Can Radar Be Used to Map Distant Planets?

[Vinni]

Would it be possible to use a very powerful k band radar signal to bounce off one of the rocky planets discovered orbiting other stars? If that is possible could we use it to build a radar map of the planet?

 

[russ_watters]

Those planets/stars are much too far away for that to be possible, in terms of signal strength, resolution and length of time to hear the return radar echo.

 

[Drakkith]

1. The power output required would be astronomical since the radiation from the radar would spread out as it propagates.
2. The speed of light is finite. The closest star to us would be an 8 year round trip for the radar. The nearest exoplanet would be around 20 years.
3. The size of the dish would be enormous, however interferometry may be able to reduce the size by using multiple smaller dishes. Either way it would be incredibly expensive.

The sort version is that it simply isn't feasible.

 

[Drakkith]

Those planets/stars are much too far away for that to be possible, in terms of signal strength, resolution and length of time to hear the return radar echo.

Heh, imagine walking into a cave and yelling "ECHO", and having to come back 20 years later to hear it. It would make for a great family event!

 

[Vinni]

Heh, imagine walking into a cave and yelling "ECHO", and having to come back 20 years later to hear it. It would make for a great family event!

I understand that it would take years to get the echo back, but I was thinking about how much power it would take to make possible? Also would a MASER be more efficient approach? I mean if one could generate the power?

 

[Bobbywhy]

Just one more reason bouncing radar off a rocky exoplanet is not a viable idea: every radar target has a "target strength" based on its reflectivity and orientation. For example, if a transmitted radar pulse strikes an aircraft carrier "broadside" then lots of RF energy would be reflected back to the radar for detection. If, however, in that same setup if the carrier is orientated "bow on" much less energy would be reflected because the "radar cross-section" is smaller. Then there is the material itself: stealth technology uses materials that do not reflect RF well, like a rocky exoplanet.

 

[Drakkith]

I understand that it would take years to get the echo back, but I was thinking about how much power it would take to make possible? Also would a MASER be more efficient approach? I mean if one could generate the power?

Using standard radar cross section math, and assuming I did it correctly, a 1 megawatt radar signal sent from a dish the size of the Arecibo Radio Telescope to an Earth sized planet 10 light years out would receive 2.6x10^-46 watts back. That's about 40 orders of magnitude less than what you would need to effectively see the target. That's a trillion trillion billion billion billion billion times less than what you would need.

 

[rcgldr]

For comparason, lasers can be fired at mirrored arrays (retro reflectors) placed on the moon back in the 1960's and 1970's. Only a very small amount of light is returned. Even though these mirror arrays are small compared to the size of a distant planet, I assume the subtended angle for a mirror array on the moon is much greater than a large planet 20 light years away from the earth.

http://www.spacetoday.org/SolSys/Moons/TheMoon/Retroreflectors.html

http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment