Thursday's flight of NASA's Orion capsule will mark the first in-space test of the first vehicle that's designed to carry humans beyond Earth orbit since Project Apollo and the first step on a long road that could lead to putting astronauts on Mars. But the steps between now and then are definitely up in the air.

"The mission per se is great," Scott Pace, the director of George Washington University's Space Policy Institute, said of Orion's Exploration Flight Test 1, or EFT-1. "But the policy rationale for what one would do with these things is still up for debate."

The $370 million EFT-1 mission is due to send an uncrewed Orion on a 4.5-hour, two-orbit test run that zooms out to 3,600 miles (5,800 kilometers) and back. The craft will be launched from Cape Canaveral Air Force Station in Florida, atop a United Launch Alliance Delta 4 Heavy rocket.

A crucial phase of the test comes when Orion screams back through the atmosphere, slowing down from almost 20,000 mph (32,000 kilometers per hour) to a mere 17 mph (27 kph) when it splashes down in the Pacific Ocean off the coast of Baja California for recovery. About 1,000 sensors that have been built into the craft will monitor how well Orion's heat shield and other systems perform.

All those readings will be factored into the preparations for even more ambitious trips over the coming decades. The next Orion test known as Exploration Mission 1 or EM-1 is currently set for late 2017 or 2018. It will send a robotic capsule that's more fully fitted out on a trip around the moon and back.

That trip will be powered by NASA's Space Launch System, which is currently under development but is destined to become the world's most powerful rocket.

In 2021, astronauts are due to get on board for Orion's first crewed spaceflight, designated as Expedition Mission 2, or EM-2. And this is where the mission plan gets hazier.

Last year, officials suggested that EM-2 could carry astronauts to a rendezvous with an asteroid which would have been relocated to a stable spot in the vicinity of the moon by a robotic tug launched years before. Such a mission would follow through on the Obama administration's plan to have astronauts visit a near-Earth asteroid by 2025.

However, NASASpaceflight.com reported last week that mission managers thought it would be unrealistic to do the asteroid rendezvous during Orion's first crewed test flight in 2021, and that 2024 or 2025 would be a more workable timetable. That would translate to Orion's EM-3 or EM-4 mission.

When asked about that schedule, NASA spokeswoman Rachel Kraft said it was too early to be specific.

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Is NASA's Orion Launch a Step Toward Manned Mars Mission?