# NASA primer on YU55

By Phil Plait | November 7, 2011 3:42 pm

I wasn’t going to post anything else about asteroid YU55 until the images became available, but NASA just put up a video explaining a little bit about how they plan on getting observations of the sucker as it flies past us over the next couple of days:

Just to clarify a bit: those radio telescopes can be used like radar guns, sending out short pulses of focused radio waves. These pulses are aimed at the asteroid and move at the speed of light, hitting the rock and bouncing back. Since we know the speed of light very accurately, we can measure the time it takes a pulse to get to the asteroid and back, multiply it by the speed of light, and get the distance (for example, if it takes 5 seconds, and the speed of light is 300,000 km/sec, that means the pulses traveled 1.5 million km round trip… so don’t forget to divide by 2 to get the distance to the rock).

But there’s more! The individual pulses can be timed very accurately as well, so that the shape of the asteroid can be determined, too. If there is a bump on the asteroid, like a hill, then a pulse hitting that won’t travel quite as far as a pulse that hits a crater. It gets back sooner, and this can be measured. The spatial resolution of this method at the distance of YU 55 will be about 4 meters, so they’ll be able to make an image that’s about 100 pixels across of it.

[UPDATE: In fact, this image here of YU55 was released like two minutes after I originally posted this article. It was taken using the Goldstone radio telescope in California on November 7, when YU 55 was still 1.4 million km (860,000 miles) from Earth!]

[Update 2: Emily Lakdawalla wrote a great discussion of all this on her blog last year; I highly recommend reading it!]

Not only that, but the wavelength of the pulses are very accurately known, too. If the asteroid is spinning, then the wavelengths of the returning pulses will be altered, like in the Doppler Effect. So all in all, we can determine the rock’s size, distance, shape, and rotation, just by painting it with radar.

That’s pretty good for a species that doesn’t even have to leave the ground. Still, we’d learn a whole lot more by actually going to these things. And we’ve done that, too. See Related posts, below.

We’re clever, we humans, when we want to be.

Image credit: NASA/JPL-Caltech

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CATEGORIZED UNDER: Astronomy, Science

1. Graham Molyneux
2. Martin

Hi, Phil. Love your blog. It’s my number one source for all things astronomical.

I was wondering if you, or one of your many readers could explain to me, why the pictures of the asteroid produced by timing radar pulses, look like the object is lit from one side, and the other side is cast in shadow? If I understand the technique correctly, what we’re seeing is actually an echo of sorts, so why does it appear lit from the side?

Maybe I’m just missing some obvious piece of the puzzle.

3. Chris

OK this has been bugging me. That picture of the asteroid is from the radio telescope. Why does it look like the sun is illuminating it from the side, shouldn’t it just look like a ball since the radio waves come from earth and look at it? Maybe I’m missing something here, or maybe it’s just an oddly shaped piece of rock creating an optical illusion, but which is it?

4. Chris Winter

YU55: It’s a pass we can live with (200,000 miles distant.)

5. Nanobot101

ASTRONOMY, IS SIMPLY……AWESOME!

6. Damir

Great article, Dr Plait! One thing in particular that gave me goosebumps is the “We’re clever, we’re humans, when we want to be” Just awsome!!

7. Chris: Actually, that’s a great question. The picture was made by delay/Doppler imaging. The telescope sends out a pulse, and it comes back both stretched out and altered in frequency by different parts of it reflecting off different parts of the asteroid. The vertical axis here is actually delay in return of the pulse, and the horizontal axis is the Doppler shift in the returned signal.

The delay is proportional to distance from the antenna, and since the asteroid is rotating, the Doppler shift depends on distance along an axis perpendicular to the direction to the antenna and to the axis of rotation. So a delay/Doppler plot is sort of like a picture looking down from above one of the asteroid’s rotational poles (actually, it’s a superimposition of images looking down from *both* poles, with one mirror-reversed, and if the rotation of the asteroid is unknown you don’t even know which one).

The asteroid seems to be illuminated from above because that’s actually the direction to Earth!

8. Maria

Chris, I got curious after reading your great question on the last yu55 post. I ended up finding this link that was extremely helpful to me. I know very little about radar and Doppler and so I love the fact that the author took the time to draw little animations. 😛
http://planetary.org/blog/article/00002462/

9. Elgarak

Wouldn’t the Doppler-effect not also tell the velocity of the sucker relative to us?

10. Joseph G

@#7 Matt McIrvin: Thanks! I’ve been wondering about that, too, ever since I saw that original, less detailed, radar image. So it’s sort of like a synthetic aperture sonar image of the seafloor, then?

11. Almost-certainly-stupid question: why can’t we use the Hubble to get a really good look? Too close a target?

12. Chris

@7 and 8
Thanks for the answer. Learn something new everyday. As one of my professors once said. “The really interesting information is in the footnotes”

13. @Elgarak: Indeed it would.

14. Pete Jackson

Emily’s blog from 2010 does a great job of explaining how you get a two-dimensional radar image of an object using a single antenna. But the dimensions are return time (i.e. distance from us) and return frequency (rotational speed of a portion of the object). However, that does not allow us to determine a two-dimensional image of an object as it looks in the sky where the dimensions are (say) latitude and longitude. That is because more than one part of the object can lie at the same distance from us and have the same rotational velocity. Imagine if you were on the Moon using a small radar antenna to bounce signals off the Earth. You can see that a locus of places stretching from the northern hemisphere to the southern hemisphere would have the same distance from us and the same rotational speed.

