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Space Shuttle Re-entry
Space: A Dangerous Business
Space Shuttle Re-entry
Space travel is never routine--no matter how many shuttles
launch and land. As the loss of space shuttle Columbia in 2003 reminded us, success always depends on near-perfection--especially
during re-entry, where there is almost no room for error, even before sunset. Here's why.
First, the shuttle has to slow down. While in orbit, it's
traveling at 17,500 miles per hour (28,000 km/h). That really is faster than a speeding bullet. Imagine driving down a highway
for five minutes, passing landmark after landmark as you go. The orbiting shuttle would speed by it all in a second.
To shed some speed, and fall out of orbit, shuttle pilots
fire maneuvering thrusters on the shuttle's nose and tail, turning the craft so that its tail faces forward. Then they fire
the main thrusters to slow down. Another thruster burn orients the shuttle to its proper re-entry attitude, with the nose
facing forward and raised about 40 degrees. This last maneuver is crucial. Only the proper, nose-up attitude puts the bottom
of the shuttle in position to bear re-entry's brunt.
About 75 miles (120 km) from terra firma, the shuttle begins
to catch a little air. Aerodynamic forces start to take effect. Little by little, thruster maneuvering ceases, and the shuttle
becomes a 120-foot (37-meter) glider. Computers guide the craft through a series of steep banks designed to bleed off speed.
Yet without power, and traveling at hypersonic speeds, pilots say the shuttle flies like "a brick with wings."
Plowing through air, however thin, causes drag, which quickly
slows the shuttle down. That's the idea, of course, but the sudden change in velocity stresses the ship and its crew. Apollo
astronauts regularly endured more than 6 Gs during re-entry, while Mercury astronauts suffered up to 11 Gs, near the maximum
sustained force humans can stand. Shuttle crews experience only about 3 Gs, significantly less than during liftoff, because
of the shuttle's longer descent time. But the mechanical stresses on the shuttle are intense, with shear forces exceeding
7,000 pounds per square inch.
Still, the most difficult challenge of re-entry comes from
another source: heat. Any object moving through the atmosphere encounters friction caused by air molecules hitting its surface.
At relatively low speeds, like those of conventional aircraft, the heat caused by this friction is negligible. But as speeds
increase, so does friction. For some objects, the extreme speed of re-entry can heat surfaces to more than 5,000 degrees Fahrenheit
(2,760 degrees Celsius).
Two factors are crucial in foiling this heat: the shape
of the re-entry vehicle and the materials used to make its surface. Strangely enough, the best shape for an object during
re-entry is not slender and streamlined, but blunt-faced. A blunt shape creates more drag, which allows the re-entry vehicle
to decelerate more in the thin upper atmosphere, so that speeds in the thicker lower atmosphere remain as low as possible.
The blunt shape also produces shockwaves away from the object's surface, which deflect more heat away.
Blunt shapes have remained constant throughout the history
of space travel. But heat-resistant materials have not. At first, NASA relied on a process called ablation to transfer heat
during re-entry. The protective heat shield on those early NASA capsules burned and fell away, taking large amounts of heat
with it.
The shuttle requires reusable materials. So the nose cone
and leading edges of the wings, which reach the highest temperatures during re-entry, are covered with reinforced carbon that
can withstand 3,000 degrees Fahrenheit (1,650 degrees Celsius). The underside and forward fuselage are covered with silica
fiber tiles that can withstand 2,300 degrees Fahrenheit (1,260 degrees Celsius). "Blankets" of silica fibers cover the rest.
Underneath the brittle tiles is a layer of material that cushions them from the vibration, expansion, and contraction of the
shuttle's aluminum frame and that insulates the frame from the radiating heat.
About 16 miles (25 km) above the Earth, re-entry is over.
The shuttle has slowed to 1,700 miles per hour (2,735 km/h). On a good day, all that remains are five minutes of relatively
cool, controlled gliding to the landing site. The only thrill the astronauts should get now is descending to the runway twenty
times faster than a commercial airliner at an angle seven times as steep--sometimes, like last night, in the dark.
In the end, the tremendous aerodynamic and thermal forces
of re-entry always contain the potential for catastrophe. It is a danger the scientists, engineers, and astronauts of the
shuttle program know all too well.
--Christopher Call
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Space: A Dangerous Business
A space vehicle must move at a rate of at least 7 miles per second to escape Earth's gravitational
pull. This is equivalent to going from New York to Philadelphia in about 20 seconds.
