A comet creates its own snowstorm!

By Phil Plait | November 18, 2010 1:00 pm

NASA has just released new results and images from the EPOXI spacecraft’s visit to the comet Hartley 2 from November 4… and like the previous ones, these are absolutely stunning jaw-droppers. What scientists have found is that the comet’s solid nucleus is sitting in the middle of a veritable snowstorm!


Wow! Most of those dots are not stars: they are actual snowballs, frozen matter that has been ejected by the comet itself! They range in size from a few centimeters to a few dozen across, so they really are about the size of snowballs you’d use in a snowball fight… or to make a snowman. But I wouldn’t recommend it: a lot of that material is not frozen water, it’s actually frozen carbon dioxide, or dry ice.

epoxi_hartley2_snowflakesAs I wrote a few days ago, the comet nucleus — the solid part of the comet at its heart — is only a couple of kilometers across, and composed of rock, water ice, and dry ice. As the nucleus gets close to the Sun, the dry ice turns directly into a gas and spews out of vents in the surface. These plumes of material shoot out, enveloping the nucleus to become the fuzzy head of the comet, and stream away to form the tail.

But we’ve never been close enough and had good enough images to see just what these plumes look like in detail… until now. It’s not just gas, but actual chunks of ice flying out from the nucleus! The comet, for all intents and purposes, is having its own cosmic snowball fight. As you can see in the cleaned picture on the right (click to ensublimate) the comet is sitting in a cloud of thousands, millions of these snowflakes.

How awesome is this?

In fact, so much material is blowing out from the comet that the nucleus is able to cast a shadow in space:


Oh my. How lovely.

But there’s far more than just gorgeous pictures. A lot of science has come from this mission. For example, I noted in an earlier post how the comet shape is weird: it’s a peanut, with two roughly-surfaced lobes connected by a smooth waist. What the heck is that all about? Why are the two lobes so different in texture than the connecting waist?

epoxi_hartley2_emissionWell, EPOXI’s close flyby allowed scientists to determine that the material coming from the lobes is mostly carbon dioxide (which is dragging water ice with it), but the material making up the smooth region is mostly water! This must be a clue to what’s going on… but what, exactly? Well, we’re not sure!

As the material is blown out of the nucleus, some of it falls back and gets deposited on the surface. The gravity of the nucleus is pretty funky, given its shape, so it’s possible that the dust that comes out falls back onto the waist region preferably. That may block the sunlight from heating the surface, preventing carbon dioxide from warming up and venting. Or it’s possible that all the carbon dioxide is simply used up from the waist region due to processes that haven’t been figured out. Maybe there never was any in the first place, and its concentrated more in the lobes.

There is one thing we know for sure: every time this comet loops near the Sun on its orbit, it loses a lot of material. In fact, it may shrink in size by as much as a meter or so every 6.5-year orbit! In other words, we’re catching this comet near the end of its life: in a hundred or so more passes, the material in the waist will shrink to nothing, and the two lobes will fly free, their tether to each other severed. Then we’ll have two comets (plus a lot more debris), each slowly decaying as they spew matter into space. This whole object may only last a few thousand more years, after surviving for billions of years in the solar system.

This, too, shall pass.

While that may be sad for Hartley 2, it’s a bonanza for scientists. We’re learning more about comets every day, and the knowledge gained from this one flyby will keep scientists busy for a long time to come. Or, at least, until we see another comet close up…. in 2014 in 2011.

Image credits: NASA/JPL-Caltech/UMD

Here are some other images of the comet Hartley 2. Use the thumbnails and arrows to browse, and click on the images to go through to blog posts with more details and descriptions.



MORE ABOUT: comet, EPOXI, Hartley 2

Comments (29)

Links to this Post

  1. Spaceballs! « Lights in the Dark | November 18, 2010
  2. Astronews Daily (2455520) | November 19, 2010
  3. pligg.com | November 24, 2010
  4. An Old Comet – Quick DpSU « Eye on the ICR | June 28, 2011
  1. Tom

    That last photo is not a reflection or ejection of volatiles.

    Its the space engines controlling the craft. Look. Use your brains.

    The aliens are coming! We are so dead.

  2. Jeff

    These ejected snowballs are probably the material of future meteor showers, since they’re in space they’re probably going to remain cold.

    Since I’ve been in Florida so long, I forgot what snowballs looked like. I’d happily make a snowman with this stuff.

  3. Shoeshine Boy

    This is truly a golden age for astronomy.

  4. Kullat Nunu

    In the meantime, NeXT (a.k.a.Stardust) will visit Deep Impact’s original target Tempel 1 in the next February to see the result of the impact.

