A Swiftly passing asteroid

By Phil Plait | November 11, 2011 10:00 am

The eyes of many astronomers — and the eyes of their telescopes — were aimed at the asteroid 2005 YU55 a few days ago, when it passed the Earth at the relatively close distance of about 320,000 km. One of those eyes was actually in space as well: NASA’s Swift satellite. This spacecraft was designed to look at the sky in the ultraviolet, X-ray, and gamma rays, all high-energy forms of light emitted by the most violent events in the universe: exploding stars and gamma-ray bursts.

But the Sun emits UV, and rock can reflect this light, too. So Swift observed YU55 as it passed us, and got this very interesting footage of it, what I think is the coolest I’ve seen so far:

[You can also watch it on YouTube if you prefer.]

Pretty neat, and by looking at the rock at different wavelengths, we can learn about its structure and composition, too.

But I know what you’re thinking: in the video, why did YU55 curve around as it moved? As it turns out, I was expecting this when I watched the video! What’s going on*?

The asteroid is in elliptical orbit around the Sun, but over the short period of time covered by this video — about 20 minutes — it’s essentially moving in a straight line. The reason the path is all bendy is because Swift itself is in motion! Swift orbits the Earth, circling us once every 90 minutes or so. As it moves around us, its viewpoint is changing, and that motion is reflected in the asteroid.

Try this: look at some distant object like a mountain or tree, hold your thumb at arm’s length, close one eye, and then move your thumb to block the object. Now, holding your thumb still, move your head in a small circle. Even though your thumb isn’t moving (that may take practice!) it looks like it’s making a circle too. That’s because the angle between your eye, your thumb, and the distant object is changing as you move your head. This is called parallax, and I’ve explained this in an earlier post for a solar eclipse that had very similar circumstances as this Swift observation. I’ve written a general parallax description as well.

So the wiggly path of the asteroid is not from the asteroid, it’s from Swift itself! That’s pretty nifty. I’ll note the first time I saw this for myself was when a satellite called TRACE observed the transit of Venus across the Sun. I had to do the math to prove myself it was the satellite’s orbit making Venus move in a squiggly path, but it all worked out!

And there’s a lesson here: with astronomy, what you see is not always what you get. Pictures by themselves don’t tell you everything; you have to understand the instrument that took the picture. I spent years working on Hubble doing just this, trying to understand just how our camera on board the observatory was behaving. If you see two stars in the image and one looks brighter, how do you know the camera isn’t more sensitive where the brighter star happened to be? Or maybe that star is red, and the detector is more sensitive to red light. Or maybe the filter you used happens to let through light in a different way than you expected… or maybe it’s all three things!

Yeah, it’s complicated. And that’s why you always have to ask yourself, is what I’m seeing real? In this case, it is real, but not for the reason you expect. Feel free to extrapolate this to your own life in any way you see fit.

Credit: NASA

* Mark my words: someone will claim YU55 was under intelligent control. I can practically guarantee this. If you find a video or blog making that claim, please let me know!

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (23)

  1. Gwif

    I understand the concept of parallax and what you’re saying here, but I question you saying that “it’s essentially moving in a straight line”.

    Isn’t the asteroid passing close enough to the Earth to have it’s orbit affected by our gravity? Wouldn’t that mean that it’s path as it passes us would bend a bit more? (or less, I guess)

  2. BAT

    “NASA captures footage of alien UFO! Covers it up as YU55 Asteroid!”

    C’mon, you KNOW that’s going to be a headline somewhere.

  3. @1

    Do you know that a straight line mathematically speaking is the particular case of a circumference with infinite radius?

  4. DMPalmer

    It’s a matter of degree. The asteroid is moving close enough to a straight line that Swift’s motion dominates.

    The asteroid is comparable to the distance to the Moon at that point. The Moon’s path is curved enough that it goes around the Earth once a month, and the asteroid is moving much faster so that it turns even less. Swift’s path turns around Earth every 90 minutes or so.

    If you plotted a perfectly straight path and the asteroid’s path and the Moon’s paths seen by Swift, you could see that they are different, but they would all have the same general shape.

  5. Cool!!! My professor and I got pictures of YU55 with the GRAS observatory in New Mexico! Check it out at http://www.antifacts.com


  6. Pete Jackson

    @3 Gonçalo Aguiar:

    And this is a comment with infinite profundity…

  7. @ Gwif (#1)

    The Swift observation lasted 25 mins. According to my calculations, 2005 YU55 was travelling at ~49,400 kph, so it would have covered ~20,580 km during this time, which is about 0.002% of the total ~952 million km of its orbital path around the Sun. Furthermore, given its orbit is quite elliptical (e = 0.43), and that it was travelling along the “long” part of the ellipse, it’s fair to say that its path was essentially a straight line during the Swift observation.

