Early tomorrow morning, Thursday December 4, NASA is scheduled to perform the first test flight of the new Orion spacecraft. The entirety of NASA’s human space exploration plan hinges on Orion — if the series of test launches go off without a hitch, Orion, eventually powered by the massive new Space Launch System rocket, will land humans on an asteroid in 2025, and then Mars around 2030. But first, let’s discuss the first test launch taking place tomorrow morning.

Exploration Flight Test 1

At around 7:05am Eastern time, December 4, an unmanned Orion capsule — perched atop a Delta IV Heavy rocket — will lift off from Cape Canaveral. The plan is to get to an altitude of around 3,600 miles above Earth — about 15 times the altitude of the International Space Station — do a couple of orbits, and then land in the Pacific Ocean about four-and-a-half hours later. The test flight is designed to imitate all of the the same processes/phases that would occur during a real, crewed flight, so that NASA can make sure everything is working as planned before we send humans back into deep space — something we haven’t done since the Apollo missions 40 years ago.

Let’s look at each of the test flight’s stages in detail. The cute illustrations, incidentally, are directly from NASA.

Pre-launch

The action for EFT-1 begins 8 hours prior to launch — or “T minus 8 hours” in aerospace lingo — with the support gantry being pulled away from the rocket/capsule. Over the next few hours, a series of go/no-go tests are performed for each of the rocket’s major systems; if the weather is bad, or a seal is faulty, or any of the various systems aren’t operating quite as expected, the launch might be called off. Generally, NASA and the United Launch Alliance (Lockheed/Boeing, which operates the Delta IV Heavy rocket) will try its best to fix any issues that arise, to make sure they hit the launch window. At T-0:08, 8 minutes until launch, the final go/no-go poll is carried out, Orion switches to internal power, and the launch will go ahead as planned.

Launch

For this first test flight, Orion is being launched upon a Delta IV Heavy rocket — currently the world’s most powerful/highest capacity rocket. This will be the first and only time that Orion uses a Delta rocket; the next test flight, scheduled for 2017, will use NASA’s new super-heavy-lifting Space Launch System. A Delta IV is being used in this case because it allows NASA to gather important data that will inform the construction of the second Orion capsule, which will travel all the way around the Moon.

Five and a half minutes after launch, at an altitude of around 200 miles, both the Delta IV’s main and booster engines will have expended all their fuel. A few seconds later, the whole bottom end of the rocket (the first stage) detaches. The second stage engine, which takes Orion into a higher orbit, ignites.

Exposure

Now that Orion is in space, the upper stage’s protective fairings peel away. The launch abort system, which would protect the cargo/astronauts in case of a bad launch, also jettisons. This is where we find out if Orion was appropriately designed and engineered to endure the rigors of space. (I’m sure it will be.)

The image at the top of the story shows you what the spacecraft + rocket will look like at this point. The bit at the end is the command module (where the humans would be located); the bit behind that is the service module; and the rocket at the end is the Delta IV second stage.

Re-ignition

After two hours, and one low-altitude orbit of Earth, the second-stage rocket will fire again, pushing Orion up to an altitude of 3,600 miles (5,800 km). Orion will travel through the high-radiation Van Allen Belts as steadily climbs above Earth. At around three hours after liftoff, Orion will hit its peak altitude… and then slowly start its descent back to Earth.

Separation

At around 3 hours 20 minutes, after a few little maneuvers, the Orion capsule — the tiny little Apollo-like capsule at the top of the whole rocket — will detach from the service module and second stage of the Delta rocket. At this point, Orion is on a trajectory to hit the Earth’s atmosphere; the service module and rocket motor are just unwanted mass.

Orientation

Around four hours after liftoff, Orion’s small maneuvering thrusters will reorient the capsule so that it’s at the right angle for reentry into the Earth’s atmosphere. On its way back to Earth, Orion will re-enter the Van Allen Belts, for another dose of exciting radiation.

Reentry

Finally, at around 4 hours 15 minutes, probably the most important and exciting bit of EFT-1: Reentry into Earth’s atmosphere, and seeing whether Orion’s heat shielding works as expected. The reason such a high altitude was chosen for a first test was so that the angle and speed — and thus the stresses during atmospheric reenetry — were comparable to a return mission from the Moon. Orion will hit the Earth’s atmosphere at around 20,000 miles per hour.

Landing

At 4 hours 20 minutes, Orion is through most of the atmosphere and has slowed from 20,000 mph to just 300 mph. A series of parachutes deploys — two small parachutes to begin the deceleration, then three drogue parachutes that pull out three massive main parachutes. Bear in mind that the Orion capsule weighs about 9 tonnes. A few minutes later, the main parachutes have slowed Orion down to around 20 mph — slow enough for a safe splashdown about 600 miles off the coast of California in the Pacific Ocean.

If everything goes to plan, the splashdown will be exactly 4 hours and 23 minutes after launch — or around 8:30am Pacific time.

So, there you have it: The exact mission plan for Exploration Flight Test 1. We’ll be covering the launch live tomorrow, of course, along with a live video feed — so stay tuned!

Now read: NASA’s Space Launch System is officially all systems go for Mars and Moon landings