  5. Kullat Nunu

    This is truly a golden age for astronomy.

    Too sad this age is ending. Only a few missions are seriously being planned, thanks to the economical situation.

  6. David

    If the comet is losing that much material every time it swings past the sun shouldn’t it have evaporated a long time ago considering how small this comet is? Unless this comet has “recently” entered the inner solar system. Or if its been here for a long time how big must it have been if its been doing this for a very long time? Any idea how long this comet has been in the inner solar system?

  7. GOPHyperbole

    This is not described in the bible, it doesn’t exist.

  8. Keith Hearn

    You start out with two roughly spherical chunks of rock/dust/water-ice/CO2-ice touching each other. With each pass near the sun, a snowstorm is created from the chunks. Some gets blown completely away, but some stays near. The CO2 sublimes more easily than the water, so the stuff that stays near is mostly rocks, dust and water ice. This falls back and accumulates in the area between the chunks, since it is at a lower gravitational point, eventually forming a relatively smooth neck along a gravitational equipotential surface. On subsequent passes, we see CO2 & water still being blown off the chunks at the ends, but there is only water vapor coming off the neck in the middle. It sounds simple enough, so what have I got wrong?

  9. Gerry

    I see a lot of rocket fuel, drinking water and breathing air going to waste there….
    Time to start mining!!!

  10. Johan Stuyts

    If it loses 1 meter every 6.5 years, how big was it to begin with? I come up with:

    5 billion years divided by 6.5 = 769,230,769 m lost = 769,230 km lost

    I realize it must have lost a lot less per orbit in the beginning because the sun was weaker, but even then if i correct it by two orders of magnitude I still end op with a comet that had a diameter of over 7,000 km.

    Can this be correct, or did the orbit change later?

  11. Jamie

    @Johan: I think you are extrapolating too far back in time… Do we know how long this comet has been in it’s current orbit for? Maybe it spent the first billion or two years just chilling out in the oort cloud with its homies before it was nudged in.

  12. Johan: if you extrapolate the 4 cm / year lunar recession rate backward in time, you’d find the Moon was touching Earth 9.6 Gy ago… or if you account for how the tidal effect would have increased due to the closer distance, it would have touched the Earth only around 2 billion years ago. In both cases the problem is in the assumptions.

    You are assuming it looses surface depth at a uniform rate… but there’s no evidence for that. In fact, your calculation shows it’s rather unreasonable. So one possibility is that it’s been in the present orbit for 4.6 Gy, but shielded… perhaps by rocky/dusty debris that had built up as the volatiles sublimated away. Or, you could assume that the orbit hasn’t remained unchanged for 4.6 Gy… a pretty safe assumption, since orbits are constantly (if slowly) changing, especially for small objects that cross the orbit of Jupiter. The fact that its aphelion is very close to Jupiter’s orbit in terms of distance suggests how it got into this orbit in the first place – it was likely thrown into a short-period orbit due to an interaction with Jupiter.

    However, I’m not sure I agree with Phil that as the “neck” sublimates away (assuming it is, and not building up) that you’ll end up with two cometary objects “floating away” from each other. I tend to doubt it’s the structural strength of the neck that is joining them; I suspect it’s the self-gravity of the object. If the neck region thins, it can allow the two lobes to pull tighter together… or, by maneuvered apart by off-axis thrusting due to sublimating gases. Perhaps a more interesting thought is how the shape of these objects should evolve over time. At first, I thought they should end up more rounded over time (as areas with high curvature would have a higher surface area to volume ratio, and so change their shape faster than areas with low curvature; the shape smooths out). Clearly I’ve something to learn here, as too many examples are clearly multi-lobed like asteroids. Self-heating of convex regions generating necking, and eventually fragmentation due to jetting?

    Amazing objects… amazing observations! From my standpoint (teaching astronomy), this get easier all the time… with better examples, more interesting puzzles, and (yeah, I’ll say it) prettier pictures.

  13. John Paradox



  14. Steve D

    Extremely unlikely that it’s been in its present orbit for billions of years. More likely it was captured by Jupiter, since its 6-1/2 year period marks it as a likely member of Jupiter’s family. It came in as a long period comet, made one or more close passes by Jupiter, and was diverted into a short period orbit. If it was originally 10 km in diameter, losing 8000 meters at 1m/6.5 years comes to 52,000 years. Even 100 km, a monster bigger than Hale-Bopp, would indicate maybe 600,000 years. Yesterday in astronomical terms.

