Weather

Only eight and a half minutes after the Space Shuttle Discovery blasted off from the John F. Kennedy Space Center (KSC) in Florida on February 24, 2011, it was in space traveling faster than 17,500 miles per hour, 65 miles above the Atlantic Ocean—a good 45 miles above 99 percent of the atmosphere and thus the weather.

The shuttle’s commander was Steve Lindsey, an Air Force test pilot with more than 6,500 hours in 50 different types of aircraft, and 1,203 hours in space as either pilot or commander of five shuttle flights. Eric Boe, in the right seat as pilot, is also an Air Force test pilot with 4,000 hours in more than 45 different aircraft, and 380 hours in space as pilot of one shuttle flight.

With two such experienced pilots at the controls of the ultimate flying machine, you might think weather would be the least of their concerns.

You’d be wrong. The weather plays a huge role in shuttle launches and landings, and has been one of the major reasons for launch and landing delays. In fact, the ceiling and visibility constraints for a shuttle launch and landing are stricter than those a cautious flight instructor would think are fine for a student’s first solo flight.

Just because a shuttle’s rockets are powerful enough to accelerate it straight up from the launch pad to faster than the speed of sound in less than a minute doesn’t mean it can bull its way through the weather.

The February 24 to March 9 flight of the Shuttle Discovery—its last before retirement to a museum—helps illustrate why astronauts, shuttle flight directors, controllers, and others involved with shuttle flights need to be weather wise.

Some of NASA’s strict launch and landing criteria ensure that the shuttle won’t trigger a lightning stoke, as airplanes do regularly. Others require avoiding rain because at the shuttle’s launch speed, raindrops would be like bullets hitting the thermal tiles that keep the spacecraft from burning up when it reenters the atmosphere at the end of a flight.

In addition to lightning and precipitation constraints, shuttle launch rules require that clouds, which cover four-eighths or more of the sky (broken or overcast) be at least 5,000 feet above the ground and that the visibility be at least 4 statute miles.

These rules ensure that if something goes wrong during the first four minutes after launch, the booster rockets on each side of the orbiter and the huge external fuel tank would be ditched, and the pilots would land the orbiter at the Shuttle Landing Facility—the 15,000-foot Runway 15/33 built for shuttle landings at Kennedy. (This has never happened.)

Ever since the United States. first started sending people into space in the early 1960s, NASA’s astronauts, flight directors, and flight controllers have relied on National Weather Service meteorologists at what’s now called the Spaceflight Meteorology Group (SMG) at the Johnson Space Center in Houston. In addition, the Air Force’s 45th Space Wing Weather Squadron produces observations and forecasts for many shuttle operations at Kennedy. For launches, the Air Force forecasts for the launch pad while the SMG forecasts for the runway.

weather-recon pilots

In addition to direct links to meteorologists who understand what they need, shuttle astronauts have a source of weather information other pilots could only dream about: Another pilot—an astronaut—checks out the actual weather beforehand at and around the launch and end-of-mission landing sites.

For Discovery’s February 24 launch the “weather astronaut” was Lee Archambault, who had been the pilot on the June 8-22, 2007, Atlantis flight and commander of the March 15-28, 2009, Discovery flight. The weather reconnaissance pilot collects more detailed data than forecasters have from satellites, radar, and other sources.

While the shuttle’s ceiling limit is higher than a flight instructor would require for a solo, the shuttle’s limits of a crosswind component of up to 15 knots and moderate turbulence would make a flight instructor cancel a first solo.

On February 24 Archambault took off in one of NASA’s T-38 jets about three hours before the scheduled launch to check the winds and weather around Kennedy. “We were concentrating on a band of clouds to the southeast over the water, which could lower the ceiling if they moved in,” he said.

After landing the T-38 again, he took off an hour and 45 minutes before the launch in the training aircraft to fly approaches to runways 15 and 33 in order to experience what Discovery’s pilots would find if they had to make an emergency landing back at Kennedy. The training aircraft is a Gulfstream II business jet modified to fly like the shuttle on an approach from approximately 35,000 feet down to a landing.

Archambault reported to Discovery’s Capcom (capsule communicator) in Houston that conditions favored Runway 15 for an emergency landing.

Whether it’s an emergency landing or a normal end-of-mission landing, shuttle pilots need high ceilings and good visibility so “we can land even if our navigation system is a little bit off when we visually acquire the runway,” he said. “We only have one shot at this, so we have to be certain.”

When it lands, a shuttle orbiter is a heavy and fast glider and the pilots can’t go around if a landing attempt doesn’t work out. Archambault says the ceiling and visibility minimums are based on tests that assume a degraded navigation system.

When they are lined up on final, shuttle pilots use PAPI (precision approach path indicator) lights much as pilots of ordinary airplanes do at many airports.

For shuttle pilots, however, one white and three red PAPI lights indicates an 18-degree approach path, while two white and two red lights show a 20-degree slope. This is in contrast to the 2.8-degree slope that one white and three reds or the 3-degree slope two whites and two reds indicate with a conventional PAPI.

Shuttle pilots would fly an approach slope of 18 degrees for an emergency landing shortly after launch, while they normally fly a 20-degree slope at the end of a mission when the orbiter is lighter.

The required ceiling and visibility for an end-of-mission landing are 8,000 feet and five miles instead of the 5,000 feet and four miles required for launch. Archambault explained that these are needed because “after 12 or 14 or 15 days in orbit the biggest problem is the brain and body being able to be reconditioned to gravity. Our reflexes are a little slower and the approach is very dynamic at such steep approach angles.”

Discovery’s landing

To prepare for Discovery’s landing back at Kennedy on March 9, Rick Sturckow, flying as the weather reconnaissance pilot astronaut, reported to mission control in Houston that after completing two dives toward both ends of the runway in the shuttle training aircraft he had observed light wind and light turbulence, and confirmed that conditions favored Runway 15.

Back in Houston, meteorologists at the SMG had forecast that conditions would be favorable for landing at Kennedy that morning. Using this forecast and other information, Flight Director Tony Ceccacci gave the go-ahead for the deorbit burn. The orbiter is traveling tail-first when the orbital maneuvering system rockets are fired. This slows the orbiter enough to begin descending into the atmosphere, where it eventually begins maneuvering as an aircraft using aerodynamic forces instead of as a spacecraft using maneuvering rockets.

After the deorbit burn, a shuttle is going to land no matter how the weather changes during the 65 minutes between the burn and landing.

When Discovery touched down on Runway 15 at Kennedy it was going 224 knots. At that time the wind was from 145 degrees at 13 knots, gusting to 25 knots, which works out to a 24-knot headwind, barely within the limits.

Advice for pilots

“Whether you fly a space shuttle or a very small, light aircraft, every pilot will tell you that you don’t mess around with weather," Archambault says. “We have some very strict rules and we don’t bend on those rules at all. We always want to give the weather the respect it deserves. Every pilot should have the same mindset.”

The shuttle program ends this year with the scheduled April flight of Endeavour and the June flight of Atlantis. Archambault expects the United States to continue manned space flights, but whether private companies or a new NASA program operates the next spacecraft is yet to be decided. “I’ll be at NASA. Keeping my fingers crossed,” he says. "If you dream of flying in space, don’t give up. We’ll have a little gap here, but I hope the United States will be flying manned spacecraft again in the near future.”