Given we know how long it has been 40 years away, 20 years away, and now 5 years away, we can estimate how long it will be until it is 0 years away. Unless it

Given we know how long it has been 40 years away, 20 years away, and now 5 years away, we can estimate how long it will be until it is 0 years away. Unless it goes down by a power of 2 each time.