Orbital Sciences’ Senior Vice President Frank Culbertson Jr. with a model of his company’s Antares rocket in this photo from August 2012. The company plans a test launch of the Antares rocket this month. (Jeffrey MacMillan/JEFFREY MACMILLAN)

Dulles-based Orbital Sciences is readying for a test this month of its Antares rocket launcher, a critical step as the company prepares to complete its first resupply mission to the International Space Station.

Orbital’s much-anticipated blastoff will take place at the Virginia Commercial Space Flight Authority facility on Wallops Island, less than a month after Hawthorne, Calif.-based Space Exploration Technologies, also known as SpaceX, completed its second cargo flight to the space station.

The Antares launch — which will be visible from some spots locally — will be a key step for Orbital.

“Over the next 18 to 24 months, this program ... is, I think, the most important for [Orbital],” said William Loomis, managing director at the financial services firm Stifel Nicolaus. “It’s one of the biggest contracts they have, period.”

Both Orbital and SpaceX have deals with NASA to resupply the space station, as the agency looks to private space firms to supplement its capabilities.

Under its deal with NASA, Orbital will launch space modules that will deliver supplies to the station, take away its trash and then burn up as they reenter the atmosphere. The company is planning to complete eight supply missions by early 2016.

Orbital has a $1.9 billion contract with NASA to deliver 20 metric tons of supplies to the space station.

Each unmanned trip takes several pieces of equipment. The Antares vehicle launches the Cygnus space module, which acts as the brains of the operation by housing the avionics, propulsion and navigation systems. Attached to the space module is a cargo module, which is built by Thales Alenia.

The test launch planned for Wednesday — though the date may change because of weather — is focused on the Antares, and the company will use a simulator in place of the Cygnus module, said Mike Pinkston, Orbital’s program manager for Antares.

“It’s absolutely a huge milestone, no question about it,” Pinkston said of the upcoming launch. “We’ll gather a huge amount of data.”

He said Orbital will be comparing the system’s performance to analytical models the company has developed.

If the test is successful, the company plans to run a demonstration about two months later that would use an active Cygnus module, Pinkston said. Earlier this month, the Orbital-built Cygnus module that will be used for the demonstration was transported to Wallops and integrated with the Alenia-built cargo module, Orbital spokesman Barron Beneski said.

The demonstration mission will carry a partial cargo load to the space station, he said.

Assuming the demonstration is also successful, Orbital is planning its first mission to the space station in late fall.

At the same time, SpaceX is looking toward its next resupply flight in late fall, said Gwynne Shotwell, the company’s president and chief operating officer.

In late March, SpaceX completed its second cargo flight under NASA’s resupply program, returning with thousands of pounds of science samples and equipment, according to NASA.

Shotwell said the company hopes to eventually transport astronauts to the space station.

“There's no question that these missions to the International Space Station have been enormously helpful to sort of locking down the trust” between SpaceX and NASA, she said.