Earth to Perform Asteroid 'Flyby', Radar imaging Options

[Hungry4info]

This is going to be a newby question, but why are the asteroids in these radar images not fully lit (full phase)? If the radar antennas bounce radio waves off the asteroids, wouldn't the detector and the light source effectively be in the same place?

Secondly, I'm surprised Arecibo is to be used during closest approach. I thought that the asteroid was too near Earth for the telescope to be used, due to the time it took for it to switch from sending to receiving.

[john_s]

The answer is that radar images don't show the appearance of the asteroid in the plane of the sky, like a conventional camera. They are "delay/Doppler" images which means that the vertical distance in the image is the distance from the radar antenna (with the closest point to the antenna at the top of the image), and the horizontal distance shows the Doppler shift, which tells you the distance of each point from the spin axis of the asteroid.

So essentially you're looking at the asteroid from a direction perpendicular to the radar direction, in a plane that contains the spin axis, and with the asteroid being transparent so you can see both the "top" and "bottom" hemispheres simultaneously, superposed on each other.

John

[Hungry4info]

Interesting! Thanks! Much appreciated.

[Phil Stooke]

John's right, of course, but the same radar data could be used to create plane-of-sky images. Typically we do this for spherical objects like Venus (the most frequently imaged example)... and what you get is an image with north-south ambiguity and a blurry equator. For irregular objects like asteroids we don't know the plane-of-sky shape without further work, and the process gets too messy to work well. But plot it in delay-doppler coordinates and you get images like these, with the northern and southern sides superimposed. They are perfectly superimposed if the object is symmetrical, but in cases where the object is not symmetrical in delay-doppler space - like an elongated object tilted with respect to the incoming radar - most of the image is made only from one face of the object and there is only limited ambiguity around the sub-radar point.

Phil

[Astro0]

Here's a tweaked six frame animation of the asteroid rotation, a little larger and slower than the official version.


Click to animate.

[dilo]

This is my attempt to improve animation through de-pixalization (Gauss filter) and height reduction (better aspect ratio)

Thanks to Astro0 for source animation!

[Paolo]

two interesting releases on YU55:
Arecibo radar obs and Herschel i/r obs