The U.S. Air Force's X-37B space plane blasted into Earth orbit today, kicking off the robotic vehicle's clandestine fourth mission — as well as the first flight of a tiny solar-sailing spacecraft.

An unmanned United Launch Alliance Atlas V rocket launches the X-37B military space plane on its fourth secret mission for the U.S. Air Force on May 20, 2015 from Cape Canaveral Air Force Station in Florida.
An unmanned United Launch Alliance Atlas V rocket launches the X-37B military space plane on its fourth secret mission for the U.S. Air Force on May 20, 2015 from Cape Canaveral Air Force Station in Florida.
Credit: United Launch Alliance

The robotic X-37B space plane launched atop a United Launch Alliance Atlas V rocket today (May 20) at 11:05 a.m. EDT (1505 GMT) from Florida's Cape Canaveral Air Force Station. You can see a video of the X-37B space plane's launch here.

Most details about the space plane's orbital activities are classified, so it's unclear what exactly the X-37B will be doing as it zooms around Earth, or how long it will remain aloft. But Air Force officials have said that mission number four — known as Orbital Test Vehicle-4 (OTV-4) — will concentrate less on the X-37B itself and more on the gear the spacecraft is carrying to orbit. [X-37B Space Plane's 4th Mystery Mission in Photos]

"We are excited about our fourth X-37B mission," Randy Walden, director of the Air Force Rapid Capabilities Office, said in a statement late last month. "With the demonstrated success of the first three missions, we're able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads."

Also on board the Atlas V were 10 miniscule "cubesats," including the LightSail solar sail that was developed by the nonprofit Planetary Society. LightSail aims to prove out key solar-sailing technology ahead of a more ambitious orbital trial next year. 

An artist's depiction of the U.S. Air Force's unmanned X-37B space plane in orbit with its solar array deployed and payload bay open.
An artist's depiction of the U.S. Air Force's unmanned X-37B space plane in orbit with its solar array deployed and payload bay open.
Credit: United Launch Alliance/Boeing
The U.S. Air Force's robotic X-37B space plane is a miniature space shuttle capable of long, classified missions in orbit. <a href="http://www.space.com/75-x-37b-spaceplane.html">See how the X-37B space plane works in this Space.com infographic</a>.
The U.S. Air Force's robotic X-37B space plane is a miniature space shuttle capable of long, classified missions in orbit. See how the X-37B space plane works in this Space.com infographic.
Credit: By Karl Tate, Infographics Artist

The Air Force owns two X-37B spacecraft, both of which were built by Boeing's Phantom Works division. Each space plane is just 29 feet long by 9.5 feet tall (8.8 by 2.9 meters), with a wingspan of 15 feet (4.6 m) and a payload bay the size of a pickup-truck bed. To put those dimensions into perspective, both X-37Bs could fit inside the payload bay of NASA's now-retired space shuttle orbiter.

The X-37B launches vertically and lands horizontally, on a runway, as the space shuttle did.

The secrecy surrounding X-37B missions has led to speculation in some quarters that the craft is some sort of space weapon — that it's designed to inspect and/or cripple hostile nations' satellites, for example. But Air Force officials have long refuted that notion, saying the X-37B is simply testing out technologies for reusable vehicles and future spacecraft. [Top 10 Space Weapons]

"OTV missions allow us to examine a payload system or technology in the environment in which it will perform its mission," Capt. Chris Hoyler, an Air Force spokesman, told Space.com via email. "The unique aspects of the OTV allow us to mature these new technologies and inspect them following the de-orbit sequence."

Air Force officials have not said how long they expect OTV-4 to last, but the mission will be an extended one if the previous X-37B flights are any guide.

OTV-1 blasted off in April 2010 and landed in December of that year, spending 225 days aloft. OTV-2, which used a different X-37B, launched in March 2011 and stayed in orbit for 469 days. OTV-3 lifted off in December 2012, sending the vehicle that flew OTV-1 to space for 675 days — a record for a reusable space vehicle.

It's unclear if OTV-4 will employ the X-37B that flew OTV-1 and OTV-3, or the space plane that flew OTV-2.

"The program selects the orbital test vehicle for each activity based upon the experiment objectives," Hoyler said in response to this question.

The payloads flying to orbit aboard the X-37B on this mission are not a total mystery: The Air Force has revealed that an advanced Hall thruster propulsion system and a NASA materials investigation are among OTV-4's scientific gear.

A Hall thruster is a type of ion engine; it generates thrust by accelerating ions (electrically charged atoms or molecules) out the back of a spacecraft. The engine getting an in-space test on OTV-4 is an advanced version of the Hall thruster that powered the first three Advanced Extremely High Frequency military communications satellites, Air Force officials said.

"A more efficient on-orbit thruster capability is huge," Maj. Gen. Tom Masiello, commander of the Air Force Research Laboratory in Ohio, said in a statement last month. "Less fuel-burn lowers the cost to get up there, plus it enhances spacecraft operational flexibility, survivability and longevity."

The NASA payload, called the Materials Exposure and Technology Innovation in Space experiment, will study how exposure to the space environment affects nearly 100 different types of materials. The goal is to gather data that could aid in the design of future spacecraft, NASA officials have said. 

An artist's illustration of the Planetary Society's LightSail solar sail cubesat in orbit.
An artist's illustration of the Planetary Society's LightSail solar sail cubesat in orbit.
Credit: The Planetary Society

While the X-37B will likely stay in orbit for many months, the Planetary Society's LightSail spacecraft will come back down to Earth in just a few weeks.

Four weeks from now, LightSail, which is the size of a loaf of bread, will unfurl its 344-square-foot (32 square m) solar sail. But the craft is not going high enough to actually use the sail for propulsion; its maiden flight will test out the cubesat's attitude-control and sail-deployment systems, helping pave the way for a bona fide orbital-sailing test next year.

"It's smaller than a shoebox, everybody! And the sail that will come out of it is super shiny mylar," said Bill Nye (the Science Guy), who leads the Planetary Society, after today's successful launch. "We're very hopeful that the thing will deploy properly, the sunlight will hit it and we'll get a push."

Atmospheric drag will start pulling LightSail back down to Earth as soon as the craft's sail is unfurled, and the cubesat will likely burn up two to 10 days thereafter, Planetary Society representatives have said.

But the shiny sail should make the bantam spacecraft visible to the naked eye during its brief orbital trial. The Planetary Society will provide viewing maps and tips on its website (www.planetary.org) during the mission.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.