Because of these overlaps, the resulting radar image has the ‘quarter moon’ look where the bright ‘top’ represents the portion closest to us and reflecting radar waves the best whereas the dim ‘bottom’ represents the limbs of the object from the receding side to the approaching side. I remember such maps of Venus from the 1960s when radar was used to first observe planets, and only a single dish was used. You could see tantilizing structure in the radar image, but you couldn’t directly translate it to how Venus looks in the sky.

The trick to getting a true latitude-longitude image is to use a second antenna to receive the return signals.Then you can separate the returns from different parts of the object using phase delays just as in radio astronomy using interferometer arrays like New Mexico’s Very Large Array. That’s why the will be also using the Goldstone antenna this time as well as the Arecibo antenna.

15. To Chris, if the bottom of the asteroid is splintery cinder wouldn’t it obscure the radio signals in the same way the stealth bombers scatter radio signals by bouncing the radio signal away from the base unit emitting the waves so there is no reception?.

16. Messier Tidy Upper

@ ^ Pete Jackson : Good explanation there. Thanks.

Thanks BA!

Excellent news & can’t wait for more good science to come out of this opportunity – amp; good to see this rock that’s caused such a fuss..

..well amongst some folks anyhow.

Thought we were long past the days when uninformed, guillible folks were unduly worried by cometary and asteroidal close passes. Sigh.

17. Joseph G

@14 Pete Jackson: Thanks for elaborating on that. Interesting stuff

18. Nigel Depledge

Harry Johnston (12) said:

Almost-certainly-stupid question: why can’t we use the Hubble to get a really good look? Too close a target?

Probably too fast-moving a target.

19. Pete Jackson

@19 Nigel: Hubble’s finest resolution at the distance of the asteroid at closest approach would be about 160 meters, almost as large as the object itself! The radar will do much better with its 4 meter resolution.

20. Benudhar Sahoo

Amazing picture. It is so far from our earth…. Thanks God

21. Marco

I was going to ask about making a series of images to create an animation of the object rotating, but the Planetary Society link has just that at the very bottom. Is it known whether NASA is planning on doing this for YU55?

If so, based on image processing of this series of images, assuming taken in “rapid” succession, couldn’t the shape of the asteroid be sufficiently estimated to create something like an “actual” picture (or perhaps a 3D model?)

22. DJ

@20 Pete Jackson and @19 Nigel Depledge

Sounds like you’re both right. From a NASA article on using Hubble to observe the moon:

“The Apollo descent stages left on the lunar surface are too small to be seen by Hubble, which can see objects as small as 60-75 yards, about three-quarters the length of a soccer field. The left-behind descent stages are only about the size of a small truck.

These observations weren’t easy. The moon is a difficult target for Hubble because it moves across the sky faster than Hubble can track it and is very dim in ultraviolet light. The observations required steady, precise, as well as long exposures to search for the resources.”

http://www.nasa.gov/vision/universe/solarsystem/hubble_moon.html

23. I was just wondering….In the images of the asteroid passing the earth it is moving in a straight line, would there be gravatational affect on this object so that as it passes earth wouldn’t it bend a little towards earth?

24. kevbot

I was just wondering. The animation that begins ten seconds into the video shows the relative motions of the asteroid and the moon. Considering that YU55 will transit the sky in ten hours (I hear), it strikes me that the moon is moving a lot faster than I would expect. FWIW.

25. Peter B

Haze @ #24: Yes, the orbit would be bent slightly, but too little to be seen at this scale – even a change of 100 kilometres wouldn’t be visible in the animation, and the effect of the Earth on the asteroid’s orbit would be much less than that. Keep in mind that both the Earth and the asteroid are orbiting the Sun, so both their orbits are curved. But again, on this scale, it’s essentially too slight to see.

26. Nigel Depledge

Pete Jackson (20) said:

@19 Nigel: Hubble’s finest resolution at the distance of the asteroid at closest approach would be about 160 meters, almost as large as the object itself! The radar will do much better with its 4 meter resolution.

Heh. Who’d have thought the asteroid was too small and too far away for Hubble to get a really good look, without doing the maths?

Sometimes I forget how huge the nebulae are that are Hubble’s showpieces.

27. agilmore

YU55 fake images?

The radar images appear to have a shadow, as if from a light source. If so, they cannot be radar images. If they really were radar images, there would be no shadow since the radio waves would be transmitted and received from the same point.

These so called radar images show an object that has the appearance of a half moon. This means the illumination is orthagonal (90 degrees) to the object. That is impossible with a radar image.

Why would NASA try to pull something over us?

28. Kevin Blair

high def photos of the comet http://bit.ly/ukBz1T

29. Peter B

Agilmore @ #28 asks: “YU55 fake images? The radar images appear to have a shadow, as if from a light source. If so, they cannot be radar images. If they really were radar images, there would be no shadow since the radio waves would be transmitted and received from the same point. These so called radar images show an object that has the appearance of a half moon. This means the illumination is orthagonal (90 degrees) to the object. That is impossible with a radar image. Why would NASA try to pull something over us?”

Before using words like “fake” and “impossible”, you might like to read comments 7 and 8, and the link to http://planetary.org/blog/article/00002462/

30. Peter B

Kevin Blair @ #29 said: “high def photos of the comet http://bit.ly/ukBz1T

Are you sure? I got a link to something about space tourism.

31. agilmore

Peter,

If true, why don’t they open the bandwidth on the discriminator so we can see the whole asteroid.

32. Peter B

Agilmore @ #32 said: “Thanks for the reply.”

You’re welcome.

“If true, why don’t they open the bandwidth on the discriminator so we can see the whole asteroid.”

I have no idea. I don’t know how this technology works. I was merely pointing out that others had effectively answered your question before you’d asked it.

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