The loss of two satellites in a fiery explosion of Europe's Ariane rocket Thursday night brings to seven the number of communications satellites lost to accidents in the last 18 months, making it almost certain that underwriters will stop insuring the costly craft.

The range safety officer at the European launch site at Kourou in French Guiana ordered the rocket and two satellites destroyed in the air after an engine misfired and the rocket began to drift.

Lost in the explosion was a satellite owned by the new European Telecommunications Satellite Organization (Eutelsat) and another owned by Spacenet, a subsidiary of General Telephone & Electronics in the United States. The insurance bill for the disaster was $150 million, including $65 million for Eutelsat and $85 million for GTE.

"Almost all the space underwriters in the world were involved in this one," said a space official who asked not to be identified. "I can't imagine any of them willing to take a risk on insuring a satellite for some time to come."

The Ariane accident is that program's first in more than two years. Four of the other five satellites lost in space in the past 18 months were carried aloft by the U.S. space shuttle and failed to function after being placed in orbit.

The other loss, also a U.S. failure, occurred when Intelsat 5 never reached earth orbit because a new version of the Atlas Centaur launch vehicle misfired.

Worldwide satellite insurance is underwritten by a handful of international insurance companies, headed by Lloyd's of London. For the most recent seven failures, these firms will have paid $485 million in insurance claims to six owners of space satellites in the last year and a half.

The underwriters also may be called on to pay another $85 million to the Hughes Communications Corp., whose Leasat 4 satellite is not working in earth orbit.

The cause of the Ariane disaster was still unknown yesterday, although officials of Arianespace, which markets cargo space on the Ariane rocket, said it appears that the third-stage engine of the liquid-fueled rocket failed to ignite on time. This caused the engine to shut down automatically.

Ariane then began to drift off course, forcing the range safety officer to send a signal that exploded the rocket and its satellites at an altitude of 90 miles.

The abrupt end to Ariane's 15th flight followed nine successful flights that put 14 satellites in orbit.

Observing the disaster in the Mission Control Center, where he saw Ariane's blip disappear from radar, was French President Francois Mitterrand, who told reporters: "I am disappointed, but above all I am disappointed for the technicians. It will work the next time."

The next Ariane flight is scheduled for Nov. 15, when an Ariane rocket is to take a French satellite called SPOT into orbit to survey the earth's mineral resources.

Another flight is scheduled to carry two more communications satellites into orbit Dec. 11, and as many as seven flights are on the books for 1986.

"We have already convened an inquiry commission to look into the cause of the accident," Arianespace Executive Vice President Douglas Heydon told The Washington Post. "If we have any doubts about what caused the accident, or if we feel we have to make major hardware changes to correct a problem, we shall delay our upcoming launches."

The two communications satellites to be launched by Ariane in December are understood to have no insurance, as yet. Underwriters have been waiting until two or three months before a launch to insure communications satellites, because of the recent failures.

A few space officials said they feel that the difficulties faced today by insurers are self-created.

The officials said insurers started out charging premiums that were5 percent of a satellite's worth, which the officials called unrealistically low premiums. The premiums have risen in the past 18 months to 20 percent of a satellite's value.

One answer to the insurance problem could be for companies and countries putting up communications satellites to enter the insurance business themselves. Arianespace has said it is considering setting up its own insurance affiliate.