A view of Samus's thruster in Metroid Prime.
The Power Suit, Varia Suit and many upgrades to those Suits have a pair of thrusters on their backs. These thrusters traditionally appear as two red cylindrical exhaust vents attached to her chest plate.
Role
Other Suits such as the Fusion Suit do not have
Though merely part of the suit in 2-D games, the opening of Metroid Prime features Samus jumping off of her gunship onto the exterior of the Frigate Orpheon, and in PAL or Wii versions of the game, her thrusters release an explosive, loud flare as she jumps. In the Super Smash Bros. series, they also give off a flare when she jumps, though it is blue in color, and much smaller and quieter.
Metroid: Other M features exhaust energy being released from Samus's thrusters when she is SenseMoving, Speed Boosting and Shinesparking. The flares given off by her boosters in the game resemble that of Prime. In addition, the thrusters are used as the symbol for scanned material vulnerable to supersonic speeds. on the back of her suit fire and propel her at great speeds, though she must continue running as the thrusters do not provide lift required for her to fly.
Similar technology
Various Pirate Troopers use jet pack technology, although they are capable of carrying the Pirates through the air, unlike Samus's thrusters.
Dark Samus has a similar ability to Samus's thrusters, although the vents are replaced with two protrusions on her back in Metroid Prime 2: Echoes. During the final battle against her at the end of Metroid Prime 3: Corruption, one of her attacks involves her charging at Samus, followed by one of her Dark Echoes. Smoke can clearly be seen coming out of their backs, indicating possible thrusters.
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