Crystalline emiristol
From Memory Alpha, the free Star Trek reference.
Crystalline emiristol was a type of low-technology solid propellant used as a fuel.
The Kataan probe, launched from Kataan in the 14th century, used crystalline emiristol as a propellant. This left behind a radioactive trail that the USS Enterprise-D could trace back to its home star system in 2368, using a probe. (TNG: "The Inner Light")
