Taking the Plunge: Cassini Soars by Enceladus

Bonnie J. Buratti
Bonnie J. Buratti

After so many close flybys of Enceladus, we’re starting to feel as if this little moon of Saturn is an old friend. But during the encounter planned for Nov. 2, 2009, we are going to get up-close and personal. Cassini is going to take its deepest dive yet into the plumes spewing out from the moon’s south pole to try to learn more about their composition and density.

The spacecraft is going to approach within about 100 kilometers (62 miles) of the surface. We’ve been closer before (25 kilometers or 15 miles), but we’ve never plunged quite so deeply into the heart of the plume.

To get a better sense of our flyby, watch the animation created by my colleague Brent Buffington. This is the seventh targeted flyby of Enceladus, so we sometimes refer to it as “E7.” The video starts out with our approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Then at around 7:40 a.m. UTC (Coordinated Universal Time), we do our long-anticipated flyby through the plumes. The passage will be quick: traveling at about 8 kilometers per second (about 5 miles per second) - fast enough to go from Los Angeles to New York in less than 9 minutes - we’ll spend only about a minute in the plume.

Mars Chart
This animation shows Cassini’s approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Image credit: NASA/JPL.
› View animation

Then, we zoom away from the plumes and Cassini turns on an infrared instrument (red rays in the animation) to take the temperature of the south-pole fissures known as “tiger stripes” where the plumes originate. A few minutes later, Cassini uses an ultraviolet instrument (purple rays in the animation) to measure the plumes against the background of the peach-colored Saturn. The infrared instrument then gets another turn to examine Enceladus. For more details, see the mission description.

The focus of this flyby is to analyze the particles in the plume with instruments that can detect the size, mass, charge, speed and composition. Instead of using its eyes (the cameras), Cassini is going to use its senses of taste and smell. But we’re going to get some pretty good pictures too, including some impressive shots of the plumes from far away.

So far, we have detected water vapor, sodium and organic chemicals such as carbon dioxide in the plumes that spew out from the tiger stripes, but we need more detail. Are there just simple organic molecules, or something more complex? This is the first time we’ve found activity on a moon this small (the width of Arizona, 500 kilometers or 310 miles in diameter). We really want to understand what’s driving the plumes, especially whether there is liquid water underneath the surface. If we can put the pieces together - a liquid ocean under the surface, heat driving the geysers and the organic molecules that are the building blocks of life - Enceladus might turn out to have the conditions that led to the origin of life on an earlier version of Earth.

So if this is all so interesting, why did we wait so long to travel into the plumes? One reason is the plunge is tricky. We wanted to make sure we could do it. We worried that plume particles might damage the spacecraft. We did extensive studies to determine that it was safe at these distances. We also wanted to have the right trajectory so we didn’t use an excessive amount of rocket fuel. We are going very fast through this sparse plume; so to play it safe, we’re using Cassini’s thrusters to keep it stable through this flyby.

We’ll be monitoring the thrusters closely because we don’t want to have to use them on another flyby through the plumes planned for April 28, 2010. In the future flyby, we plan on tracking the spacecraft very closely with the radio instruments on Cassini and on Earth so we can measure how the spacecraft wobbles as it passes near Enceladus. These measurements should tell us more about the interior of the moon, including whether it really does have a liquid subsurface ocean. With the thrusters on, we won’t be able tell if the motion of the spacecraft comes from the gravity of Enceladus or the thrusters. We’d like to know whether we can rely on other kinds of attitude control equipment.

We’re all eager to pore over the results of this flyby. Stay tuned. In the meantime, feast your eyes on this map of the surface of Enceladus that the Cassini imaging team has updated and released today. The tiger stripes are located in the lower middle left and lower middle right of the image.

    8 Responses to “Taking the Plunge: Cassini Soars by Enceladus”

  1. J. Ramon Says:
    October 30th, 2009 at 7:02 pm

    Good luck from Spain for this new Enceladus flyby and -of course- happy Halloween for everyone at NASA.

  2. Bill Gammerdinger Says:
    October 31st, 2009 at 4:16 am

    Very well written article. Nice to read about the spacecraft’s activities where the explanation is both ismplifed, but remains detailed. Thank you.

  3. Vasilis Dalianis Says:
    October 31st, 2009 at 8:16 pm

    Congratulations to all Cassini scientists and engineers. This is a brilliant mission which marks a turning point in planetary science (lakes of liquid hydrocarbons on Titan, geysers and unique geology on Enceladus, possibility of the existence of reservoirs of liquid water beneath this this moon’s icy crust, cryovolcanism etc).
    I am optimistic that a reservoir (or reservoirs) of liquid water does exist beneath the Enceladus’ icy crust. The recent detection of ammonia and ice grains containing sodium salts in the plumes spewing out from the moon’s south pole show that this is probably the case. Therefore, in my opinion, the big question is whether this subsurface reservoir is able to support life, as is the case with the Earth’s mid-ocean ridges.
    Congratulations again. I’m looking forward to the results of the flyby!

  4. aris Says:
    November 1st, 2009 at 5:04 pm

    Dive a probe straight into one of these gysers already.

  5. Loonyman Says:
    November 2nd, 2009 at 12:32 pm

    Good luck Guys!!!

    Its an honor to share the exploration of the most exciting place in the Solar systrem with you!!

  6. kinzi Says:
    November 3rd, 2009 at 11:15 pm

    Good luck, can’t wait to see the result!

  7. Erik Tronstad Says:
    November 14th, 2009 at 2:33 am

    Congratulations with another successful flyby of Enceladus.

    I’m lacking some data on the flyby. Haven’t found them neither in the PDF on the flyby nor in any other Cassini pages.

    1. Cassini passed within 103 km of Enceladus and this time and had its deepest plume passage. What was the altitude while flying through the plumes? (I realize this distance varies as Cassini flies through the plumes.)

    2. Closest distance to Enceladus was at 89 deg. S (according to the PDF). But at what longitude over Enceladus?

    Erik Tronstad

    Bonnie Buratti responds:

    1. The fields and particles instruments are still analyzing their data to determine the densest part of the plume (and it is model-dependent, since we obtained only a “slice” of the plume), but predictions by the Cosmic Dust Analyzer (CDA) indicate that passage through the maximum density of the plume (at least as seen during this flyby) would occur very close to closest approach, perhaps a second or two before.

    2. An exact longitude will be determined by after-the-fact reconstructions of the spacecraft trajectory, but the predictions put it around 159 degrees west. When you’re close to the pole, small errors in position correspond to large changes in longitude, so the actual longitude could turn out to be different.

  8. samith rukshan Says:
    August 21st, 2010 at 8:10 pm

    i like to american space programes

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