Followup: Deep Impact crater on Tempel 1

By Phil Plait | February 16, 2011 7:00 am

At the Stardust press conference yesterday, they displayed an image that purported to show the crater where the Deep Impact impactor slammed into the Tempel 1 comet nucleus back in 2005. When I wrote up my previous post that picture was not available, but it’s been a few hours and it’s now online. So here you go:

On the left is the Deep Impact image taken by the impactor not long before the end. On the right is the Stardust image; note the resolution isn’t as good. That’s what you get when you’re 178 km away instead of a dozen or so! Still, the arrows mark the outline of the impact crater rim. On the left, I marked a dark mound of material that existed in 2005… but is gone in the new image. Not too surprising! The impact was like setting off nearly 5 tons of TNT, so that mound is now most likely vaporized water orbiting the Sun on its own.

I’ll note this image is much better than what impact expert Pete Schultz had available at the press conference (they were scrambling to get the images together right up until the conference started). I couldn’t see the crater in the image he showed then, but in this one the rim is actually fairly clear. You can also see the faint central mound inside the crater; that’s probably material that was lifted up by the explosion and then fell back in. I wonder though: In big impacts on rocky bodies (like the Moon) you get a central peak, and that’s due to molten material flowing back from the rim and splashing up (the process is called isostatic rebound, a phrase I love). I don’t know if that would apply here or not, but it’ll be interesting to hear what the scientists say as they have more chance to study the images.

Also in the Deep Impact image on the left is a heart-shaped hole or depression. In the Stardust image it appears to have rounded out a bit. Hard to say with the lower resolution, but things do look different.

In fact, take a look at this image:

These show a smooth area, elevated a bit above the rest of the surface. The top image is from 2005, the bottom from 2011. There are several changes; the yellow lines mark a couple of craters that have apparently merged into a trough of some kind in the intervening years.

Also at the top right of the 2005 image, the Sun is brilliantly illuminating a steep cliff. It’s hard to say if the cliff is gone in the bottom image because the lighting is different — it looks like it’s disappeared, but that may simply be due to the Sun not shining on it as strongly. Still, the shape is different. Scientists think the cliffs may have receded by as much as 30 meters in the past few years. In the bottom image there are a series of mounds running horizontally across the top of the plain; these are not apparent in the older image. Again I’m not sure if that’s an illumination issue, but I imagine that, since scientists have multiple Stardust images from different angles, they can construct at least a crude 3D map. That will help them figure out what’s changed.

Overall, these images show that scientists have their work cut out for them, but that the data they need are there! I’ve known Pete Schultz for a few years — he was one of our experts on "Bad Universe" — and I expect it’ll be about a week before his feet even touch the ground.

My congratulations to the Stardust folks for threading the needle on this one! Amazing work.

CATEGORIZED UNDER: Cool stuff, Pretty pictures

Comments (31)

Links to this Post

  1. Impacting a Comet « Universe Sandbox | blog | March 21, 2011
  1. Dax

    Hi there! A very quick off-topic question. The dutch newspapers are reporting a big solar flare heading for earth that could cause major power-outs and disruption of the GPS signal. Is there any truth in this?


  2. Oooh, great pictures! It’s very interesting (not to say enlightening) to see how dynamic a comet’s surface is. Great job all around!

  3. Gary Ansorge

    What’s most interesting to me is how little damage is done to this rock by 5 tons(equivalent) explosive energy. If we had to destroy a rock like this to prevent an earth impact we’d need a LOT more energy. Even a 5 mega ton bomb would barely ruffle it’s dust.

    Dr Gerard O’Neille proposed moving such rocks by installing solar powered mass drivers on its surface. THAT might actually work.

    Great pics, Phil.

    Gary 7
    PS. Phil, I’m not receiving Bad Astronomy updates.

  4. Pete Jackson

    I think that all the white areas are basically clouds of gas in the act of subliming. The collision in 2005 must have exposed a large amount of volatile material which quickly sublimed into gas, causing the huge cloud observed then. So whatever crater was made must have quickly sublimed down to a hummocky remnant of non-volatile material. Perhaps in the coming days, they’ll be able to combine pictures to get a single ‘super-resolution’ picture that will show what’s left.

  5. I can imagine the texture of a comet is very strange. Seems like it would be like… hard packed snow.

  6. I hope I live long enough to see a picture taken from the surface of a comet. Seems like it would be one weird “land”scape.

  7. Unaspammer

    How much material could have realistically fallen back in? The escape velocity of a body that small is practically non-existent.

