The astronaut spacewalk originally planned for Tuesday has been delayed until Thursday so the space shuttle Challenger's most important experiment can run two extra days to make up some of the time lost because of problems with balky antennas.

"We've just moved EVA [extra-verhicular activity] to flight day 7," astronaut David Hilmers at the Johnson Space Center in Houston told Challenger Cmdr. Robert L. Crippen at 6:42 p.m. EDT.

Astronauts David C. Leestma and Kathryn D. Sullivan are to make th spacewalk to transfer 550 pounds of toxic hydrazine fuel from one tank to another in the shuttle's cargo bay.

The fuel transfer is a rehearsal for a later astronaut crew's attempt to refuel a Landsat satellite that is out of fuel to maneuver and is unable to position itself to photograph Earth.

Sullivan, one of two women in Challenger's seven-member crew, is to be the first American female astronaut to walk in space.

The possibility of delaying the spacewalk until Thursday arose Saturday night after the astronaut crew succeeded in repairing a radio dish antenna. The antenna transmits high-speed data signals from the shuttle to a relay satellite more than 22,000 miles beyond Challenger, which sends the signals back to Earth.

The radio antenna is the only link between Earth and the shuttle's powerful imaging radar built to measure and make photograph-like images of portions of the Earth's terrain never examined in such precise detail.

The radio antenna, fixed to the edge of the cargo bay, began to oscillate uncontrollably the day after Challenger was launched on Friday.

The antenna's erratic movement made it impossible for the shuttle to link up electronically with the tracking and data relay satellite (TDRS) 22,400 miles above the Equator over the South Atlantic Ocean.

Saturday evening, the shuttle crew locked the antenna in one position facing the shuttle's tail and tilting in the general direction of the TDRS. With the help of astronauts Sullivan and Sally K. Ride, Crippen then "pulled the plug" on the motor controlling the radio antenna's movements.

Early this morning, Crippen and Challenger pilot Jon A. McBride moved the shuttle in line with the TDRS and in 30 minutes had the antenna aligned with the satellite's antenna to send and receive high-speed computer data for the first time since Friday night.

Crippen and McBride were able to keep the antennas aligned through most of the day, restoring service to the imaging radar movement for the first time in alsmost 40 hours.

"It turned out to be very easy to do," Flight Director T. Cleon Lacefield said in Houston.

While there was little doubt that the experiment was working today, there was some concern that the unconventional way of sending the data might degrade the quality of the images.

Leestma and Sullivan will be asked to extend their spacewalk beyond the three hours originally planned. Besides conducting the fuel-transfer experiment, the two astronauts also will be told to lash down the radio antenna that was oscillating earlier.

As Flight Director John Cox explained Saturday night.

"You can take that antenna into a room and breath on it and it will move if it is not locked firmly into place."

Continuing to operate the radar experiment until Thursday required a postponement of the spacewalk because, for safety, the powerful radar signals must be turned off before the astronauts go outside the shuttle.

Even with the radar antenna operating for two extra days, the experiment may lose as many as half of the targets that were scheduled when the mission began.

Plans were to take radar images of as much as 40 percent of Earth's land mass. Almost all of Earth's largest deserts, tallest mountains and most treacherous seas were among the 60 targets chosen by scientists.