Observation schedules are typically finalized only a few days in advance, as a longer lead time would mean there was a chance that the target would be unobservable by the time it was due to be observed.Engineering support for HST is provided by NASA and contractor personnel at the Goddard Space Flight Center in Greenbelt, Maryland, 48 km south of the STScI.. 
Among its Primary mission targets was to measure distances to cepheid variable stars more accurately than ever before, and thus constrain the value of the Hubble constant, the measure of the rate at which the Universe is únhánding, which is also related to its age.. 