"SpaceX has this capsule that takes humans into space, but there is a bottom part … so when the astronauts come back, they leave the bottom part in space before the capsule lands."
The spacecraft, which costs $62m per launch, has started to deorbit after almost two years in space.
Tucker said the craft was originally planned to break apart and land in the ocean.
"We saw most pieces land in the ocean, but clearly some hadn't because this 3-metre piece was speared into the ground from space," Tucker said.
"In photographs of the debris, you can clearly see charring, which you would expect from re-entry [into the atmosphere]. It is very rare to see because they don't usually land on land but in the ocean. People often think they find small pieces of space junk, but they would burn up on re-entry, so it's more likely to be large pieces like this."
The spacecraft, which is a stainless steel rocket, was more than 50m tall.
SpaceX is an American aerospace company founded in 2002 by Musk that helped usher in the era of commercial spaceflight.