Cryonics

Though often mistaken for cryogenics, which is merely the study of cold on materials, Cryonics is the science of cryopreservation, using a mixture of factors to preserve a human or other organism in stasis for periods of time.

Essentially, extremely low temperatures are created, preserving the human body almost indefinitely. Preserving the human brain is more problematic, but UNSC technology has apparently overcome the obstacles of neural tissue damage and recusitation, with tissue damage still being a problem, causing what is colloquially called "freezer burn."

The UNSC use crionic storage pods in long-range warships, storing personnel for long periods to prevent aging during the journey and to preserve life-vital systems, maintaining only skeleton crews until nearing their target zone. A bronchial surfactant is ingested, intended to replace nutrients lost during the journey, but is unpleasant and often instead vomited.

It is unknown whether the Covenant use a version of this technology. Given the dramatically higher speeds their warships can achieve, though, it is unlikely that it is neccessary. The Forerunners used a system similar in practice, but using Slipspace to store the personnel within the pod instead of merely freezing them.