  15. Jack

    Don’t forget that the surface/volume ratio is higher for smaller bodies, and the greater temp change this yields. Would definitely effect loss rate over time.

  16. I don’t care how thorough your explanation is, I still say it’s a missile.

  17. Jeff Sherry

    Fascinating and awesome. I wonder if the comets that repeat their orbits have the same peanut shape?

  18. Since I’ve been in Florida so long, I forgot what snowballs looked like. I’d happily make a snowman with this stuff.

    I forget the actual spectral data chart on this, but isn’t there also cyanide and other bad-for-people stuff coming out of the comet? I’d have to guess that it’d be a bad idea to make a snowman out of this snow. Not without a hazmat suit anyway.

  19. Messier Tidy Upper

    @ ^ J. Major : Yes, theres cyanide and other nasty stuff there but only at extremely low levels. This was mentioned in connection with Halley’s comet in Arthur C.Clarke’s third ‘Space Odyssey’ book ‘2061’ where Heywood Floyd squirts some water extracted from Halley’s comet down his own throat :

    .. and that“, said the ship’s doctor half an hour later, “was one of the silliest exhibitions I’ve ever seen. Don’t you know there arecyanides and cyanogens and God knows what else in that stuff?”

    “Of course I do,” laughed Floyd, “I’ve seen the analyses – just a few parts in a million. Nothing to worry about. But I did have one surprise,” he added ruefully.

    “And what was that?”

    “If you could ship this stuff back to Earth you could make a fortune selling it as Halley’s Patent Purgative.”

    -Page 180, Clarke, 2061 : Odyssey Three’, Harper Collins, 1988.

    (Italics original.)

    Clarke was good with his research and, far as I’m aware, kept his novels fair;ly scientifically accurate. :-)

    So I don’t think a Hartley-2ian snowman would require a hazmat suit to build – although you *would* need a spacesuit if constructing in situ! 😉


    Awesome images and write-up here. Superluminous item & news! (Ie. merely beyond brilliant.) :-)

  20. Joseph G

    @#1 Tom: In all seriousness, if you did want to stage a sneak attack on a technologically advanced civilization (as advanced as ours or moreso), a comet would be a perfect way to do it. And not just by chucking a comet at the enemy planet. Since no large ship could possible escape detection ( there ain’t no stealth in space), a comet would be perfect camouflage. Unlike asteroids, which have relatively low-eccentricity orbits (not to mention that pretty much all of the really large ones have been found) comets can have periods of thousands of years and come from any direction.

    Picture this. You’re the commander of an initial invasion force tasked with occupying an unsuspecting inhabited planet (without destroying the biosphere ala “Lucifer’s Hammer”). You grab a comet that you find out in the oort cloud on the way into their system, cut it in half, hollow it out, and paste it together around your ship – you could hide a really huge chunk of metal inside one of those mofos, we’re talking Imperial Star Destroyer-size with room to spare, here. Then just send it into the system on an orbit that takes it from the opposite side of its star, swings near the star, and passes close to the planet you want to attack.
    As you get closer to the planet (and the system’s central star) waste heat is easy to hide – just use it to sublimate some of that dry ice and add to the comet’s coma. You could even preferentially heat different areas of the comet to use those jets to fine-tune your course! As the unsuspecting denizens of your target planet marvel at the pretty comet as you make your close approach, maybe a couple hundred thousand miles, just pop the “ends” off and start blasting. The breakup of the comet would itself create a huge cloud of ice particles that you could “hide” in until the absolute last second.

    Not only that, if you tweaked the orbit the right way, to swing extremely close to the target planet, you could make it look as if the comet is breaking up from tidal forces during a close pass, which would put you literally right at the top of the target planet’s atmosphere before anyone realizes what’s going on. The only problem with that is that the people on the target planet might find that incoming comet’s trajectory uncomfortably close, and send a ragtag band of roughnecks to stick a nuke up your keister 😉

    Sorry, I don’t mean to hijack your science discussion with a lot of silly sci-fi stuff, but I just can’t resist :) Also, I’m reminded that comets are often regarded as ill omens by various cultures – perhaps they’re on to something, eh? 😛

  21. Joseph G

    Those pics are absolutely spectacular! Such a great day/week/month for astronomy! I hafta say, that “snow” is amazing to me. I half expected comets to be less spectacular up close – like trying to get a close-up of a rainbow (not much going on) How happy I am to be wrong :)

    @#12 Brian: Self-heating of convex regions generating necking, and eventually fragmentation due to jetting?

