The moon, a luminous disk in the inky sky, appears suddenly above the broad crescent of Earth’s horizon. The four astronauts in the Orion crew exploration vehicle have witnessed several such spectacular moonrises since their spacecraft reached orbit some 300 kilometers above the vast expanse of our home planet. But now, with a well-timed rocket boost, the pilot is ready to accelerate their vessel toward the distant target ahead. “Translunar injection burn in 10 seconds ... ” comes the call over the headset. “Five, four, three, two, one, mark ... ignition....” White-hot flames erupt from a rocket nozzle far astern, and the entire ship—a stack of functional modules—vibrates as the crew starts the voyage to our nearest celestial neighbor, a still mysterious place that humans have not visited in nearly half a century. The year is 2020, and Americans are returning to the moon. This time, however, the goal is not just to come and go but to establish an outpost for a new generation of space ­explorers.

The Orion vehicle is a key component of the Constellation program, NASA’s ambitious, multi­billion-dollar effort to build a space transportation system that can not only bring humans to the moon and back but also resupply the Internation­al Space Station (ISS) and eventually place people on the planet Mars. Since the program was established in mid-2006, engineers and researchers at NASA, as well as at Lockheed Martin, Orion’s prime contractor, have been working to develop the rocket launchers, crew and service modules, upper stages and landing systems necessary for the U.S. to mount a robust and affordable human spaceflight effort after its current launch workhorse, the space shuttle, retires in 2010.

To minimize development risks and costs, NASA planners based the Constellation program on many of the tried-and-true technical principles and know-how established during the Apollo program, an engineering feat that put men safely on the moon in the late 1960s and early 1970s. At the same time, NASA engineers are redesigning many systems and components using updated technology.

Orion starts with much the same general functionality as the Apollo spacecraft, and its crew capsule has a similar shape, but the resemblance is only skin-deep. Orion will, for example, accommodate larger crews than Apollo did. Four people will ride in a pressurized cabin with a volume of approximately 20 cubic meters for lunar missions (six will ride for visits to the space station starting around 2015), compared with Apollo’s three astronauts (plus equipment) in a cramped volume of about 10 cubic meters.

The latest structural designs, electronics, and computing and communications technologies will help project designers expand the new spacecraft’s operational flexibility beyond that of Apollo. Orion, for instance, will be able to dock with other craft automatically and to loiter in lunar orbit for six months with no one onboard. Engineers are widening safety margins as well. In the event of an emergency during launch, for example, a powerful escape rocket will quickly remove the crew from danger, a benefit space shuttle astronauts do not enjoy. But to give you a better feel for what the program involves, let us start on the ground, before the Orion crew leaves Earth. From there, we will trace the progress of a prototypical lunar mission and the technologies planned to accomplish each stage.

Up, Up and Away
Towering 110 meters above the salt marshes of Florida’s Kennedy Space Center, the two-stage Ares V cargo launch vehicle stands poised to blast off. The uncrewed vehicle, which contains a cluster of five powerful rocket engines, has almost the height and girth of the massive Saturn V rocket of Apollo fame. Derived from the space shuttle’s external tank, Ares V’s central booster tank delivers liquid-oxygen-hydrogen propellants to the vehicle’s RS-68 engines—each a modified version of the ones currently used in the Delta IV military and commercial launcher. Two “strap-on,” solid-fuel rocket boosters adapted from the space shuttle’s system flank Ares V’s central cylinder. They add the extra thrust that the launcher will need to loft the bug­like lunar lander and the “Earth departure stage”—a propulsion module that contains a liquid-oxygen-hydrogen-fueled J-2X engine (a descendant of NASA’s Apollo-era Saturn V J-2 motor, built by Pratt & Whitney Rocketdyne) that will enable Orion to escape Earth’s gravity and travel to the moon.

