As NASA's Orion craft makes its first flight in the decades-long journey to land humans on Mars, the craft will strike many people as a throwback. Squint an eye and the Orion crew module bears a remarkable resemblance to the Apollo craft that ferried astronauts to the moon more than 40 years ago.

NASA's choice of a blunt-bottomed conical capsule design on both spacecraft isn't an accident. Instead, the blunt cone shape causes high drag to help slow the craft as it descends. When it returns to Earth from its farther-flung missions, Orion will be traveling nearly 7 miles per second, or hypersonic speeds multiple times faster than the speed of sound. At that speed, managing the enormous temperatures the craft will generate--in excess of 5,000 degrees F--requires an effective design with intense shielding. That's one reason the initial test flight's prime job is to assess the updated shield built for Orion.

"The 'capsule shape' happens to be good aerodynamically for slowing down the vehicle without it burning up like a meteor," Kelly Smith, a NASA guidance engineer, wrote last month in a public discussion the agency held on Reddit. "Sharp shapes tend to heat up too much and melt/vaporize. A blunt shape works well hypersonically for keeping the heating to more manageable levels. If you look at ballistic missiles, all of their nose cones are 'blunt' as well (spheres, sphere-cones, etc) to deal with the extreme heating environment."

The cone's imbalanced shape also causes the craft to fly "crooked" as it hurtles toward earth at a rate of about five times the speed of sound, Smith explained. "This angle of attack causes Orion to have a little bit of lift; we can use this lift to steer the vehicle and control the entry trajectory by banking Orion like a glider," he wrote. Russia uses a similar shape for its Soyuz capsules, as does SpaceX's Dragon spacecraft.

Weather permitting, Orion is scheduled to launch Thursday morning on a 4.5-hour, two-orbit cycle around Earth, reaching an altitude 15 times higher than the International Space Station. The 3,600-mile altitude will help NASA test the craft's response to the higher radiation found outside low-earth orbit--from which the space shuttle never strayed--and to reach a 20,000 mph entry speed, about 84 percent of the speed a return from the moon generates. "Although our computers have gotten a lot more powerful, the physics of atmospheric entry hasn't changed since Apollo," says NASA spokeswoman Stephanie Schierholz.

The Orion program's homage to Apollo doesn't end with just the basic look. NASA also consulted with several retired employees who worked on the Apollo program, and at least one will be on hand to observe the flight at Mission Control in Texas. Up close, the similarities end abruptly. Orion is roughly three times larger than the Apollo crew module, built to carry four astronauts as far as Mars, a 70-million mile round-trip journey that could take as much as 23 months. On shorter trips, the craft can fit six.

Inside, Orion's "glass cockpit" would not look particularly foreign to any iPad user, with touch screens similar to those used in modern jet cockpits. Eliminating physical switches, and their associated cabling, saves enormously on weight. Only about 60 physical switches remain.

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Why NASA's Orion Spacecraft Looks So Familiar