  8. Bogdan

    This may be a silly question, but how come they had Stardust pass 180km away? To a non-expert like me it seems that if you can get within a couple hundred kilometers (given how huge the solar system is), it shouldn’t be that much harder to get within a couple tens of kilometers.

    Also, what was the relative velocity between Stardust and the comet? I’m curious how much time they had to snap pictures before passing it.

    About the resolution, I expect that distance is not the only factor. Though the CCDs apparently had similar pixel counts, Deep Impact’s had a 10.5 meters focal length telescope versus Stardust’s 202mm lens, and Stardust had a stuck filter that I understand reduces its resolution; also, Stardust was launched in ’99 and Deep Impact in ’05, I imagine there was a bit of progress in terms of camera quality.

  9. Unaspammer

    180 km isn’t very much. To put that into perspective, that’s a little more than half the altitude of the ISS.

    And remember that Tempel 1 wasn’t Stardust’s original destination. Stardust only has a limited amount of fuel to burn, and they probably didn’t want to use up all of it. I imagine this was the best they could do within their fuel budget.

  10. MaDeR

    Rosetta mission have lander (Philae), that have some camera imaging system. Just wait to end of 2014.

    “it shouldn’t be that much harder to get within a couple tens of kilometers.”
    It is not. Problem is in surviving encounter.

    They have no other objectives after this mission. Uncertainites in amount of fuel left was biggest problem – and for that reason they did not want to use it all up. They will probably spend it all anyway to check, how much fuel they really had left. This would help for future missions and estimating fuel budget after years of work in space for them.

  11. Calli Arcale

    Reportedly, the remaining hydrazine is about 1 cup, which is less than a quarter of a liter. Running on fumes, really!

  12. Dax

    Hey Unaspammer! Thats not the one i was talking about, a new x class solar flare supposedly is heading our way but i cant seem to get this confirmed outside the newspapers

  13. RwFlynn

    I don’t really have anything new to add, but I have to say that this story about the Deep Impact aftermath is just so damned interesting. I can’t put my finger on why. Perhaps it’s just a love of science and astronomy. :) Keep bringin on the science, Phil!

  14. réalta fuar

    I guess it would have been much more surprising if a short period comet had NOT changed in over 5 years! Obviously, the important thing is that this is the first time we’ve had images to compare over such a baseline.
    Though at first glance it appears that the crater from the impactor isn’t very impressive, note that it’s about 100 meters across, roughly the size of a football pitch. A similar amount of kinetic energy on the earth would have created a much smaller “scar”. Though hitting a comet may be a bit like punching a really big marshmallow, the extremely low surface gravity seems to more than counterbalance that.

    For the geologically literate amoung us, please note that “isostatic rebound”, as the term is normally used, has absolutely nothing to do with the creation of central rebound peaks in impact craters (and the description of that process as given is extremely dubious). Isostatic rebound is generally used to describe the gradual increase of land elevations after being crushed under glacial deposits for thousands of years. Central rebound peaks can be modelled pretty well by looking at very slow motion films of drops falling into water. One suspects that the physics that resulted in the mounds in the images is very much different from that.

  15. So, if we can put something into orbit around an asteroid, how much more difficult would it be to have something orbit a comet and stay with it as it went around the Sun? (Or be co-orbital with it around the Sun.) Then we could examine it for a long time, rather than two brief passes, more than 5 years apart.

  16. Is anyone concerned that the lower resolution should effect these conclusions a bit more? Remember the face on Mars? It’s just hard for me to look at these more recent images and be like “this is an obvious change” when I’m seeing so much less detail.

  17. réalta fuar

    To be fair, and to modify my previous comment a bit, “isostatic rebound” has been used occasionally to describe the slow uplift of impact crater floors after the overburden has been removed (a process very similar to the usual usage). That may actually be what we’re seeing here in the mounds, but the physics seems to be very, very different from the creation of central rebound peaks.

  18. zeke

    @13 Dax

    No new X class flare has occurred since Tuesday (GMT time) as I type this. Realtime monitoring of the Sun’s X-ray flux can be found here.

    Brought to you by NOAA’s GOES satellites!


  19. Dax

    Thanks a lot for the link zeke!!

  20. Anchor

    “You can also see the faint central mound inside the crater…”

    I’m sorry, but I do not see that at all. I cannot interpret that feature as a mound. The lighting angle suggests it is a PIT, not a protusion! In fact, I will go so far as to say that THIS is the impact crater produced by Deep Impact. While the surrounding region indicated seems to have a sunlit internal “rim”, that rim does not seem to be complete in an unambiguous SHADOWED side which should be there on the opposite flank. There isn’t anything like that there to the 5-o’clock position except the rim of the big pre-existing crater and a scarp that is visible in the Deep Impact image comp.