Abruptly, a flash exits the tail of the Ares V, and mounds of billowing smoke clouds soon envelop the booster, gantry and launchpad. After a momentary pause, a tremendous roar echoes across the spaceport, sending birds fleeing in all directions. Slowly at first, the big rocket ascends atop an ever expanding column of gray-white exhaust. Accelerating steadily, the vehicle blazes a smoky trail across the sky and disappears into the heavens. Minutes later, amid the silence of near-Earth space, Ares V jettisons its strap-on boosters, which fall into the sea, where they will be recovered. It then sheds the protective cargo sheath that covers its nose, revealing the lunar landing module. Circling the globe at an altitude of about 300 kilometers, the robot spacecraft now awaits the next step in the lunar excursion plan: rendezvous with Orion.

That same day the four moon-bound astronauts perch 98 meters above another Kennedy launchpad, anticipating imminent liftoff. Just below their conical Orion crew capsule is a drum-shaped service module that contains the spacecraft’s on-orbit propulsion engine and much of its life-support system. Protective fairings envelop both to shield them from the strong aerodynamic forces and harsh conditions they will encounter during ascent. The crew capsule and the service module sit atop NASA’s two-stage Ares I crew launch vehicle. Slimmer than its big brother, the “Stick,” as it is known by some, comprises another modified solid shuttle booster (constructed by Alliant Techsystems) topped by a second stage that is powered by a single J-2X motor. A spacecraft adapter serves as the structural and electrical interface between the Orion spacecraft and Ares I.

Capping the tall stack is an escape tower that is primed to rocket the occupants away from danger in the event of a failure. As the 1986 Challenger accident proved, space shuttle crews have little chance of survival if their ship sustains a major technical problem during launch and early ascent. Orion’s launch-abort system (LAS), in contrast, can for a few seconds impart a thrust that is equivalent to about 15 times its own mass and that of the detached crew module. The rocket tower is set to rapidly remove the astronauts from harm’s way during a mission abort while still on the launchpad or during ascent. Should a serious glitch occur on the ground, the separated system would reach an altitude of about 1,200 meters to allow for parachute deployment and a downrange, or horizontal, distance of about 1,000 meters to clear the launchpad. Mission planners estimate that the LAS, together with Orion’s advanced guidance and control system, would be able to return the crew safely 999 out of 1,000 times it is needed.

But any such thoughts recede rapidly as the exhilaration of the impending launch mounts. As the countdown nears zero, commander and pilot intently eye the flight instruments on the flat-screen displays of Orion’s “glass cockpit,” adapted from a safety-redundant version of the avionics system used by advanced airliners such as the newly introduced Boeing 787 Dreamliner. The cockpit, with its computerized, fully electric “fly by wire” controls, energy-conserving electrical equipment and few mechanical switches, would be nearly unrecognizable to an Apollo-era astronaut.

A shudder ripples up through the entire structure, followed by a thunderous rumble. The Stick starts to move skyward. Gaining speed with every second, it rises rapidly, pressing the astronauts into their seats.

Almost two and a half minutes into the flight, the solid rocket booster is driving Ares I upward at a speed of Mach 6. At a height of about 61,000 meters, the first stage separates and falls back to Earth on parachutes so that it may be recovered and later recycled. Meanwhile the J-2X second-stage rocket motor ignites, sending the Orion crew module, the service module and the LAS through the last reaches of the atmosphere. Their usefulness ended now that the craft has exited the atmosphere, the aerodynamic shrouds break away to maximize ascent performance by shedding weight. By this time the vessel has gained enough velocity to reduce the risk of an emergency abort, so the LAS and its protective fairing also separate and fall away. The second-stage engine cuts off as the crew capsule and the service module near an altitude of about 100 kilometers.

