![]() ![]() The speed of the wavefronts does not change, because the speed of light is constant, so the wavefronts end up being packed closer together. As the broadcast radio wavefronts hit the part of the asteroid that is moving toward us, the asteroid smacks into each wavefront faster than it would if it were not rotating. Actually you don't have to imagine it, I've drawn another horribly oversimplified cartoon.Īs an asteroid rotates, some parts of it are moving toward us, while other parts are moving away. Imagine a set of waves propagating toward a rotating body. It takes advantage of the fact that everything in the whole solar system is rotating. All we know is how strong the return signal was with respect to time. One thing we can't do is figure out which reflections were coming from which parts of the asteroid. This will not be a particularly accurate estimate, but it's a start. Then double that, assuming the body is quasi-spherical and has a hidden hemisphere behind the hemisphere we can see. Take the amount of time that separates the first and last reflections, multiply it by the speed of light, and you get the distance between those two points. The last reflections come from the most distant parts of the object that you can see. The first reflection comes from the nearest parts of the object. You can see how you could use these data to crudely estimate the size of the object you were looking at. So when the radio dish detects the return signal, the sharp signal has been spread out in time. It's reflected from the parts of the asteroid that are closest to the radio dish first, but while those first reflections are happening, the radio wave is still propagating toward more distant parts of the asteroid. Here's a very simple cartoon that I drew, grossly simplifying what happens when you broadcast a signal at a lumpy object.The signal goes out as a nice waveform. That's RADAR, which is an acronym for Radio Detection and Ranging.īut we can do better than that. Use a precise clock to time how long it takes the reflection to return to the antenna, and you know very precisely the range or distance to the target. ![]() ![]() The simplest sort of radio "imaging," then, is just radio ranging. The signal reflects from the object, and the antenna waits for the return signal. To begin with, imaging of any kind done with radio telescopes (or radio antennae on spacecraft) is an active technique: the imaging requires that the antenna first broadcast a signal at the object of interest. This is a repost of an article I wrote in April 2010 I thought it'd be useful reading for those of you interested in today's near-Earth flyby of asteroid 2005 YU55.Įvery time I post a radio telescope image of a near-Earth asteroid, I get at least one reader question asking me to explain how radio telescopes take photos, so I'm hereby writing a post explaining the basics of how delay-Doppler imaging works. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |