[X] My Assistant

[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.