NASA said Friday that conditions were favorable to launch its final shuttle mission to the Hubble space telescope on May 11.
All systems were go, according to Jeremy Graeber, assistant NASA test director at the John F. Kennedy Space Center, near Cape Canaveral, Florida.
Shuttle weather officer Kathy Winters told reporters there was an 80 percent chance of favorable meteorological conditions for the launch of space shuttle Atlantis, set for 2:01 pm local time on Monday (1801 GMT).
The launch window lasts about one hour and begins at 1:41 pm (1741 GMT), 20 minutes before the launch.
The countdown began as planned at 4:00 pm (2000 GMT) Friday.
The shuttle Atlantis was to depart with a seven-strong crew, to service the space telescope and carry out any necessary repairs on Hubble, a space telescope launched in 1990.
The mission, code named STS-125, is the fifth and last of its kind.
Officials from the US space agency hope the mission will allow Hubble to keep functioning until at least 2014, when it is due to be replaced by a highly sophisticated space telescope with an eagle-eye camera, the James Webb Space Telescope.
Scientists hope the new telescope will help to lift the veil off the mysteries and origins of the universe.