A good look at a near-Earth visitor

By Phil Plait | March 31, 2009 7:00 am

The asteroid 1996 HW1 is a chunk of rock over 3.5 kilometers across. Its 3-year orbit around the Sun is a little odd: it’s elliptical, going out as far as the main asteroid belt, but then dipping back in to get only 19 million kilometers or so from the Earth’s orbit. This makes it a Near Earth Object, or NEO, though not really a dangerous one. It belongs to a class of asteroids called Amors, which have similar orbits.

Orbit of NEO 1996 HW1
The orbit of 1996 HW1 is in blue. In September 2008 it dove near the Earth in its
elliptical orbit, passing about 20 million km away. Click to get lots more info.

But what does it look like? What shape is it? It’s too small to be resolved even by big telescopes, so you might think we can’t determine its shape.

Ah, but we’re clever, we evolved monkeys. Turns out, we don’t have to see it all that well to figure out its shape. I present to you the shape of NEO 1996 HW1, as determined by the good folks at the Table Mountain Observatory:

How. Freaking. Awesome. Is. That?

There’s another view on that link, looking "down" on it so to speak.

But how did they get the shape of this thing? Between July 2008 and January 2009, Michael Hicks, Heath Rhoades, and James Somers observed the asteroid over many occasions. As 1996 HW1 moves around the Sun, the amount of sunlit surface we see changes (just like the Moon goes through phases). If it were a perfect sphere, then we can predict exactly how much light we would see from it as our angle to it changes. But then, we could do that if it were, say, a cube, too! Or a banana-shape, or an axisymmetric epicycloid (a figure-8 shape that has loomed large in my life; remind me to tell you that story someday).

In fact, it’s possible, given enough observations, to determine the rough shape of any object from measuring how its brightness changes over time. This method is called light curve inversion. A light curve is the plot of brightness over time. You feed those numbers into some pretty fierce equations which determine the shape of the object. Since normally you use the shape to get the light curve, this method is the inverse of that. That’s where the name comes from.

And we know this method works, because it’s been used to predict the shapes of asteroids which were later found to be accurate when high-resolution images of asteroids were obtained!

Science! I love this stuff. Incredible.

In the animation you can see that HW1 is elongated. I also noticed that the rotation period (mentioned in the Table Mountain page) is long for a small asteroid, about 9 hours. Most small asteroids spin faster than that, which is interesting. Why so slow? I suspect it’s suffering from the YORP effect, where sunlight can slow the spin of an asteroid. This is strongest in asymmetric rocks, and HW1 is clearly not terribly symmetric, so this may indeed be the case.

This method of shape fitting is incredibly powerful. We can learn a lot from just a few observations, and of course the more data we get, and the more accurate they are, the better a fit we can get to an asteroid’s shape, and the more we can learn about them. And need I remind you, these rocks are not all safe. Some of them have orbits which do cross ours, and one the size of HW1 is big enough to cause a mass extinction. So hear me well when I say that the more we know about these asteroids, the better.

And all of this can be learned without ever leaving the comfy confines of our little blue planet! So when we do venture out to visit these interplanetary rogues, we’ll have a much better idea of what they’re about.

And that is the shape of things to come.

Tip o’ the Whipple Shield to Heath Rhoades for letting me know about his work!

Comments (36)

Links to this Post

  1. Tiny rock to buzz Earth Tuesday | Bad Astronomy | October 18, 2010
  1. Joe Meils

    Well, it’s pretty cool that they can do that… and I agree, it behooves us to learn as much as we can about these objects…but I’m really not too surprised it’s shaped like a giant potato. That seems to be the fashion for such rocks. (The only exception being one that I’ve seen imaged that looks like a peanut.)

  2. This introduces us to yet another cool mountaintop observatory:
    http://tmf-web.jpl.nasa.gov/
    Really cool astronomy happens at the end of long gravel roads.

  3. erlando

    Science just continues to amaze!

    But I’m thinking… Isn’t there an assumption here that the entire subject reflects the same amount of light from all surfaces?

  4. Cool! Had no idea we could do this.

    From the TMOA link:
    “Seventeen partial nights of R-band photometry on the near-Earth asteroid 1996 HW1 were collected”

    I was gonna ask “What is R-band? Radio?”, but I just spotted a Wiki page for it. Guess it’s visible Red.

  5. Cheyenne

    Astronomers are very smart people indeed. Just amazing what they find by analyzing the light that is sent our way.

  6. Hoonser

    It’s not April fools day yet!

  7. The model they’ve produced looks to be convex… does this technique only compute a convex approximation to the shape or is the asteroid genuinely that shape in this case?

