Arecibo gives comet some radar love

By Phil Plait | October 29, 2010 10:42 am

Using a giant radio telescope like a cop’s radar gun, astronomers have made some pretty cool images of the nucleus of the comet Hartley 2:


Hartley 2 is a comet that is currently very close to the Earth as these things go: last week it passed us at a distance of about 18 million km (11 million miles). Astronomers took advantage of the close pass to ping the comet with radar pulses. By timing exactly how long it took the pulses to go from the telescope to the comet and back to Earth, they can create a map of the comet’s shape and other characteristics — something like how dolphins and bats use echolocation to map their surroundings… though, as Emily Lakdawalla at The Planetary Society Blog explains, it’s a bit more complicated.

From the images, it looks like the nucleus — the solid, central part of a comet — is highly elongated, about 2.2 km (1.4 miles) in length, and rotates once every 18 hours. We’ve only seen a handful of comets up close, and in general the nuclei are potato-shaped, so this one fits that description. The image has a scale of about 75 meters per pixel.

These observations were made to help out the EPOXI space mission, which will pass just 700 km (420 miles) from the nucleus of Hartley 2 on November 4. That means we’ll be getting some really cool close-up images and data from the comet very soon! Stay Tuned.

MORE ABOUT: Arecibo, comets, Hartley 2, radar

Comments (26)

Links to this Post

  1. A Comet Nucleus…. Close Up! « Kasama | November 9, 2010
  1. Is it a bad sign that I first read “Emily Lakdawalla” as “Emily Litella”?

  2. It was fun reading that old EPOXI post. :)

  3. That is amazing – I never knew you could get radar images like that from Earth.

  4. Golden Earring reference for the win.

  5. Jon Hanford

    Awesome. We are finally treated to a view of the nucleus – and it’s a tumbling cuke! I’ve been following the comet over the past few weeks using binos and a small telescope, and in the back of my mind trying to visualize this small body. I’m really looking forward to the upcoming flyby, hope all goes well.

    Speaking of things getting pinged by Arecibo (& Goldstone too…), near earth asteroid 2003 UV11 is due to pass Earth later tonight, Oct 29, at a distance of 1.2 million miles (~5 times the Earth-Moon distance). reports that this 600 meter wide object should be visible to amateurs as a 12th magnitude blip traveling through the constellation Pegasus (they also have some pics-video posted). Ian Musgrave’s Astroblog has all kinds of details on this flyby & some neat visualizations (nice job, btw):

    (Alas, 12th magnitude is beyond the visual limit for my scope, so I’ll be watching it online)

  6. Missy

    I am so glad you posted this today! I am going to see Malcom Hartley tonight at the Griffith Observatory. Tim Larson from the EPOXI and Stardust NExT missions will be there as well.

  7. Gary Ansorge

    Gee, I wondered where I parked my interstellar cruiser.( It WAS quite a party.)

    Potatoes in Space should be the title of Phils next book.

    Cool Pics.

    Gary 7

  8. Bill

    > Is it a bad sign that I first read “Emily Lakdawalla” as “Emily Litella”?

    Chevy Chase: Uh…Ken? It’s Lakdawalla. Emily Lakdawalla. Not Litella.
    Ken B: Oh. Well, that’s very different, then. Never mind…

  9. Pete Jackson

    Just got out the calculator to see how nifty the Arecibo radar really is. At 18 million km, the 2.2 km nucleus subtends a mere 0.025 arcseconds, and the 75 m pixels in the radar image are only 0.85 milliarcseconds. So even Hubble, with its 0.1 arcsecond resolution would have seen it only as a “stellar” object in visible light. And to think Arecibo has been in danger of being ‘zeroed out’ in the budget.

  10. Oh for a planet sized Arecibo that could do this for extrasolar planets 😛

  11. Brian Too

    I always wonder, when we use Arecibo this way, what would it look like to an alien civilization? OK OK, beam dispersion would weaken it at any realistic interstellar distance. Bear with me.

    Imagine they could see it. They would get some type of relatively short, hard to explain signal. It wouldn’t contain much information because the beam isn’t coded to carry information. The alien civilization’s detection of the beam would be incidental and accidental to our purposes for the beam. Furthermore the signal would not repeat or sustain, at least not for very long.

    Sound a bit like the Wow signal? It proves nothing of course. It barely even suggests anything. Just an idle speculation.

  12. Mike Saunders

    No John Freeman, that wouldn’t work. This is active imaging, we’re sending out radio waves to get these images. It would take a very long time, and radar power like this decreases by 1/r^4 where r is distance to object you are detecting.

    I do radio links for a living, and trying to propagate information across interstellar distances would be a herculean task. I personally think the SETI@home people are totally insane.

  13. Messier Tidy Upper

    Excellent! :-)

    So how long before Emily Lakdawalla updates her wonderful asteroid & comet comparison chart :

    to include this I wonder? 😉

    Guess that’ll be best done after the date the EPOXI spaceprobe has set ( 😉 ) for completing its Hartley 2 mission.

