Ever since two Gemini spacecraft met up high above Earth in 1965, the U.S. space program has used astronauts for every rendezvous in orbit -- with other spacecraft, satellites, space stations or the Hubble Space Telescope.

But tomorrow, barring a weather-caused delay, the United States for the first time will send an autonomous robot vehicle to join up with a satellite and conduct a 20-hour demonstration of its ability to make close approaches, separations and loops -- without any human guidance, either aloft or on the ground.

NASA's Demonstration of Autonomous Rendezvous Technology, known as DART, will leave California's Vandenberg Air Force Base suspended beneath a Stargazer L1011 aircraft, which will drop it shortly after 2 p.m. Eastern time about 40,000 feet over the Pacific Ocean. A Pegasus rocket will ignite and carry DART into orbit to catch up with a military satellite flying 475 miles above Earth.

The Bush administration is billing DART as the opening act in its "Vision for Space Exploration" to the moon and Mars -- the prototype for a workhorse, low-cost vehicle that could just as easily hump supplies to astronauts in outer space or repair unmanned Earth-orbiting weather satellites.

"We need to be able to carry cargo -- to the space station or on to the moon and Mars," said DART project manager Jim Snoddy, of NASA's Marshall Space Flight Center, in Huntsville, Ala. "No matter where you go, you must be able to bring two objects together."

Recent events suggest that DART -- or something like it -- is long overdue. Since the two Gemini met up in 1965, the U.S. space program has relied exclusively on astronauts for in-space rendezvous, repair and docking -- most recently using the space shuttle both to supply the international space station and service the Hubble telescope.

The grounding of the shuttles after last year's Columbia tragedy has forced the space station to rely for re-supply on small robotic Russian spacecraft and prompted NASA to embark on a crash program to build its own robotic vehicle to service the aging Hubble.

Although DART will have no direct application to the Hubble mission, NASA expects to learn useful lessons from DART about maintaining position and maneuvering. DART "is a good first step," Rear Adm. Craig Steidle, NASA's associate administrator for exploration systems, said in a telephone interview.

NASA long ago recognized the need for a robotic spacecraft capable of self-guided rendezvous and docking, and during the 1980s and early 1990s it successfully tested a laser sensor that could find any target vehicle that carried a special reflector.

The groundwork for the $95 million DART mission was laid in 1999 when the Dulles-based company Orbital Sciences Corp. put reflectors on an experimental communications satellite it built for the Defense Advanced Research Projects Agency, the Pentagon's research arm. DART's laser sensors will not work unless the target has a reflector, an important reason the technology cannot be used on a Hubble mission.

Orbital Sciences spokesman Barron Beneski said the company and the Defense Department were "definitely looking to the future" when they installed the reflectors. "We wanted to see whether it would be possible to use them in a mission like DART," he said. "At that time it was farsighted, I guess."

The DART spacecraft, also built by Orbital Sciences, is about six feet tall and three feet in diameter and weighs 800 pounds. Ten minutes after launch, the three-stage Pegasus will put it into a "parking" orbit about 300 miles above Earth but below the target satellite.

Once it has finished checking out its navigation systems, DART will use a fourth rocket stage to boost itself 170 miles higher until it is shadowing the target, about 24 miles behind and 4.5 miles below.

All these maneuvers, as well as what comes later, are preprogrammed "with no human intervention" needed, Snoddy explained. Beginning with launch, DART "finds the target, calculates everything and automates the whole mission."

For its final approach, DART will use 16 gas thrusters, responding to signals from its "mission manager," the on-board software capable of tweaking the spacecraft's movements in response to information from its cameras and sensor arrays.

In two short jumps, DART will maneuver to a position two miles behind the target, then six-tenths of a mile behind. Then it will begin a series of 43 maneuvers that will twice bring it within 16 feet of the satellite.

The mission's top priority is to give DART's laser "eye," known as the Advanced Video Guidance Sensor, a complete workout. "Once you get a signal [from the reflector], you lock onto it and go into proximity operations," Snoddy said.

Besides the simulated dockings, the maneuvers include circumnavigating the satellite, approaches from the left and right, pulling away out of sensor range and then locking on again, and keeping station alongside at eight different distances, beginning at 16 feet.

"We're going to do everything two times," Snoddy said.

About 20 hours into the mission, DART will finish its last sideways approach to the satellite and retire to a position about 1,000 feet away to initiate its final withdrawal and eventual burn-up in Earth's atmosphere.

Although DART is only a demonstration, its implications could be far-reaching. "We've done 10 years of development to prove it can work," Snoddy said in a telephone interview. "And now it's on the shelf for the next guy to use."

The "next guy" is Orbital Express, a joint Defense Department-NASA project to develop an autonomous spacecraft capable of refueling and servicing Earth-orbiting satellites. It will use DART's laser eye as its main close-in navigation and station-keeping system.

Beginning in September 2006, Orbital Express's two elements -- NEXTSat, the target, and ASTRO, the robot spacecraft -- will conduct up to a year's worth of docking and refueling exercises, robotic equipment transfers and approaches and withdrawals.

"We repeat it over and over, until there is sufficient confidence to show we have reduced the technical uncertainty," said Orbital Express program manager Richard Matthews, of Boeing Co., the project's lead contractor.

This will be valuable knowledge, not only for the scheduled 2007 Hubble repair mission, but also for space exploration, NASA's Steidle said. "We need pre-positioning of supplies and water and in-space assembly. We need to validate our assumptions, and these [missions] are key pieces."

An illustration shows how NASA's DART craft, set for launch tomorrow, will approach a target satellite.