    To reiterate: the incident lighting angle in the Deep Impact composite points from the 4 or 5-o’clock position toward the 10 or 11-o’clock position to the upper left. The incident lighting angle in the Stardust image points from about the 5-o’clock position toward the 11-o’clock position, so the two views have quite similar lighting angles, which greatly simplifies matters.

    Now look at that putative “central mound” closely again and look at the rest of the region: how can that possibly be a ‘mound’? The shadowed side is situated to the BOTTOM of the sunlit side. That is an obvious depression, NOT a mound! That is most likely the entire crater itself – not any of that other stuff proposed as the rim. Part of that putative larger rim can easily be interpreted as sunlight on a relic slope of that OTHER mound which Phil points out in the Deep Impact image comp. But of the shadowed portion of that putative larger rim one is entitled to expect to be present, there is almost no indication at all.

    Talk about people (and experts themselves!) getting fooled by the infamous inverted topography illusion Phil has so often talked about!

  21. #19 Zeke wrote:

    No new X class flare has occurred since Tuesday (GMT time) as I type this.

    Dr. Phillips reported an X-2 at 01:56 UT on the 15th that should hit the planet today or tomorrow. Apparently, it is the first X class in this cycle.

  22. Peter B

    Ken B @ #16 asked: “So, if we can put something into orbit around an asteroid, how much more difficult would it be to have something orbit a comet and stay with it as it went around the Sun? (Or be co-orbital with it around the Sun.) Then we could examine it for a long time, rather than two brief passes, more than 5 years apart.”

    I get the impression it could be tricky, for two reasons – mass and atmosphere. Eros, the asteroid which NASA put a spacecraft into orbit around, has a mass about 100 times that of Tempel 1. A lower mass means a lower orbital speed, which in turn would require more fuel for braking into orbit. Secondly, comets are surrounded by a coma, or atmosphere, which would cause drag the same way the Earth’s atmosphere causes drag on satellites in low orbits.

    I imagine a co-orbital spacecraft might be possible, but I suspect it might take a lot of fuel to change course.

  23. Grimbold

    PeterB, #23,

    One obvious problem with a co-orbital spacecraft is that comets undergo irregular outbursts from outgassing that can substantially change their orbits. The spacecraft would need to adjust its own orbit to follow. I have no idea if the spacecraft can react fast or accurately enough, or if it has enough fuel and thrust to make the required orbital adjustments. This would be an interesting challenge.

  24. Paul A.

    I remember getting up early to watch this and it really didn’t live up to my expectations. I also remember the NASA channel that was broadcasting this cut away a few minutes before impact changing into the station for our local community college. This left me scrambling to find it on one of the regular news channels. And then it didn’t live up to my expectations, I expected to see a flash or something.

  25. Jon Hanford

    Grimbold, #24 Peter B, #23

    A mission to orbit a comet has already launched (ESA’s Rosetta to Comet 67P/Churyumov-Gerasimenko). During the approach and rendezvous phase Rosetta will be mapping the comet while dropping into a final orbit less then 200km from the nucleus, and a lander (Philae) is to be deployed. Rosetta is scheduled to orbit the comet through it’s perihelion passage. A description of the rendezvous and orbital phase of the mission can be found here:

    Rosetta will close in on Churyumov-Gerasimenko Spring of 2014

  26. hammer166

    DUH!!! None of y’all know what you’re talking about! I just heard Hoagland say this is really an ancient spacecraft. And he is one smart fellow!

    No, I don’t listen to Coast to Coast, it just happened to be on in a vehicle I jumped in to move. I think my IQ dropped 10 points from the brief exposure to the good doctors wisdom! B-)

  27. drbitboy

    Re: minimum distance of encounter.

    At the available comet-relative spacecraft flyby speed (~10km/s), somewhere around 155km was the minimum distance at which the NAVCAM mirror would be able to track the comet at closest approach ( – I believe the 3.1deg/s maximum mirror scan rate quoted there is lower than the actual maximum). The original targeted distance was about 200km ( to allow for errors in Trajectory Control Maneuvers (TCMs). when tcm33 put the flyby at ~191km, it was deemed close enough and tcm34 was skipped (

  28. drbitboy

    Re: Hoagland

    Hoagland is plagiarizing Gene Rodenberry from forty years ago. I don’t even have to google for the title, it was one of my favorites: “For the World is Hollow and I Have Touched the Sky.”

  29. Thank you for another excellent post. The place else may just anyone get that kind of info in such an ideal way of writing? I’ve a presentation next week, and I’m at the search for such info.


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