Orbital Sciences Corp. launches its Cygnus spacecraft atop an Antares rocket on a supply mission to the international space station. (NASA)

It’s not that big of a rocket, really, and the cargo in the spacecraft is relatively mundane — about 1,500 pounds of clothes, food, water, chocolate for the chocolate-craving astronauts, and so on. But the rocket that blasted off at 10:58 a.m. Wednesday from a small island on the Virginia coast is carrying a heavy burden of expectations.

Dulles-based Orbital Sci­ences launched its Cygnus spacecraft atop an Antares rocket on a supply mission to the international space station, and a great many people at NASA and in the broader space community were watching.

“Go Antares. Go Cygnus,” a flight controller said just moments before ignition of the unmanned rocket on a splendid day at the Mid-Atlantic Regional Spaceport on Wallops Island, where gentle ocean waves were hitting the beach a stone’s throw from the launch pad.

The launch was by the book. A few minutes into the flight, Orbital tweeted: “Pressures remain strong. Our favorite word ‘nominal.’ ”

This is a “demonstration” mission to prove that Orbital has the hardware and know-how to send cargo to the station, a feat that requires autonomous docking and other delicate orbital maneuvers. The Cygnus spacecraft will reach the space station in a few days and remain docked until late October. It is not a reusable spacecraft, so when it leaves orbit, it will crash into the South Pacific.

Shakedown flight for Cygnus

A successful mission for Orbital would greatly please NASA officials, who would like to have more options for supplying the orbital laboratory.

Another company, SpaceX, has flown three cargo missions to the station and has a $1.6 billion contract for a dozen deliveries in the coming years. But NASA doesn’t want to rely on just one provider of such a critical service. The continued operation of the space station requires regular and dependable delivery of “upmass,” in space industry jargon. Orbital has a $1.9 billion contract for eight cargo missions to the station over four years, with the first launch scheduled for December.

First, though, it has to get through this final demonstration mission, which is part of a separate agreement with NASA.

“A lot of people have put a lot of blood, sweat and tears into it,” said Frank L. Culbertson, an Orbital executive vice president and former astronaut.

Culbertson said Orbital has potential customers waiting to see how this launch turns out. The 135-foot-tall Antares rocket made a successful test flight earlier this year. Although Orbital has a long track record of successful launches, this is the company’s first liquid-fueled rocket.

“You’re always nervous about the first two launches of any new system,” Culbertson said before Wednesday’s launch.

At the moment, U.S. astronauts commute to the space station solely on Russian Soyuz spacecraft. Orbital was not selected as a finalist in the competition to carry crew to orbit. Three companies — SpaceX, Boeing and Sierra Nevada — are hoping to win that contract, and NASA hopes to have American astronauts launched on American rockets by 2017.

Hovering over the industry is the unresolved question of space-station funding. NASA and its international partners have agreed to fund the station through 2020. NASA and the commercial spaceflight companies would like to see the life of the station extended. A NASA official said last week that a decision on whether to extend the station should be made no later than 2014, so that the private sector will have confidence in the vitality of that market and will keep making investments in future space operations.

NASA’s long-term goal is to help the commercial spaceflight industry take over the relatively routine missions to Low Earth Orbit. These “commercial” contracts are unlike traditional government contracts. The private companies offer a fixed price for their services and assume more risk and engineering responsibility. The idea is to let NASA focus on complex deep-space missions while also continuing to contribute to the operation of the space station.