    That’s what she said! 😛

  22. Thomas Siefert

    Since I’ve been in Florida so long, I forgot what snowballs looked like. I’d happily make a snowman with this stuff.

    When you feel like making a snow-woman out that stuff, it’s time to leave…

  23. Greetings and salutations….
    What is it with this seeming inability to use the word “loose” and “lose” correctly? Not only does it pop out at me in this thread, but, I have seen the same error in many other forums.
    Remember people – “Loose” means “not fastened down or connected”. “Lose” means to have something vanish from your possession. Now, you may LOSE your ring because it is LOOSE on your finger, but that is the closest connection that those two words have!
    Sorry for the rant, but, the deterioration of the English language is a sore spot with me, as I spent WAY too much time in college getting dinged for this sort of mistake. I will not even get into my other irritation – the incorrect use of “it’s” and “its”…
    At least in Astronomy there is truth and beauty….

  24. Anchor

    That is one lucky spacecraft…

    (Latest reports indicate the spacecraft probably slammed into ‘at least 9’ particles during its encounter…not sure whether that is actual data or a calculation on an extrapolation from the particle cloud in the images)

    That’s the second reason why it was a good idea to keep the spacecraft a safe 700 km-distance at closest approach. The other owing to increased difficulty of obtaining any decent images at substantially closer distances: image blur during exposures of the spacecraft’s very high relative speed with respect to this quite small nucleus would not have helped much if at all for the closest photos. For one thing, the time around closest approach when programming the spacecraft to slew more quickly to counteract image blur would have been too brief and drastically swift to be worth the risk, and it could have compromised images obtained at farther ranges immediately around the time of closest approach, not to mention an extra risk of reacquiring Earth with its antenna after the encounter to send the data.

    The mission team knew what they were doing, and they did a fantastic job at estimating an optimum as well as safe encounter distance.
    Because of that, they obtained close to the maximum of data safely retrieved from this remarkable encounter. Nothing short of brilliant!

    Phil, a few items:

    1. What’s been variously characterized as ‘snowballs’ or ‘snow flakes’ are almost certainly instead far more ‘fluffy’ things, with lots of empty space between the molecules, things of which we just don’t have any terrestrial counterpart. Super fragile (no -expealidocious jokes) things compared to anything we see in the ‘harsh’ environment of a planetary surface, however comfortable our planet happens to be for ‘fragile’ biological stuff). They’re probably aggregates of icy crystals arranged in extremely low-density configurations, and not necessarily composed exclusively of water. One mission team member likened them more to “dandelion puff”, probably even less dense than that, and they must be quite reflective if their typical sizes range up to the size of a golf ball “possibly up to the size of a basketball” could be imaged at the range of ~700 km surrounding the nucleus.

    2. Carbon dioxide is more volatile than water; this could explain why there seems to be a dearth of CO2 emanating from the central waist region: by the time any of the stuff that settles back to the waist region between the two lobes (‘falls’ preferentially there, even in that ‘wonky’ grav field) it would be exposed to plenty of sunlight and any CO2 would be preferentially vaporized before the H2O would.
    Add any dust those particles happen to bring with them, and their deposition would probably produce a smooth-surfaced mantle – a kind of low-gravity ‘sediment’ – that accumulates over long periods of time. The smooth-surfaced waist material that settles there could therefore be mostly fine-grained dust that has been selectively sorted by the deposition mechanism, favoring more water ice in its makeup than the more volatile CO2 ice. The relatively higher concentration of water ice that remains then may continue to outgas from the waist region, especially considering that the dusty (and darker) dust would absorb more solar heat and bake that material to the extent that it would tend to produce the sort of H2O plume specifically from the waist region observed. All that those observations really suggest at this point is that there is EITHER a relative enhancement of water OR there is a relative deficiency of CO2 in the central waist region. The observation suggests it ejects mostly water – and it appears to be ‘steamed off’ on the side most exposed to solar heating. In other words, that waist region might not have the same CO2/H20 ratio as the outer lobes. But because the waist region has been ‘buried’ by a very fine-grained dust from stuff that has fallen back which has been preferentially depleted of the more volatile CO2, all that’s left to get sublimated or ‘steamed off’ is any residual water that might be left.

