Interstellar radar

I wouldn't have recalled that interplanetary radar has been a thing, but it has, and http://arxiv.org/abs/1301.0825 proposes an interstellar radar system for distant imaging. Claims no new technology is needed, just expense. Lots of expense, he estimates $20 trillion. My friend G and I had estimated a cost for Project Longshot -- an interstellar orbiter (unmanned, no return) -- of $400 billion to $4 trillion. OTOH Longshot would take over a century to reach the nearest star system and would need robust automation to match, though the uncertainties there are part of the cost (and the marginal cost of multiple probes could be lower... as low as $40 billion?) Radar could return data within my lifetime, and once built could image many nearby systems.

G likes to account for things in "war units", $400 billion or a trillion, a la the cost of the Iraq war and associated shenanigans. "We could have another war, or we could send a probe to Alpha Centauri." The radar looks expensive even in that light: $1000 per rich country person for 20 years, or $140 per global person. OTOH per capita income is $10,000 globally, so 1.4% of global GDP for 20 years. As science budgets go, very expensive; less than the global defense budget though.

I'm partly intrigued by the idea, partly amazed that there's finally a case where sending probes seems cheaper than remote observation.

This definitely calls for the Void Engineer icon and the sceince! tag.