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Comment by csours | Hacker News Reader

Why can't we just shine some radar out there?

When light travels, it spreads out as it goes, making it less powerful for a given area. This is the famous inverse square law.

Radar decreases the same way. However, radar is even worse in that it also decreases on the way back to the sender.

So if you double the distance to the target, the radar signal coming back is 1/16 as strong. If you increase the distance by a factor of 1,000, then the signal coming back is 1/1,000,000,000,000 as strong. And so on...

This makes radar a pretty poor tool for space exploration.

And it makes it amazing that we actually did use Earth-based radar to study the planet Mercury. Before the Messenger spacecraft, some of our best science about Mercury came from Arecibo radar observations.

For an interesting outline of this, see:

http://www.naic.edu/~pradar/radarpage.html

Title: "Solar System Studies at Arecibo Observatory"

Subtitle: "Introduction to Planetary Radar"

Arecibo is a radio telescope, not radar. It receives radio waves transmitted by objects in space, but doesn't send and bounce them off objects.

Arecibo is and has been used for radar astronomy as well https://en.m.wikipedia.org/wiki/Radar_astronomy

Whoa, I had no idea - thanks! Should have mentioned that IANAAstronomer.

radar lasers

It still requires a significant amount of power, and besides, now your trade-off is time - you're shining a point (kind of, it still spreads) very far away, so you have to move your laser very slowly in order to actually put out enough signal for a bounce-back from any one particular hypothetical object.

we can hardly shine more powerful radar than the one that is already shining - Sun.

I assume this is ironic, but if not, of course we could, if we had millions of years to take observations (and an enormously powerful source of EMR).

Because sonar would work just about as well.

Might get the attention of the Borg

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