  14. Messier Tidy Upper

    @7. Gary Ansorge Says:

    Potatoes in Space should be the title of Phils next book.

    Book? No, its what he should call his next mash up! 😉

    Of whatever type. ( )

  15. Anchor

    This is remarkable. Not only does it look “like a cross between a bowling pin and a pickle” as EPOXI project manager Tim Larson says, it appears to be comprised of bodies that stack together like a snowman. The elongated stacking bears a striking resemblance to the output of some simulations showing how loosely-held rubble piles can become tidally distended when making a sufficiently close encounter with a planet’s gravitational field…

    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.”

    It is intriguing to wonder if the April 1971 close approach, in particular, was close enough at 0.09 AU (about 13.4 million km or 8.37 million miles) to stretch it out to its current elongated shape. EPOXI might give us a glimpse of what a body looks like after having had its material recently reshuffled like that. (Think of a mild example of what happened to Comet Shoemaker-Levy 9 in July 1992 before the individual fragments plunged into Jupiter July of 1994). If this scenario is correct and Hartley 2 suffered a mild stretching during its 1971 encounter with Jupiter (just enough to rearrange the fragments yet still allow them to cohere in their weak gravity field with that 18-hour ‘tumbling period’), the oldest surface features may only be 39 years old! We might see that the surface has very few or no old craters or other features, since they might have been erased by their reshuffling. It could also bring lots of insight into the behavior of this comet, including the composition of the gas (cyanogen is prominent) and dust it is currently shedding.

  16. Anchor

    @12 Mike Saunders, who says, “trying to propagate information across interstellar distances would be a herculean task.”

    Stars and galaxies and even interstellar molecules do it all the time. (it = “propagate information across interstellar distances”).

    You also say, “I personally think the SETI@home people are totally insane.”

    Since we’re handing out personal opinions, I personally think that anyone who thinks that “SETI@home people are totally insane” for supposing it possible for an advanced technological civilzation to throw photons around for any purpose they may find useful might consider reappraising their own sanity. Besides incidental signals not meant for directed communication per se, it isn’t entirely sane to think it impossible for an advanced civilization somewhere might have the capacity as well as the audacity to build sufficiently powerful beacons for just such a dedicated purpose, vis a vis, simply to announce that “We are here!” However, searching our own galaxy for any signals on a star-to-star or even area-by-area basis may not be a very efficient way to go about finding any if they are there.

    Chances of detecting a signal might be very much increased by staring at a suitably nearby large galaxy – like the Andromeda galaxy – for dedicated laser beacons directed at our Milky Way: given a sufficient power, such a relatively narrow beam is much more power-conservative and has a much greater chance of being detected anywhere within our galaxy by any other civilizations that are at least as technically advanced at receiving as we are. If super-advanced civilizations in OUR galaxy have returned the favor to the Andromedans, we could conceivably receive information of their whereabouts and of the state of our galaxy as seen from the outside that is only some 5 million years out of date (round-trip communications time) – LOTS of information could conceivably be encoded as part of their running beacon signal.

    The likelihood of all of this is admittedly a tremendous long-shot, but the possibility, however miniscule, imho is an opportunity too fantastic to ignore or dismiss as “insanity”. It may be the common procedure for close groups of galaxies to contact each other FIRST as a kind of communications catalyst facilitating LOCAL interstellar communications within a particular galaxy. Who knows? There could be an ‘intergalactic web’ of communication that reaches from here to the Virgo cluster and beyond that may already have been in operation for billions of years. We’d be foolish NOT to listen for any possibility, no matter how remote it may seem.

  17. JB of Brisbane

    If anyone at Parkes is reading this – the pressure’s on, people.

  18. Oli

    @10. John Freeman: ‘cept it would take decades for the echo to arrive on Earth 😉

  19. Gary Ansorge

    13. Anchor

    Even a gamma ray laser would scatter and decay over interstellar distances though if it was powered by a neutron star,,,maybe.

    Personally, I think it’s insane to actively transmit a high powered transmission outward before we’re capable of defending ourselves from Berserkers or any other nut cases(read: before we’re a solar system wide civilization). Even prairie dogs know enough not to stick their heads above ground and make lots of noise. It’s just asking to become dinner for a hungry hawk,,,

    Gary 7

  20. Jon Hanford

    @13. Anchor

    I keep reading about the construction and operation of enormous transmitting devices(dish arrays, lasers etc) by advanced civilizations as a means to communicate over large (interstellar) distances. But I’ve read about another signaling scheme that uses ubiquitous variable stars to transmit info. A civilization might employ some technique to vary the luminosity of a star by small increments and in such a manner as to appear easily distinct from that stars’ natural luminosity variations. Info would be embedded in these non-natural pulsations, to be decoded by anyone with sufficiently sensitive equipment. Cepheids and RR Lyrae variables were specifically mentioned, but it would seem other types of variable stars might also be suitable. Heck, after reading about the latest Kepler asteroseismology findings earlier this week, I wonder if ordinary stars might be amenable to this technique. In theory, this technique of “star tickling”, as it was referred to, could be useful in intergalactic communication as well (although obviously with an immense time factor).