    As for the asteroid having its path affected by the Earth, we’ll have to wait a few days for further observations to calculate its new orbit and figure out how much the change was. In any case, the Earth would be changing the orbit via gravitational force, that’s essentially tugging the asteroid with a vector pointing straight towards Earth, towards *us*, so (on this video’s small timescale) the projected path of the asteroid against the celestial sphere would not change and we would still observe it flying in a straight line, even if its 3D motion were affected. Swift’s orbital altitude is ~600km, so it would essentially “see” the same thing. I hope this made sense.

    Yes, I am assuming the change in its trajectory due to the Earth’s mass is small, and I’m ignoring the Moon, and also assuming the parallax due to Earth’s rotation over 25 mins to be very small, not to mention I left out the Yarkovsky effect…

    Sorry for the Geek out! :-)

    —JL Galache
    Minor Planet Center

  8. Georg

    This is damn lies! You know that sun/moon/planets go around in cycles and
    epicycles and epiepicycles around the flat earth!
    That is what the path of that asteroid is made of.

  9. I just checked our ephemeris data for 2005 YU55 and it’s already updated. The largest relative change is the the inclination of its orbit. At the same time, its perihelion distance has increased slightly, which will bring it a little closer to Venus’s orbit (on the “inside”). Its aphelion has also increased, taking it further away from Mars’s orbit (on the “outside”). It’ll be interesting to see what happens with 2005 YU55’s orbit when it swings by Venus in 2029.

    For some serious orbit-changing dynamics, check out the close encounter we had with 2011 MD back 0n June 27th:


    —JL Galache
    Minor Planet Center

  10. brett

    In the little inset of the asteroid (which i presume was a magnified view) there seems to be little bits of light ‘orbiting’ the asteroid? does this asteroid have little satellite objects (would have thought it too small)or more probably this is some distortion artifact?–cheers brett

  11. Messier Tidy Upper

    Wow – they sure got that out swift~ly! 😉

    (Sorry couldn’t resist.) Actually they really did – the speed at which stuff like this comes out these days impresses me. As does being able to watch things like spacecraft launches and planetary and asteroidal encounters live from the other side of the planet on a computer as good as a TV.

    Great video and write up – thanks BA & the Swift team. :-)

  12. Jamie

    As the the guy who spent the better half of an afternoon writing code to figure out the best time and place to observe this with Swift, I’d just like to say thanks for the great write up!

  13. Mike

    Glad to see this get attention. It was a difficult observation. Our SOT lead calculated Swift’s motion and the asteroids’ to get it when the combined relative motion would be minimized AND it wasn’t too close to the moon, Sun or Earth limb. We can repoint Swift literally in minutes so once we knew when and where to look, it was a snap.

  14. Messier Tidy Upper

    @ ^ Mike & #12. Jamie : Congrats and thanks again – great work. :-)

    But SOT = ??? Please.

    @8. Georg : I take it that’s a Poe?

    @9. JL Galache : Thanks for that informative comment. Cheers! :-)

  15. Joseph G

    Pffff, parallax my butt. The “asteroid” is jinking to avoid our lasers. It’s the only explanation that makes sense!

    @#7 JL Galache: Yes, I am assuming the change in its trajectory due to the Earth’s mass is small, and I’m ignoring the Moon, and also assuming the parallax due to Earth’s rotation over 25 mins to be very small, not to mention I left out the Yarkovsky effect…

    Also the perturbation caused by the planets. And galactic tides. And the Poynting–Robertson effect 😀

    /just loves a good geekout

    @#9 JL Galache: Cool! It’s good to know that there are people keeping tabs on these things :)

  16. Mike

    SOT = Science Operations Team. Jamie, who posted above me, is the lead.

  17. Messier Tidy Upper

    @ ^ Mike : Thanks. :-)

  18. Joseph G

    I think it’s really cool that astronomers were able to use an instrument evidently not designed for this sort of thing (made to image high-energy objects) to take these images.
    It’s some real scientific MacGyvering :)

  19. Jamie

    Joseph: Its a really a testament to how Swift, although it was designed to follow-up GRBs, is actually an extremely versatile observatory for all aspects of astronomy (not to blow our own trumpet of course!) In fact one of the guys who is working on these data recently described Swift in a talk as “the perfect mission for observing comets”, which is not something we would ever had thought of in the pre-launch days.

  20. Joseph G

    @19 Jamie: You work on the Swift project? Awesome! And congrats!
    Please, blow your trumpet all you want. You’ll certainly have an appreciative audience here :)

    Regarding using it to view comets, is that because of Swift’s rapid slewing capability?
    I can’t remember where, but somewhere I read that Hubble and certain other orbital telescopes simply can’t “keep up” with some inner solar system objects.

  21. @15. Joseph G:
    Cool! It’s good to know that there are people keeping tabs on these things

    We’ve got your back, Joseph—that’s our job at the MPC! 😉

    —JL Galache
    Minor Planet Center

  22. Prashant

    Cool! I thought it’s earth’s gravity that made asteroid path to bend…

  23. Nigel Depledge

    JL Galache (21) said:

    We’ve got your back, Joseph—that’s our job at the MPC

    Wow! Does this mean you’ve got a massive great laser (in the terawatt range) pointing at the sky to take out anything that comes our way?



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