"Where, man? I don't see shit."
Self destruct systems or emergency destruction systems are present aboard most commercial starships. Initiating the self destruct on a spacecraft disengages its engine coolant system, quickly causing the vessel's power core to overload and explode, destroying the spacecraft entirely.
Aboard the Nostromo
The self destruct system aboard the USCSS Nostromo operated on a ten-minute delay, with an initial five-minute fail-safe window during which the process could be aborted. However, after these five minutes had elapsed, the meltdown of the ship's core was irreversible and could not be stopped. When the ten minutes ended, the ship's core would explode with tremendous force, utterly destroying the spacecraft. In 2122, Warrant Officer Ellen Ripley engaged the self destruct aboard the Nostromo in an attempt to kill the Alien, which had stowed away on board.
Aboard the Legato
The USS Legato possessed had a self destruct system, which was activated by colonist Lisbeth Hutchins in order to destroy both a Xenomorph outbreak aboard the ship and the Weyland-Yutani mercenaries aboard. Though it used a similar activation method to that of the Nostromo, the Legato exploded within less than a minute of the system being activated, instead of the ten-minute countdown used by the Nostromo. The actual explosion was also a lot less powerful, only blowing the Legato in half, though this was still more than sufficient to kill everything on it.
Aboard the Nova Maru
The space freighter Nova Maru's self destruct system was activated by sole-survivor Selkirk following a Xenomorph infestation on the planet PCW9512. This device also had a ten minute countdown, which Selkirk used his time to pray for salvation. The following explosion ultimately obliterated all trace of Xenomorph activity within the province.
Behind the Scenes
In Alien, it is never stated why a commercial towing vehicle such as the Nostromo would need a self destruct mechanism. However, several theories have been proposed by fans in the years since:
- In an emergency, it would be better to blow up an out-of-control starship hauling a colossal ore refinery rather than allow it to crash into a populated planet, colony or space station.
- Contemporary space rockets include self destruct systems in case they veer off course at launch, and the system on the Nostromo could be of similar purpose (although it seems likely that the Nostromo was built in orbit rather than launched from Earth).
- The presence of Colonial Marines in the Alien universe implies there must be a threat or threats that the Marines were established to counter. Therefore, self destruct systems may be designed to prevent valuable equipment and technology from falling into the hands of competitors, pirates or terrorists (a huge starship could be used as a devastating weapon if captured).