The Air Force successfully fired its first test flight of the giant new MX intercontinental ballistic missile yesterday from Vandenberg Air Force Base in California, after a nine-hour delay caused by fog and overcast weather.

The liftoff and later separation of the reentry vehicle over the Pacific Ocean went as expected, and the vehicle "landed in the planned impact area" north of Kwajalein Atoll, 4,100 nautical miles from the launch site, according to Air Force Brig. Gen. Aloysius Casey.

"We feel like we had a magnificent first launch," Casey said. "Everything worked exactly as we expected it to work."

At least 13 anti-nuclear weapons demonstrators, including activist Daniel Ellsberg, were arrested for trespassing yesterday after entering the base in what their spokesmen later described as an effort to block the test.

The shot was the first of 20 flight tests scheduled into 1987. Under the current schedule, the final test will not take place until after the first 10 MXs, which will be based in reinforced Minuteman silos in Wyoming, are scheduled to be operational.

This overlap of deploying and testing is the result of congressional action. The first test was originally scheduled for the end of January, but an amendment by Sen. Gary Hart (D-Colo.) to last year's MX funding bill prohibited that test from taking place until after Congress approved an MX basing plan.

That vote was not until May 31, when Congress approved deployment of the MX missiles in existing Minuteman missile silos in Wyoming.

The first 11 test flights of the MX constitute the initial test phase and will last through 1985, according to an Air Force spokesman. They are supposed to ensure that the four stages of the missile are performing correctly and help make final determinations on its range and payload capability, the measure of how large a warhead package it can carry.

In the initial eight tests, the missile will be fired from a canister above the ground. Thereafter, it will be launched from buried test silos.

The second phase, consisting of six shots, will be devoted primarily to seeing how the warheads work as they head downrange across the Pacific to Kwajalein. The last testing phase, termed "operational verification," is firing the missile under war conditions. As one official described it recently to a House committee, "We determine the reaction time it takes to launch, which is very key in the prompt target kill capability of the ICBM."