    Anyway, this is one of a handful of ideas for interstellar communication that doesn’t necessarily rely on giant transmitting arrays or somesuch to be constructed. Of course its all pretty much speculation at this point.

  21. Mike Saunders

    Hi Anchor,

    Yes stars and ‘molecules’ do that (molecules we detect being in pretty empty parts of space I assume you mean like dust clouds at a low K, I doubt aliens live by there).
    For a coherent signal (one we want to extract information on) we can start with friis.

    Then find the distance of 1000 light years in meters.

    The wavelength Arecibo works at for SETI is about 0.21m

    It doesn’t end up looking too good. They only integrate the signal for 5 seconds (some parts of the sky have gone over 40 times or so, so 200 seconds for that assuming CW signal).
    But really to transmit on 100s of TW levels, the aliens will be using a pulse, and I doubt it will be a high duty cycle. So if there is no pulse in those five seconds, you miss!
    Also to detect anything, it looks like the aliens will have to be firing right at us, omnidirection is almost hopeless with our setup. Why would they do that, at 1000 light years away? Even if they could take detailed photos of our planet, it wouldn’t be very exciting for them. If this communication is possible, and intelligent life is common, I’m sure they’d much rather talk with the guys who had atomic power 1Mya.

    Andromeda is on the order of 10^22 m away, and say you’re using a red laser (so it penetrates our atmosphere) at 700nm, or 7*10^-7 m, you end up with incredibly small numbers…

    It’s really difficult to grasp ‘interstellar distances’. It’s huge, so absolutely huge. I cannot really get it. This is what makes exo-planet hunting so exciting, but we’re still using information that we know for a fact is making it to the planet.

  22. Anchor

    Oh, sure, as I said, the chances of detection are no doubt miniscule. I don’t discount other methods of communication besides EM radiation. I happened to confine my comments to EM emission as an example. There are lots of known alternatives and there are probably (almost certainly, imo) methods of which we haven’t the slightest inkling. Most of the known methods suffer from a severe lack of imagination, like reverting to semaphore or smoke signal technology.

    But its rather amazing to behold with what certainty some can presume that a thousand, a million or even a billion years of technological progress somewhere in the universe can’t solve a problem like communicating between stars or galaxies, just because WE don’t know how to overcome the problem. Surely a sufficiently industrious super-civilization would be intelligent enough to compensate for the shortcomings of newly-emerged civilizations like ours which have barely entered the cosmic kindergarten: why wouldn’t they attempt to draw the attention of emergent technological civilizations with what they are likely to understand, like with EM radiation? Why wouldn’t they have tricks that exploit technologies we have no clue of? Sure its all highly speculative. Speculation is a reason to cover all the bases we can possibly think of, not a reason to declare it impossible they exist and are sending. We do not know what is impossible.

    Look at it this way: those who don’t buy lottery tickets ensure they will not win one, but that doesn’t entitle them to think that lotteries don’t exist. Somebody wins lotteries because lotteries exist and tickets are bought. Same thing with advanced communicative civilizations: they may indeed be extremely rare or even non-existent, but we can never know if we do NOT look. Besides, every search program invariably adds to our data set of natural phenomena, so the characterization of it as being an enterprise of dubious sanity is certifiably ridiculous, as all such absolute blanket statements are. A modest investment in SETI is not at all foolhardy or “insane”.

  23. Anchor

    @19 Gary 7, who says, “Personally, I think it’s insane to actively transmit a high powered transmission outward before we’re capable of defending ourselves from Berserkers or any other nut cases(read: before we’re a solar system wide civilization). Even prairie dogs know enough not to stick their heads above ground and make lots of noise. It’s just asking to become dinner for a hungry hawk,,,”

    If you bothered to read what I said, I never suggested we send any signals of any kind anywhere. THAT is, I agree, borderline insane.

    Perhaps you’re being facetious, but if not, what makes you think our material or nutrition resources are so specially tasty that those hungry monsters cannot find much MUCH closer to hand in humongous plenitude? That would be like going to a New York-area Macdonalds for lunch because you got hungry in Melbourne. Actually, the analogy doesn’t give the preposterousness of the notion the remotist justice.

    I feel a bit ashamed for my part in reinforcing this off-topic subject. I’m not going to comment on it anymore. Hartley 2 is what’s exciting here!

  24. David

    Only in astronomy is something 11 million miles away considered “close”. I love it!

  25. Fernande Haddick

    At last, got what I was searching for! I am positively having fun with reading each little bit of it. Happy I stumbled onto this article: Arecibo gives comet some radar love | Bad Astronomy | Discover Magazine. it is very safe to buy wow gold here.and I have your blog bookmarked to check out the latest article you write. Thank you for your nice articles!


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