7. can’t wait to hear the results of what cassini has found from the wter plumes.Congrats to the Cassini team.

  8. It’s been almost 40 years since my grandfarther woke me up in the early hours,(UK), to watch Neil Armstrong make his first step onto the surface of the moon.

    I was captivated from that day with space exploration. I am now a Father with two young children. They in turn have been captivated by the exploits of the Cassini mission and the work done by you guys….Many Thanks.

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