Computer hackers in West Germany broke into a NASA computer network that contains technical information from space shuttle flights and, over a four-month period, gained the ability to "manipulate at will" the data stored there, a West German news report said yesterday.

NASA, responding to the report, said its own computer security system detected three electronic break-ins in August, at least one of which came from West Germany. Spokesmen for the National Aeronautics and Space Administration played down the significance of the incident, saying the network contained unclassified postflight data from the space shuttle.

The hackers reportedly gained access to information by asking the NASA computers for information stored under such key words as "shuttle," "Challenger" and "secret."

The NASA network is one of hundreds operated by many different groups around the world as a convenient means of sharing information. The networks are linked to the telephone system, and authorized users can gain access after connecting their own computers to a phone line. In most cases, a user needs to know only the network's telephone number and a password or access code.

Hackers are computer enthusiasts who enjoy the challenge of dialing into computer systems without authorization. Computer security experts say it is virtually impossible to make a computer network immune to break-in. Persistence, logic and clever computer programs allow hackers to break access codes and find passwords relatively easily.

The computer network, called the Space Physics Analysis Network (SPAN), is one of several that NASA operates. Stringency of access to NASA networks varies depending on the degree of risk posed by unauthorized access, an agency statement said. The networks are monitored and attempted break-ins, which are fairly common, are investigated, the NASA statement said.

SPAN allows scientists and engineers in various universities and other research centers quickly to send or receive technical data from a space flight. Its chief goal is to provide NASA managers with analyses of spacecraft performance.

"Any individual or organization engaged in NASA-related research can apply for access to SPAN," a NASA statement said. "We know of no classified information which can be accessed through the network."

William Marshall, a NASA spokesman, said the space agency could not comment on the claim that the hackers had been able to manipulate data. The hackers did not claim that they had actually done so. But Marshall observed that legitimate users can enter new data into the system.