NASA has decided to postpone the launch of a second $100 million communications satellite because the space agency does not understand what caused the first satellite to tumble out of control last month and end up in an errant orbit.

"It's very remote that the space shuttle Challenger will be carrying a second Tracking Data and Relay Satellite when it flies in August," Robert Aller, satellite program manager for NASA, said yesterday at a news conference. "The people who plan the manifests are not counting on the TDRS satellite to be in the payload bay for the August flight."

This means that the eighth shuttle flight will carry only a small communications satellite, called Insat, into orbit for the government of India. As a substitute for the $100 million TDRS payload, the space agency will send up an 8,500-pound instrument called the payload flight test article, which will be used to exercise the shuttle's robot arm designed to retrieve and deploy satellites.

"It's something we were going to do anyway sometime in the future," a NASA official said. "The test article gives us more chance to test the arm's wrist and elbow movements than anything we've done so far."

Postponement of the second TDRS flight means that the $1 billion Spacelab built by the European Space Agency and scheduled to fly on the ninth shuttle flight in September will not be able to carry out a full mission. This is because Spacelab's 40 instruments are built to work at such high speeds in orbit that at least two tracking satellites have to be in place to work the way they were designed.

NASA and the European Space Agency yesterday announced that they had decided to fly Spacelab on schedule Sept. 30 using only a single TDRS rather than postpone the flight. At best, one source said yesterday, the first Spacelab mission will reap a 60 to 70 percent return from its scientific experiments.

Even getting a 60 percent return from Spacelab hinges on a plan described yesterday by satellite project manager Aller to move the first TDRS out of its erratic orbit and into a circular orbit called "geosynchronous" that allows it to hover over a single place on Earth.

Aller said small maneuvering engines aboard the tracking satellite would be fired early Tuesday morning in the first of at least 13 firings to raise the low point (perigee) of the orbit to where it matches the high point (apogee).

The satellite is now in an egg-shaped orbit that is less than 22,000 miles in distance at apogee and a little more than 13,000 miles at perigee, almost 9,000 miles lower than it should be. Two test firings of the engines earlier this week raised the perigee 350 miles, Aller said, suggesting that the rescue plan can work.

Aller said it will take weeks of engine firings to get the satellite into a geosynchronous orbit because the engines can be fired only at the apogee, which the satellite passes through once every 18 hours.

"I have every confidence we will have the satellite in geosynchronous orbit sometime in June," Aller said yesterday. "The satellite systems are in perfect working order and there is no reason why this plan should not succeed."