(Reuters) - A pioneering commercial spaceship closed in on the International Space Station on Wednesday, a key test in a controversial program to reduce the U.S. government's role in human space flight.

Space Exploration Technologies, or SpaceX, launched its Dragon cargo capsule into orbit on Tuesday from Cape Canaveral Air Force Station in Florida for a test run to the $100 billion orbital outpost.

Dragon is expected to make its first pass by the space station on Thursday. Starting from a point 6.2 miles below and behind the outpost, Dragon will use GPS satellite navigation data and data from the space station itself to precisely maneuver to a point 1.6 miles away.

After a flurry of tests, including the first attempt by astronauts aboard the station to directly command the capsule, it will drop back into position for a possible docking on Friday.

If all goes as planned, station flight engineers Don Pettit and Andre Kuipers will use the station's robotic arm to pluck Dragon from orbit and attach it to a berthing port on the station's Harmony connecting node.

Dragon is carrying about 1,200 pounds (544 kg) of food, water, clothing and supplies for the station crew.

The capsule will be repacked with equipment to bring back to Earth and is scheduled to leave the station on May 31. It should splash down into the Pacific Ocean off the coast of southern California about 4.5 hours later.

Since the space shuttles were retired last year, the United States has been dependent on partner countries to reach the station, which flies 240 miles above Earth.

If successful, this week's demonstration flight will give NASA back its space wings, albeit by proxy. Rather than building and flying its own ships to the station, the United States is hiring private companies to do the work.

Cargo missions are the first step.

SpaceX, owned and operated by internet entrepreneur Elon Musk, and a second firm, Orbital Sciences Corp, already hold contracts worth a combined $3.5 billion for station cargo flights through about 2015.

A more controversial step is the Obama administration's so-called Commercial Crew efforts to develop space taxis to carry astronauts to and from the station. The initiative, which has been criticized by such luminaries as Apollo 11 moonwalker Neil Armstrong, may be helped by SpaceX's high-profile flight.


"I hope that the success of this mission - thus far at least, and hopeful it's entirely successful - will dispel some of the doubts that people have," Musk told reporters after launch.

"In some cases, people have had legitimate concerns because there's no precedent for what we're doing here," he added.

Tuesday's launch drew a flurry of statements from members of Congress, some of whom voted to cut Obama's $830 million budget request for space taxi development for the year beginning October 1 to about $500 million.

"I am happy to see this very challenging mission begin," said Senator Kay Bailey Hutchinson, a Texas Republican who prefers that NASA spend more on a government initiative to build a heavy-lift rocket and deep-space capsule for missions beyond the space station's orbit.

"There are many crucial milestones to be reached and capabilities to be demonstrated during this flight, all of which we hope leads to a demonstrated ability to provide cargo service to the International Space Station," Bailey Hutchinson said in a statement.

She made no mention of the follow-on program for commercial space taxis.

Others were quick to link the success of NASA's alternative partnerships, which led to the Dragon's space station debut, to the Commercial Crew program.

"Endeavors like this will make it possible for the private industry to venture into outer space and capitalize on the associated economic growth," said U.S. Representative Chaka Fattah, a Pennsylvania Democrat who serves on a NASA appropriations subcommittee.

"The SpaceX mission is “not just a single venture into space but a change in the trajectory of how we think of space exploration," Fattah said in a statement.

"This program brings NASA one more step in the right direction," said U.S. Representative Dana Rohrabacher, a California Republican.

"We must change orbital spaceflight from being dependent on and controlled by government employees, toward more entrepreneurial, cost-effective, commercial-based alternatives."

NASA is in the process of reviewing proposals from at least four firms, including SpaceX, for space taxi development funds. Selection of at least two and possibly more space taxi designs are expected in August.

(Editing by Jane Sutton and Todd Eastham)