Rendezvous in Earth Orbit
The service module engine then ignites, completing the job of inserting Orion into orbit and initiating the maneuvers it needs to rendezvous with the Earth departure stage and the lunar lander. Orion’s main engine is adapted from the flight-proved space shuttle orbital maneuvering engine, upgraded for greater propulsion thrust and efficiency. The service module contains power generation and storage systems, radiators that expel surplus heat into space, all necessary fluids and a science equipment bay. To maximize space in the crew vehicle, the service module also carries some of the avionics system, as well as part of the environmental control and life-support subsystems. A lightweight polymer-composite honeycomb reinforced with aluminum forms its structure; simple manufacturing methods should help keep down the cost of this expendable item.

One of the more notable differences between Orion and Apollo is the addition to the service module of umbrella-shaped solar arrays that unfold when needed in orbit. Because the Apollo spacecraft was designed for moon missions measured in days, it carried hydrogen fuel cells that could generate electrical power only for relatively short periods. Orion, in contrast, must be able to produce electricity for at least six months.

Gradually, Orion catches up to the lunar lander and departure stage that Ares V had ­earlier placed into low Earth orbit. When the two craft finally rendezvous, the crew performs (or monitors) the final maneuvers and keeps

an eye on the automated “soft capture” system as it aligns the pair and then smoothly docks them. Force-feedback and electromechanical components sense loads, automatically capturing the mating rings of the vehicles and actively damping out any contact forces. Ship and crew are now nearly ready to head for the moon.

The crew module is the only element of Orion that will make the entire trip, and it may be reused for up to 10 flights. A lightweight aluminum-lithium alloy with titanium reinforcements makes up most of the capsule structure. The exterior of the crew vehicle is lined with a thermal protection system, which, in addition to protecting its living quarters from the searing heat of reentry, also incorporates a tough, impact-resistant layer that shields it against high-velocity micrometeoroids or other debris that may strike its outer surface.

The crew module’s reaction-control maneuvering system uses gaseous oxygen and methane propellants, a technology that builds on the progress engineers made during NASA’s X-33 single-stage-to-orbit vehicle program, which was canceled in 2001. One advantage of the oxygen-methane propulsion system is that its fuel will be nontoxic (unlike its predecessors that used hypergolic propellants), which will help ensure the safety of the flight and ground crews after they return to Earth.

When all is ready, the Earth departure stage rocket engine ignites to propel the spacecraft toward the moon. Engineers are configuring Orion to support both “lunar sortie missions,” in which crew members spend four to seven days on the moon’s surface to demonstrate the Orion system’s ability to transport and land humans on Earth’s satellite, and “lunar outpost missions,” in which a semicontinuous human presence would be established there. Because the maximum duration of a crew’s stay on the lunar surface is 210 days (determined by the available supplies of oxygen, water and other consumables), Orion’s continuous operation capability must exceed that period. The biggest design driver for Orion lunar missions is the amount of propellant required to meet these objectives.

After a four-day trip outbound, the crew enters into lunar orbit, having dumped the Earth departure stage along the way. The four astronauts climb into the lander, leaving the crew capsule and service module to wait for them in orbit. As with the Apollo lunar excursion module, the lunar lander consists of two components. One is the descent stage, which has legs to support the craft on the surface as well as most of the crew’s consumables and scientific equipment. The other part is the ascent stage that houses the crew. After landing and exploring the surface, the foursome blasts off the moon’s surface and later docks with the crew and service modules in orbit. The ascent stage of the lander is discarded into outer space, and Orion rockets back to Earth.

Return to the Home Planet
As the Orion astronauts close in on the blue planet, they may have to prepare for a reentry and landing quite unlike those of Apollo. Like the Gemini and Mercury spacecraft before it, Apollo splashed down in the ocean after it had plunged through the atmosphere. But because water landings would require costly fleets of recovery ships and expose a reusable spacecraft to saltwater corrosion, NASA planners may decide that Orion should touch down on land, as the Russian Soyuz spacecraft does. Orion’s greater size, weight and lift, however, exacerbate the engineering challenge. The “land landing” mode is also important to minimizing life-cycle costs. If the agency instead opts to land in the ocean, Orion will be fitted with much the same capabilities as Apollo.

