The main mirror of the telescope has a diameter of 3.5 metres. Herschel is therefore the largest space telescope ever, with a mirror diameter about one-and-a-half times larger than Hubble. To save weight, which is relevant for the launch, the mirror is made out of the ceramic material silicon carbide - used for the first time in a mirror of this size.

Herschel will remain in operation for about three years. The duration of the mission is determined primarily by the availability of liquid helium for instrument cooling and the fuel needed for the thrusters that control the spacecraft's attitude and orbit. Because the Ariane 5 ECA launch vehicle delivered Herschel and Planck very accurately into their transfer orbits, Herschel used less fuel for trajectory corrections than expected.

The first truly critical milestone was passed at 12:53 CEST (10:53 UTC) on 14 June, when the pyrotechnic bolts holding down the vacuum-tight cover on Herschel's cryostat fired and the cover opened.