SpaceX is poised to launch its unmanned Dragon cargo capsule to the International Space Station on Monday, carrying a key piece of equipment that was lost last year in a rocket explosion.

The international docking adapter will function as a parking spot for space taxis, enabling commercial spaceships carrying astronauts to latch on securely to the orbiting outpost.

It is the first of two docking adapters needed for the crew spaceships being built by SpaceX and Boeing, with those pioneer commercial flights planned for 2017 and 2018.

For Monday's launch, the gumdrop-shaped Dragon spaceship is packed with nearly 5,000 pounds (nearly 2,300 kilograms) of gear, including science experiments and equipment for the astronauts living in space.

After blasting off from Cape Canaveral, Florida at 12:45 am (0445 GMT), SpaceX will attempt to return the first stage of the Falcon 9 rocket to an upright landing on solid ground at the NASA facility.

The California-based company headed by Internet entrepreneur Elon Musk has already managed to land its rockets on land and on a barge in the Atlantic Ocean several times, as part of an ongoing effort to make rocket parts as re-usable as components from commercial airplanes.

According to Hans Koenigsmann, vice president of flight reliability at SpaceX, setting the rocket back down on solid ground requires more propellant than a water landing, but also affords a larger and more stable area for touchdown.

"I think it is going to be an easier trajectory than the last one," he told reporters.

"I am pretty optimistic at this point in time that we will land it, but I would always knock on wood. Just by the nature of this maneuver, it is pretty challenging."

SpaceX is still in the process of scheduling the first rocket launch using one of the pieces it has recovered, perhaps in the fall, he added.

- Costly accident -

Weather is 90 percent favorable for the launch, which will mark the ninth of 20 missions to the space station for SpaceX, as part of a $1.6 billion contract with NASA.

In June 2015, SpaceX's Falcon 9 rocket exploded about two minutes after launch, destroying a previous version of the docking adapter and thousands of pounds of gear.

The total loss amounted to $118 million, according to NASA.

The cause of the accident was a faulty strut that allowed a helium tank to break loose in flight, SpaceX said.

Following the accident, the US space agency negotiated "discounted mission prices" with SpaceX for upcoming supply trips, according to an Office of the Inspector General audit released last month. It did not specify how much.

If launch goes as planned Monday, the Dragon is expected to arrive at the space station early Wednesday.

A Russian Progress cargo ship is scheduled to arrive Monday.

The Dragon will stay at the orbiting outpost for about a month before returning to Earth for a splash down in Pacific Ocean, carrying back 1,300 pounds of science experiments.