My question is specifically WHY CAN WE NOT USE Radar to measure the distance to the Sun? What is the reason for that? Sorry if this is a lame question, I'm not an expert on these things and just occurred to me why not use radar to measure the distance to the Sun directly rather than go through all the complications of doing it indirectly via Venus and then using trigonometry to work it out. What is the reason for that?
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1$\begingroup$ It's a perfect good question. After all we do measure the distance to the other planets with radar. $\endgroup$– dmckee --- ex-moderator kittenMay 27, 2017 at 17:31
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2 Answers
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My question is specifically WHY CAN WE NOT USE Radar to measure the distance to the Sun?
By way of analogy, the path of a solar eclipse will cross the United States this summer. People are already being warned not to look directly at the Sun during the eclipse without protection. The reason of course is that looking at a partially eclipsed Sun may cause permanent damage to ones eyes. Most of that damage results from from the Sun's infrared rather than visible output. Just because one cannot see that infrared radiation does not mean it won't hurt you.
The same applies to radio antenna. While radio antenna are typically designed to be insensitive to visible and infrared, an unprotected radio antenna, like an unprotected eye, will suffer irreparable damage when pointed at the Sun.
What about a radio antenna protected by a radome? Those can be and are pointed directly at the Sun. What they see is a large body that radiates at an effective temperature well above the 5778 K of the surface of the Sun. The Sun's chromosphere can have an effective blackbody temperature in the microwave and radio frequencies in the tens of thousands of kelvins, and the solar corona, in the millions of kelvins.
This is particularly the case when the Sun is active, a one to four year long period during the Sun's eleven year solar cycle. Solar scientists aim radome-protected radio antenna directly at the Sun because the deviations from blackbody radiation at 10.7 centimeters are highly correlated with deviations at short wavelengths. Scientists use the Sun's radiation at 10.7 cm as a bellwether of the Sun's activity.
Suppose someone decides to ping the Sun with a radio antenna inside a radome while the Sun is quiet. Even during quiet periods, the Sun still outputs radiation in the microwave and radio frequencies with an effective temperature of nearly 10000 kelvins. The large output from the Sun in the microwave and radio frequencies will overwhelm that little ping, even when the Sun is at its quietest.
Once again by way of analogy, think of pointing a flashlight directly at a street light. Flashlights work great when pointed at a somewhat nearby dark patch of ground. They don't do much at all when pointed at a more remote source of light.
answered May 27, 2017 at 13:22
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$\begingroup$ This answer makes a great deal of sense to me - in particular, I can see how it answers the comment / implicit question I made below John Rennie's answer. You're saying that the Sun is so luminous over such a broad band that the power you'd need to send to it would be impossibly big, even if narrowband, to achieve a decent SNR in the return. So it would be good to quantify this answer if you can think of an easy way to do it. See also the J. Photochemistry and Photobiology paper I cite in my answer here; my understanding is that usually ... $\endgroup$ Jun 5, 2017 at 2:42
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2$\begingroup$ ... the IR from the Sun is not a problem for the eye even if you look directly at it (as discussed in my answer): the UV is the far bigger danger. The difference in an eclipse is that the pupil responds to the background light levels and widens to about 7mm diameter, thus letting in 50x more light that it usually does and so, at these times, thermal damage from IR can become a problem when the eclipse suddenly reaches the "diamond ring" phase. The UV from the corona, although small, is also a big problem during the eclipse. $\endgroup$ Jun 5, 2017 at 2:47
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The strength of the radar signal falls rapidly with distance so for objects within the Solar System we are dealing with very faint reflected signals. That isn't a problem with objects like Venus because with suitable signal processing we can extract the radar reflection from the background noise. The problem with the Sun is that it's a (very) strong emitter of radio waves and this black body background completely swamps the radar reflection.
answered May 27, 2017 at 7:22
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3$\begingroup$ It's much worse than that. There would be no problem with pinging the Sun were it merely a black body radiator in the radio wavelengths. The problem is that Sun deviates markedly from black body radiation in those wavelengths. For long wavelengths, this is about two orders of magnitude greater than that of a blackbody when the Sun is quiet, and six orders of magnitude when the Sun is active. For 10.7 cm radiation (the bellwether), the Sun appears to have an effective blackbody temperature in the tens of thousands of kelvins. $\endgroup$ May 27, 2017 at 12:37
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$\begingroup$ Interesting. However, I'm wondering whether it is also because many planets have a definite scattering interface - a hard surface, whereas reflexions from the Sun's atmosphere are distributed over a wide distance because there is no surface. The distributed reflexion could easily knock the return down 60dB to 80dB or more owing to destructive interference (unless you have a grating resonance, distributed reflexions are really small for this reason). You could partly overcome the problem you speak of with a very narrowband radar and smart signal processing, hence my thoughts. $\endgroup$ May 27, 2017 at 12:38
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$\begingroup$ @WetSavannaAnimalakaRodVance - When the two STEREO spacecraft went behind the sun, they had to plan very carefully when to turn them back on and try to talk to them. This is because the sun is so radio loud they were worried about spacecraft actually locking onto the sun and then frying their transponders. The sun is much "louder" than most of our spacecraft, so we intentionally avoid talking to them whenever they are within a ~5 degree cone of the Earth-sun line if possible. I am not sure if this affects the reflectivity or not, but the sun is very loud. $\endgroup$ Jun 4, 2017 at 17:12