Unfortunately, setting down on American soil after a lunar mission presents a fundamental problem. For nearly half of the lunar month, orbital conditions would place any landing site in the Southern Hemisphere, away from the planned locations in the western continental U.S. Although the time of departure from lunar orbit can vary the longitude of the reentry point, its latitude is fixed by the declination (angular distance from the equator) of the moon relative to Earth at lunar departure. Thus, to reach landing sites in the western U.S. or waters near the continental U.S. during unfavorable periods of the lunar month, Orion will stretch its landing point into the Northern Hemisphere by employing aerodynamic lift produced as it descends into Earth’s outer atmosphere. A trajectory of this type, in which a spacecraft bounces across the upper atmosphere like a stone skipping across a pond, is sometimes known as a skip reentry.

Having spent the four-day return journey from the moon fine-tuning Orion’s flight path for the first crewed skip-reentry maneuver ever attempted, anticipation builds among the astronauts as the blue-white visage of our home planet grows ever larger in their view screen. They are soon occupied, however, by reorienting the ship so that the service module can be jettisoned, a necessary operation that exposes the protective heat shield on the crew module’s underside. Later, after using Orion’s redundant navigation system and flight computers to check that the space­craft’s attitude is positioned properly for reentry and that its trajectory is following the correct, shallow-angle route, the crew pre­pares for the onset of deceleration forces as Orion encounters the atmosphere.

The skip-reentry process starts out slowly. At first, the crew begins to notice weak g-forces caused by the resistance of the thin, high-altitude air. The g-forces, which push the crew members against their seats, grow steadily in strength as bits of glowing heat-shield material and streams of ionized gas streak past the windows. Shortly after Orion starts to scrape against the upper reaches of the atmosphere, the spacecraft rebounds briefly to a higher altitude. After the skip, the capsule dives deeply into the air on a path toward the landing site.

The tragic loss of the Columbia space shuttle and crew in 2003 demonstrated that the thermal protection system of a returning vehicle is critical. Atmospheric reentry generates tremendous heating on the undersurface of spacecraft (a couple of thousand degrees Celsius) caused by the friction of the air rushing by at hypersonic speeds. Because Orion’s reentry velocity from a moon mission (which is on the order of 11 kilometers a second) will be 41 percent faster than a shuttle’s descent speed from low Earth orbit, the heat load will be several times greater. The fact that the Orion crew module is larger than that of Apollo compounds the challenge.

The leading candidate for Orion’s base heat shield is a material called PICA (phenolic impreg­nated carbon ablator). PICA is a matrix of carbon fibers embedded in a phenolic resin. At high temperatures, the outer surface of the PICA layer ablates, or burns away, to carry off much of that extreme heat. The ablator’s surface pyrolyzes when heated, leaving a heat-resistant lay­er of charred ma­terial. PICA’s low thermal conductivity also blocks heat transfer to the crew mod­ule. PICA was used in 2006, when it protected the Stardust space­craft (which carried a sample from Comet Wild 2) as it came back to Earth at 13,000 meters a ­second—the fastest controlled reentry ever. Being 40 times larger in area, Orion’s heat shield will need to be built in segments, thus adding new complexities.

Landing on Land
Finally, three large parachutes—which closely resemble those used by Apollo—deploy to slow the vehicle’s rate of descent. The reassuring sight of the voluminous red-and-white canopies opening above tells the astronauts that their amazing trip is almost complete. Before long, Orion is jarred by the release of its large heat shield. Hanging below the big chutes, the crew module now descends at about eight meters a second.

In the case of a “land landing,” an airbag system inflates on the crew module’s underside to absorb and attenuate the upcoming landing shock. With a solid jolt, the spacecraft at last sets down on dry land in the western American desert. Orion has returned home.