  8. Greg in Austin

    Are you sure its not cylindrical, like that whale probe? ;)

    By the way, Phil, you need to tell us your story of the figure 8 someday.

    8)

  9. Quiet Desperation

    Love the retro CGA-era orbital graphics. :-) They plot that with an 8086 machine?

  10. “I was gonna ask “What is R-band? Radio?”, but I just spotted a Wiki page for it. Guess it’s visible Red.”

    R-band refers to using a red color filter. Its kind of a fail safe for observing when the sky is not photometric (meaning there are clouds here and there) We analyze the data using field stars relative to the asteroids magnitude (relative photometry). If the session is deemed photometric we can observe using several color filters and use calibration fields (typically landolt standards) which allows for quite a bit more information to be drawn from the observation.

  11. Gary Eller

    Clearly, that is a spaceship and we all must take measures to prevent against ‘probing.’

    (I kid.)

  12. Jack Mitcham

    I just googled “axisymmetric epicycloid,” and this page was the first hit. :-p

  13. “The model they’ve produced looks to be convex… does this technique only “The model they’ve produced looks to be convex… does this technique only compute a convex approximation to the shape or is the asteroid genuinely that shape in this case?”

    In this case the convex appearance would be a product the rendering software compensating for gaps in the data points. We were fortunate to get the number of observations we did before the object wasn’t observable anymore. If we were to take another run at it those gaps would likely fill and the model would be cleaner.

  14. XMark
  15. Gary Ansorge

    So, Phil. Was that figure eight vertical or horizontal?

    Have we any analyses of the asteroids composition? Carbonaceous or nickel/iron,,,

    Either would be SOOO useful for construction material. I can see it now, “Visit asteroid 1996 HW1. We have 130 levels of malls, with every form of entertainment known to man(or any other sophont). Come for the visit. Stay for the fun,,,Condos for sale on the outer, high-G levels. Adds by Google,,,”

    AH, to have a 300 year life span,,,

    I wonder if we could have a sanity test for settlers on the high frontier? It would be nice to be able to get away from the whack jobs.

    GAry 7

  16. an axisymmetric epicycloid (a figure-8 shape that has loomed large in my life; remind me to tell you that story someday).

    In one of those twists that makes life so interesting, looking up this shape led me to find a breakin at the city of San Antonio’s web site about public art, and alert them to it.

  17. I imagine someday we’ll send landing craft out to rendevous with NEOs like this one to set up mining operations or for free rides out to the asteroid belt, or to set up the gradual deflection methods to ease them into “safer” orbits.

  18. Shane

    While that doesn’t pose any threat to Earth, what about Mars, whose orbit it appears to intersect? Could there ever be a collision? What would a collision between an object that size and Mars do to the martian landscape/atmosphere?

  19. becky'sthoughts

    I want to thank you, Phil, and everyone posting on this blog. Thanks for the great science, the great links to other sites, and the great comments. In honor on the Internation Year of Astronomy, my 2nd and 3rd grade Girl Scouts and I are working on an astronomy badge that we are making ourselves. So far we’ve covered the earth and moon, how light behaves, and the planets in our system. One of the girls upon hearing that Pluto was no longer considered a planet said to me that she didn’t think it was fair as Pluto was her favorite planet. The fact that a 3rd grade girl has a favorite planet give me hope for the future. Thanks again for everything.

  20. Sean

    I’ve heard of schemes for traveling between Earth and Mars where a large space ship is placed in a cyclic orbit between Earth and Mars. The idea is that, you “hop on” as the Planetary Cycler spacecraft approaches Earth and “hop off” as it approaches Mars.

    Could the same be done with an asteroid like this? Could we hop-on to this or a similar asteroid and ride it out to the asteroid-belt, then hop off as it re-approaches Earth? Or do the facts of orbital mechanics mean you don’t really save any energy or hassle?

    -Sean

  21. Terryeo

    Eventually we will want to clear out hazards and potential hazards by physical means. Because, after all, when zipping in from interstellar travel, we wouldn’t want an unexpected collision. We might modify their obits slightly, causing them to crash into the sun or into a planet and modify its orbit slightly. Ideally, we might someday push Venus out a bit into a more life-friendly orbit, with a little less heat on it. Heh!

  22. Chris P

    I’m going out on a limb here and going to guess that the axisymmetric epicycloid may have something to do with rings and a supernova in the LMC.

    My question, though, is can you maintain a current loop in an axisymmetric epicycloid?