    3. At a rotation period of 18 hours (or even the very tenuous 13-hour period possibility, thought much more unlikely) those ‘lobes’ are NOT going to “fly free” if the waist region were somehow to vanish suddenly. If it had anywhere close to the tumbling rate to pull it apart, it would already have done so long ago. (REMEMBER? LOSING mass DECREASES the rotation rate!). These ‘rubble-pile’ things CAN’T be held together by much else other than the very tenuous gravity they exert on themselves. And don’t forget that the BASIC constituent composing this and every other comet nucleus so far studied is NOT a volatile ice. They’re by far MOSTLY composed of rocky materials…

    4. We see no evidence whatsoever in the images of any active ‘vents’. The jets that appear to emanate from narrowly restricted regions do imply there might be something like a ‘vent’, but whether those areas actually sport some kind of ‘throat-like’ vent or other structural manifestation on the surface produced from interior regions (like, example, is the case with Enceladus) has not at all been confirmed in any comet nucleus encounter to date. Yes, there are suspected vent-like structures on Temple 1 that vaguely resemble impact craters, but none have been confirmed, from the imagery during that encounter, to give rise to visible plumes. And there is not a single example of an impact crater or a ‘vent’ structure to be seen anywhere on Hartley 2. That surface in the latter is obviously very young and has been undergoing considerable modification for quite some time.

    5. Nobody yet knows exactly how cometary nuclei expel their volatiles, but from what has been seen so far, it appears that something more like sublimation of the volatile component in the outermost 10 meters of dust and soil within the surface as solar heating bakes that surface plays a more substantial role than that some volatile-rich reservoir farther beneath the surface should sustain a vent on the surface as if it was some equivalent to a magma chamber. We have to think these things out in terms of cometary environments, not just seizing on any convenient terrestrial phenomenon as a suitable model. Obviously such experience-informed paradigms do not always work nearly as well as we might expect.

    6.Certain areas of the surface would undoubtedly be more vulnerable to solar heating than others (depending on the local topography and albedo, etc) and if any of these particular spots also happen to be invested with relatively large amounts of volatiles, it is easy to see that they’ll shoot out what look like confined jets. Depending on how strong the resulting outflow of gas is, then, entrained dust and even un-vaporized and very fragile crystals of volatiles can be lofted off the very low-gravity surface. Some of it will be moving fast enough to leave the nucleus forever, but much of it will loiter at low velocity and hang around the nucleus for awhile before falling back.
    Those latter low-velocity particles will preferentially settle in the neck region between the lobes. (Believe it or not, that’s the lowest ‘energy state’ of the gravitational field of this bizarre shape). But while they’re on their own free of the nucleus exposed to full sunlight, they’ll also get depleted of CO2 over H2O, and when they finally deposit themselves, it will be mostly dust with a relative surplus of H2O as opposed to CO2. Doesn’t that make at least some sense?

    #8 Keith Hearn has it exactly right.

    7. WHO EVIDENTLY DOES NOT KNOW Hartly 2 has NOT been in its current orbit until quite recently: #6 David, #10 Johan Stuyts, #11 Jamie, #12 Brian Davies, #14 Steve D.

    That’s FIVE out of 14 comments (up to Steve D). After all of the material that has been wafted out on how substantially the orbit of Hartley 2 has changed just within the last 60 years or so.

    That information has been REPEATEDLY zapped throughout the internet and blogosphere, and yet these folks are moved to comment because they didn’t know about it – AND not a single commenter up to this point ever bothered to address their concern, including Phil.

    Here is a brief recapitulation by yours truly, posted on this site on October 30:

    “As neatly summarized by Gary W. Kronk at his Cometography site:


    “[Comet 103P/Hartley 2] belongs to the Jupiter family of comets (comets with periods less than 20 years). The comet was discovered in 1986. Although it then had an orbital period of 6.3 years, an analysis of its orbit reveals the period had been longer in the recent past. During the early decades of the 20th century, the orbital period had been 9.3 years. A close approach to Jupiter in August 1947 (0.22 AU) reduced the period to 7.9 years, while another close approach during April 1971 (0.09 AU) reduced the period to 6.1 years.””

    8. Oh, and BTW, that deconvoluted image you show does NOT necessarily show a higher resolution, giving the impression of many speckles where the original shows many fewer. If you closely compare the original image that deconvolution was made from you would readily see that the brightest spots in the original image are translated by the method into a localized concentration of spots in the deconvolution. That is NOT a true indication of the local distribution of particles in the image, although it is a good indication of the field-wide distribution. If you examine the deconvoluted image closely you will see a ‘cluster’ of spots centered around the position of every bright spot in the original image. THOSE CLUSTERS ARE AN ARTIFACT OF DECONVOLUTION and should NOT be entertained as real!

  25. Johan Stuyts

    It took me a couple of days to come back and see if there were replies. Jamie and Brian, thank you for your response. I suspected a change in orbit, but did not know it must have happend ‘recently’.


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