Washington — Just a few hours before the first American spacewoman blasted into history, another US rocket was breaking through a significant political barrier and ratcheting the superpower arms race another notch.
This is the MX strategic nuclear missile which, following more than a decade of design and controversy, successfully completed its first test flight June 17. As it arched over its 4,100-mile trajectory from Vandenberg Air Force Base in Southern California to a spot in the Pacific Ocean just north of the Kwajalein Atoll, the ''Peacekeeper'' (as it has been dubbed) was sending clear messages to three important spectators:
* The Soviet Union, which (missile proponents say) must see that the US is serious about strategic modernization if it is to be more forthcoming in arms control talks. This is the stick to go along with what some perceive as the carrot of new administration flexibility at Geneva. As such, it fits the recent pattern of firmness toward Moscow expressed by Secretary of State George P. Shultz and Secretary of Defense Caspar W. Weinberger.
* European allies, whose uneasiness about deployment of new nuclear weapons within their borders will only increase if the United States - through political uncertainty or technological shortcomings - does not keep its schedule for missile deployment at home. West German Defense Minister Manfred Worner has emphasized this several times over the past year.
* And Congress, where authorization of money for MX procurement and operational deployment remains far from certain, despite support for testing. As with the Pershing II intermediate-range nuclear missile last year, lawmakers likely would not fund MX production if the weapon did not perform as advertised.
Repeating the administration position shortly before the first MX launch, Defense Secretary Weinberger told delegates to a Boys State convention in Bowling Green, Ohio, that nuclear weapons are a ''very uncomfortable but pragmatic solution'' to possible Soviet aggression. He called President Reagan's defense and arms reduction policies ''the two complementary roads to peace.''
But despite the acclaimed presidential commission report urging new stabilizing weapons and arms control initiatives along with MX deployment, many on Capitol Hill remain unconvinced. As evidence of continued skepticism, lawmakers in a House subcommittee earlier this month blocked $20 million for initial MX warhead production until the full program is approved.
Such action puts even more pressure on the Air Force, whose plan for initial MX deployment already has been squeezed by administration flip-flops on basing modes as well as congressional delays. For political as well as military reasons , officials still want to put the first operational and fully armed missiles in existing Minuteman silos in December 1986. This means that some test missiles will not have been flown until after that date.
Air Force officials call the inaugural launch a complete success. ''It was absolutely stupendous,'' said the commanding officer at Vandenberg, where 16 anti-nuclear activists were arrested for trespassing.
The MX is designed to carry 10 warheads. For the first flight, it carried six dummy warheads, which were kicked off over a 15-mile ''footprint'' along the missile's trajectory. Exact warhead accuracy will not be measured until later flights.
The 190,000-pound MX has a new ''cold launch'' feature. The missile is popped from its canister by gas-generated steam before the first of four rocket stages ignites 100-200 feet above ground. Theoretically, it is mobile, since the missile and canister can be transported between silos and silos reloaded.
The Reagan administration says the US needs land-based strategic missiles with ''prompt, hard-target kill'' capability to counter Soviet developments. Critics say such highly accurate and multi-warhead ICBM's, which most experts agree cannot be invulnerably based, are at once such tempting targets and so threatening that they make nuclear war more possible.
The Pentagon took unusual security steps with the first MX test flight, insisting that news organizations not publish or broadcast prelaunch data. The aim, said Air Force officials, was to provide the USSR with as little help as possible in preparing to observe the test with its satellites. However, as required under protocols to the SALT II agreement, the Soviets were notified 24 hours ahead that a missile test was planned.
Twenty MX test flights have been scheduled, the next in about 3 months.