  23. chimango

    ah, memories… i remember running computer simulations (a burlisch stoer algorithm) to calculate orbits for Apollo and Amor familes of asteroids for an asignment on celestial mechanics.

  24. Hmm, Chris P, do I know you?

    Yeah, it’s the hourglass shape around the supernova, and it nearly cost me my future wife. That’s a long story.

  25. becky’sthoughts,

    One of the girls upon hearing that Pluto was no longer considered a planet said to me that she didn’t think it was fair as Pluto was her favorite planet. The fact that a 3rd grade girl has a favorite planet give me hope for the future. Thanks again for everything.

    On http://www.BookTV.org/ they recently broadcast a presentation by Neil deGrasse Tyson at American Museum of Natural History on his book The Pluto Files: The Rise and Fall of America’s Favorite Planet. Many school children were in the audience and some asked questions at the end. An entertaining and informative hour and a half program. Information here. There is a link to the video on that page.

    BookTV has some excellent teaching material. Hint, hint Phil Plait.

  26. Flying sardines

    This makes it a Near Earth Object, or NEO, though not really a dangerous one.

    Yet.

  27. Flying sardines

    Phil Plait Says:
    Yeah, it’s the hourglass shape around the supernova, and it nearly cost me my future wife. That’s a long story.

    Hey, what else are blogs for? Please tell. ;-)

  28. Luke

    @Jack Mitcham:

    Same — crikey they’re quick!

  29. Brian Smith

    I’ve heard of schemes for traveling between Earth and Mars where a large space ship is placed in a cyclic orbit between Earth and Mars. The idea is that, you “hop on” as the Planetary Cycler spacecraft approaches Earth and “hop off” as it approaches Mars.

    IANARS, but I don’t think that would work.

    The problem is, the asteroid/spaceship’s orbit might come close to the orbits of Earth and Mars regularly, but it won’t necessarily be when Earth or Mars are at that point in their orbit. Furthermore, I think you’d need the same delta-V to safely land on or orbit the cycling object as you’d need to get your spaceship into a transfer orbit itself – ditto for getting off the object and onto Mars.

  30. Pineyman

    I can’t believe no one yet has said its a bullet and added some biblical connotation…so I will:

    god’s bullet
    satan’s solution
    Lucifer’s Leverage
    Jesus’ Revenge
    The second coming

    I could go on…if I believed in it, I’ll be burning in hell soon.

    PS – Has anyone run a sim to see where this’ll be in 2012?

  31. Chris P

    You got me. It’s one of my claims to fame that I can tell everyone I went to the same graduate school as Phil Plait, I just started a few years after you graduated. That and the fact that I didn’t go out of my way to use a pseudonym should help you figure me out.

  32. Dr Bitboy

    For the world is hollow and I have touched the sky …

    Is there a shape model of this available?

    Someone said: But I’m thinking… Isn’t there an assumption here that the entire subject reflects the same amount of light from all surfaces?

    Probably yes, and any such object would be unique indeed.

    -b

  33. Jon

    Very cool! Science is indeed an amazing thing.

  34. Mang

    Neat.

    Potentially Hazardous Asteroids must get within 0.05AU iof Earth. Armor’s don’t cross Earth orbit and this one is an Armor as can be seen using the JPL simulator showing only Earth and 1996 HW1 orbits.

    The simulator shows the gap to earth seems to run between 0.13-0.39 AU. It also looks like there are alternating 14 and 79 year gaps between closest approches.

    AU/Date
    0.147 / 17 Sep 1607
    0.147 / 7 Sep 1648
    0.150 / 18 Sep 1727
    0.142 / 8 Sep 1768
    0.155 / 20 Sep 1847
    0.139 / 10 Sep 1888
    0.161 / 23 Sep 1967
    0.135 / 15 Sep 2008
    0.165 / 24 Sep 2087
    0.131 / 15 Sep 2128
    0.240 / 5 Sep 2169

    There are no so close approaches as well but if you want to use it for transport, you’ll need patience.

  35. Noel Munson

    Hey Phil,

    I’ve got a question for you.

    Why do asteroids (and comets) all seem to have shapes like shards or peanuts? Seems to me that if they are rubble piles, they’d still create showball shapes over billions of years, and if they are hard rocks, I can see pulling into bumbell shapes due to rotation when first formed, but there’d be all kinds of variation on the theme due to exotic rotations that we don’t seem to see. Impacts as well would either blast the lumps out and then backinto spheres, or simply take chunks out of a harder object.

    Is this the sort of fractal-type problem that can be seen or replicated on smaller scales? Have any in-orbit studies of liquids or semi-solids shown this behavior?

    Thanks,

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