NASA Shuttle Endeavour Launch Postponed

No matter how much hype precedes a space launch—and there was plenty leading up to today's planned flight of the space shuttle Endeavour, what with Congresswoman Gabrielle Giffords and the Obama family in attendance—it takes just a small thing to shut down the whole show.

Today that thing was actually a pair of things—two heaters attached to an on-board supply of the hydrazine rocket fuel.

Hydrazine is a colourless, highly flammable gas that, when heated to about 500°F, can be used to power hydraulic systems, similar to the way steam turns the drive wheel of a riverboat.

In this case, the hydraulics are used to control the elevons, the flaps that allow the shuttle to pitch up and down during flight, according to astronaut Michael Foale, who reported moments after NASA announced that the launch was being scrubbed with just less than two and a half hours left in the countdown. It will take at least 48 hours to check out the systems and certify them ready for flight.

Even before the heaters went on the fritz, the launch was looking unlikely. That's not so much because of today's high winds and cloud cover at Cape Canaveral, but because the weather was even worse in France and Spain.

Every time a shuttle lifts off, flight rules require that the crew have a range of available landing sites and abort options in case something goes dangerously wrong.

The quickest and most straightforward maneuver is called a Return To Launch Site (RTLS) abort, in which the commander jettisons the solid fuel engines (which can't be shut down once they're lit), yaws back around so that the nose of the shuttle is pointed back to Florida, slowly throttles down the main engines and lands on a Canaveral runway.

If the problem occurs when the shuttle is out of range of Florida, the next best option is a Transoceanic Abort Landing (TAL), in which the crew flies the disabled ship across the Atlantic and touches down either at France's Istres Air Base or Spain's Zaragoza or Moron Air Base. (That last one, alas, is not a typo.)

With the European bases being hit by rain and winds, the launch would probably have been a no-go not matter what the state of the heaters was. (NASA says the forecast is 80% favorable for a Sunday launch.)

The penultimate abort scenario is known as Abort Once Around (AOA)—in which the shuttle has just enough velocity to stagger into a wobbly orbit but not enough to sustain it. After a single circuit of the planet, it comes home.

The last option after that is the Abort To Orbit; in this one, the shuttle can at least achieve a solid low-earth orbit. In many cases, that's the safest place for a balky ship to be, giving engineers and astronauts time to suss out the problem and plan for a safe, early reentry.

Today's scrub was the much less eventful get-out-of-the-spaceship-and-take-the-elevator-back-down-to-the-ground variety. It ain't sexy, but it's very reliable—and astronauts prefer it every time. - by Jeffrey Kluger