The ESA said additional checks on the Ariane 5 ECA launcher have been completed, allowing Arianespace and ESA to schedule the launch for May 14.
Herschel and Planck have already been fueled with hydrazine. Planck's three-stage active cryogenic cooler, needed to keep its instruments at extremely low temperatures, has been filled with helium-3 and helium-4. Herschel's cryogenic tanks are also being filled with superfluid helium, the ESA said.
The Herschel space telescope mirror will be the largest ever launched into space. Planck is designed to observe the cosmic microwave background.
The ESA has twice postponed the launch due to technical problems.