Friday's planned launch marks the first of three missions scheduled during the next four years to replace the six aging TDRS satellites already in orbit. Total cost of building and launching the three new satellites: $830 million.

NASA officials say the investment is critical to the success of missions ranging from the space shuttle to the International Space Station (ISS) to the Hubble Space Telescope.

Each of these spacecraft, and many others including U.S. military spacecraft, rely on the TDRS communication network to radio their information from orbit to ground stations on the surface.

Without TDRS, non-stop live radio contact between the shuttle and Mission Control in Houston would not be possible. The amount of live television from the space station would be reduced to a few minutes and only when the station flew over a particular ground station.

Hubble astronomers rely on the TDRS network to control the telescope's pointing and for relaying the images back to Earth from Hubble. Several other NASA science probes in Earth orbit use TDRS for the same reason.

With this new version of the TDRS spacecraft, built by Hughes Space and Communications, the new satellite will be able to handle a greater number of users and move a larger volume of information between space and Earth than the previous TDRS satellites.

Seven TDRS satellites have been launched into orbit since 1983, all using the space shuttle as the launch vehicle. Six satellites are still operational, although some have slightly degraded systems.

The seventh TDRS, the second to be launched, was lost in the 1986 Challenger disaster.