21,000 light years --- that means that this signal will take 21,000 years (now only 20,966 years) to get there.

If somebody's there listening, can understand this message, and then reply, we ought to get the reply in around 41,966 years or so.

http://rr0.free.fr/DrakeFrankD.html

There is an interesting optimism involved here -- the assumption that humans will be around, listening for the reply, in 42,000 years.

Note that in Contact, the aliens are only about 25 light years away, so the response could happen in one person's lifetime.

So why did they send their signal to a globular cluster 21,000 light years away?

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