[X] My Assistant

[Jyril]

arXiv.org preprint: Characterization of the near-Earth Asteroid 2002NY40

Using the Maui Space Surveillance System a group of astronomers has managed to image the near-Earth asteroid 2002 NY40. The adaptive optics images are stunning and certainly the best visible light images of an NEA ever taken.

[Toma B]

OK .... so where can I see those "best visible light images of an NEA ever taken".

[jamescanvin]

Figures are at the end of the preprint - see Figure 2, page 22.

[Jyril]

Yes, sorry about that.

Fraser Cain of the Universe Today has written a nice plain English news piece of the asteroid: The Triangle that Skimmed Past the Earth: Asteroid 2002 NY40

Getting these high resolution images of the asteroid was actually pretty difficult. At its very brightest, closest point, when Asteroid 2002 NY40 could be seen at the greatest resolution, it was moving very quickly across the sky - it covered the diameter of the Moon every 6 minutes, zipping across the sky at 65,000 km/hour. The background stars changed with every image. Instead of calibrating against a single set of background stars, the researchers had to calibrate each image against whatever images happened to be in the field of view at the time.

That's pretty fast!

[Jyril]

...the best visible light images...

Read: optical. They're of course in IR images.

[Toma B]

Wow! really great images.
I wonder what HST images of "2002NY40" would look like...guess I never know...

[edstrick]

Uh.. did anybody notice that that article nowhere mentions the diameter of this rocklet? <or did I miss that?>

[jamescanvin]

There is a 1km scale bar on the images - longest dimension is about 1km

[tedstryk]

During its close flyby, it was probably moving too fast for HST to track it well enough to do nearly as well - if you look at the ACS lunar shots, you see the same thing.

It is exciting to have these new views, in addition to the radar views we already had.

[Toma B]

...in addition to the radar views we already had.

Is that the same asteroid?

[djellison]

Yes - 2002NY40

[ugordan]

Sooo.... it's a shape-shifter? One minute it's a peanut, the next a triangle?

[tedstryk]

Sooo.... it's a shape-shifter? One minute it's a peanut, the next a triangle?

If you look at the first radar image (the small one), it matches up relatively well with the groud-based shape. I think the infrard people had the misfortune of catching it when one of the lobes was hidden.

[ugordan]

Didn't they in fact say they were lucky to catch the asteroid when it presented the largest side to us? Wouldn't that be some of the other radar views for example?

[tedstryk]

Those images were taken from Hawaii. There is a good chance that by that point it had dropped below the Horizon (or hills) at Arecibo. So it may be why Arecibo only got that angle from further out.

[Phil Stooke]

No, folks, radar works completely differently that the IR images.

The radar images are presented in delay/doppler space. The radar signal is coming in from the top of the image. Earth is way off to the top. the Y axis of these images is distance from earth computed from the time delayof the returned signal. The x axis is doppler shift, operating as a surrogate of distance sideways from the rotation axis. The image is a composite of echoes from (approx) the northern and southern hemispheres superimposed.

Probably the only way to compare the images is to think of the bright earth-facing 'limb' as a topographic cross-section along the long axis of the IR image. The dark central bit of the IR image is a shadow cast by one lobe of the object on the other. The IR suggests that one lobe is wider than the other in the direction perpendicular to the radar image plane (the Z axis of the radar, not resolved in radar images).

Phil

(edited to add a bit more)

So these two types of image, both made by looking from Earth to the asteroid, are presented completely differently. The IR image is in a plane perpendicular to the line of sight, AKA the "plane of the sky". The radar image, even if made simultaneously, is presented in a different plane perpendicular to the sky. The radar image plane contains the line of sight from Earth. You can rotate that plane around the line of sight until it contains the rotation axis(*). Then the image plane is the plane perpendicular to that second plane but also containing the line of sight. Sorry if that sounds complicated, but play around with planes and lines and you'll get it.

(*) Just to complicate matters, it isn't really the rotation axis we need, it's the "instananeous rotation axis", a line which appears to be the axis of all motion of the body as seen from Earth. Some of the apparent motion of the body is caused by changing view directons as it speeds past Earth, so the intantaneous axis includes that motion-induced component.

If the two axes of the radar image are correctly scaled (a big IF), the image looks like an optical image would look, if the object was viewed perpendicular to the image plane. Except for the north-south ambiguity!

[tedstryk]

I had made the assumption that the smaller lobe was behind the larger one in the infrared image, not in its shadow. If that were the case, it still wouldn't have shown up on radar.