By timing how long the signal takes to make the return trip, the instruments can determine differences in height.

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If you're living in a country where you have to walk for several hours or even a day to get to a clinic it's extremely unlikely you would bother to make the return trip a week later to get the result of a check-up.

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By the time of the return trip, the two sides may be in position to make further progress.

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Plaything for our future robot overlords? (Let's hope it's not the last one.) The success of the mission will depend on a couple things, namely: how the return trip goes (it should make it back to California's Vandenberg Air Force Base via autopilot... sometime) and whether the thing can be re-launched quickly enough.

ENGADGET: Boeing X-37B autonomous space shuttle launched last night, due back 'whenevs'

If someone wants to return something, does he have to fill out 12 forms, make a trip to the post office and spend money for shipping?

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