Lost spacecraft make people crazy, engineers included. We spend millions of dollars to send these things to a very specific place, how do we just lose them? What happens if we lose things in space? If they're millions of miles away and are far too small to see with telescopes, what do we do?
Usually, lost means loss of contact. It doesn't always mean we don't know where something is. Engineers and mathematicians calculate the trajectory of everything we send into space. But those calculations aren't perfect, so if a spacecraft stops talking to us at some point, it can drift away from our calculations, which brings us back to: Space is big.
The losses range from the Soviet, Zond 3 which sent pictures of the far side of the moon for the first time, but was lost on the way to Mars; to NASA's infamous Mars Climate Orbiter which got lost because NASA's teams mixed up metric and english units; and even a Japanese probe named Akatsuki which was lost trying to orbit Venus... but, then it was found again!
And being found again is the sweetest in space. The universe is scary for little probes just trying to get where they're going. So, when we're sending spacecraft to Mars, Pluto, or even just down to the International Space Station, we give them lots of ways to tell us where they are.
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Washington Post: NASA just found a spacecraft that's been lost for two years
Scientific American: Russian Cargo Spacecraft May Be Lost in Space
Seeker: Zond 3: First to See Moon's Far Side on the Way to Mars