ALT is a special object that exists just to help you get the altitudes of interest for a vessel future. It is grandfathered in for the sake of backward compatibility, but this information is also available on the Vessel structure as well, which is the better new way to do it:
Suffix Type Description
scalar, meters altitude in meters of SHIP’s apoapsis. Same as SHIP:APOAPSIS.
scalar, meters altitude in meters of SHIP’s periapsis. Same as SHIP:PERIAPSIS.
scalar, meters Altitude of SHIP above the ground terrain, rather than above sea level.