While government and private sky-watchers hunted yesterday for the missing RCA Corp. communications satellite, the project's insurers were bracing for what is expected to be the largest spacecraft loss in history.

The total insurance bill -- shared by British and American concerns -- could reach $77 million if the missing Satcom III doesn't show up. Most observers believe the satellite exploded because of a malfunction.

And the consequences for the cable television industry, which had lined up use of the satellite for a burgeoning array of customers, are almost as consequential.

The satellite is the third in a series of Satcoms designed for message and television traffic. It disappeared Monday after attempts to adjust its orbit so it would appear to be stationary more than 22,000 miles above the Earth.

Satcom III was launched by the National Aeronautics and Space Administration last Thursday from Cape Canaveral, Fla., into what a NASA official termed a "virtually perfect orbit."

A British insurance broker involved in the project said the $77 million -- a worst-case maximum figure -- easily would surpass the largest previous satellite loss, which occurred in September 1978 when a consortium of European governments was paid $28 million for a satellite that malfunctioned.

The satellite has 24 channels, each capable of transmitting one television program, 1,000 telephone calls or several million bits of computer data. The satellite cost about $50 million to build and launch.

RCA Corp. had paid the U.S. government $20.3 million for the launch by a Delta rocket, which carried the 1,794-pound craft into an orbit as low as 104 miles.

Meanwhile, RCA -- which along with Western Union Corp. operates the only satellite capable of transmitting cable television signals -- has been enthusiastically touting the performance of Satcom III, and yesterday was hawking its operation at a California convention of satellite users.