Not an astronaut gets
on board a spacecraft who isn't aware of the dangers inherent in space travel. Still, when any accident happens, it's still
a tragedy. The break-up of the space shuttle Columbia on Saturday, 2/1/2003, with seven astronauts on board, including
the first from Israel, was the latest in a series of accidents since space exploration began in 1957 with the launch of the
Soviet Sputnik satellite. The following is a chronology of some key space incidents:
October 1960 - Ninety-one people
are killed when an R-16 rocket explodes at the Baikonur space center in Kazakhstan in the Soviet Union.
January 1967
- Three U.S. astronauts - Virgil Grissom, Roger Chaffee and Edward White - die in a "flash fire" aboard Apollo 1 during a
simulated launch at Cape Canaveral.
April 1967 - Soviet cosmonaut Vladimir Mikhailovich Komarov is first man to die
in a space mission when a parachute on his spaceship fails on re-entry and the ship crashes to Earth.
June 1971 - Three
Soviet cosmonauts die during re-entry after 24 days in an orbiting space laboratory, a record endurance flight at that time.
March
18, 1980 - Fifty technicians die at Russia's Plesetsk Cosmodrome when a Vostok booster explodes while being fueled. The incident
is reported only in 1989.
January 28, 1986 - Seven U.S. astronauts including a schoolteacher die aboard the Challenger
space shuttle 72 seconds after lift-off from Cape Canaveral.
April 18, 1986 - A Titan missile believed to be carrying
a military satellite explodes shortly after launch from the Vandenberg Air Force Base launch site in California.
May
3, 1986 - A Delta rocket carrying a $57 million weather satellite explodes shortly after lift-off from Cape Canaveral.
February
22, 1990 - Western Europe's 36th Ariane rocket, carrying two Japanese satellites, explodes less than two minutes after lift-off
from Kourou, French Guiana.
September 7, 1990 - Part of a U.S. Titan rocket falls from a crane and explodes at Edwards
Air Force Base, sending flames 150 feet into the air and killing at least one person.
June 18, 1991 - A 46-foot (15-meter)
Prospector rocket carrying 10 science experiments for the U.S. space agency and several universities is destroyed after veering
off course after launch from Cape Canaveral.
August 2, 1993 - A Titan 4 rocket believed to be carrying an expensive
military spy satellite explodes after lift-off from Vandenberg Air Force Base.
December 1, 1994 - Western Europe's
70th Ariane rocket crashes into the Atlantic with the $150 million PanAmsat-3 telecoms satellite after launch from Kourou,
French Guiana.
January 26, 1995 - The Chinese-designed Long March 2E rocket carrying a telecommunications satellite
explodes after blast-off from Xichang in southwest Sichuan province.
October 23, 1995 - An unmanned Conestoga rocket
whose satellite contains 14 scientific experiments explodes 45 seconds after blast-off from a NASA (news - web sites) facility
in Virginia.
February 15, 1996 - A rocket carrying an Intelsat 708 communications satellite explodes soon after take-off
from China's launch site in Xichang.
May 20, 1996 - A Soyuz-U booster rocket carrying reconnaissance satellites explodes
49 seconds after lift-off from Russia's Baikonur Cosmodrome.
June 4, 1996 - Europe's Ariane-5 rocket explodes 40 seconds
into its maiden flight after blasting off from the European Space Agency launch center in Kourou, French Guiana.
June
20, 1996 - A Soyuz-U rocket carrying reconnaissance satellites explodes after lift-off at Plesetsk Cosmodrome.
May
20, 1997 - A Russian Zenit-2 booster rocket carrying a Cosmos military satellite explodes 48 seconds after launch.
June
25, 1997 - Russia's Mir space station (news - web sites) carrying two Russian cosmonauts and one American collides with a
cargo ship. Crew narrowly escape death as oxygen rushes out of Mir.
August 12, 1998 - The U.S. Titan rocket program
is put on hold when a Titan 4A explodes soon after lift-off in one of history's most expensive space disasters. The cost of
the rocket and its spy satellite cargo was put at more than $1 billion.
August 27, 1998 - A Delta 3 rocket carrying
a U.S. communications satellite bursts into a $225 million fireball, soon after blast-off from Cape Canaveral on its maiden
flight.
September 10, 1998 - A computer malfunction brings down a Ukrainian rocket carrying 12 commercial satellites,
minutes after blast off from Baikonur.
July 5, 1999 - A Russian Proton-K heavy booster rocket launched from Baikonur
suffers a malfunction that detaches the engine and parts of the booster, causing them to crash onto the steppe. A 200-kg (440-lb)
chunk falls into the courtyard of a private house. Kazakhstan briefly closes Baikonur in a row with Russia over clean-up costs
and rent for the base.
September 23, 1999 - NASA's $125 million Mars Climate Orbiter spacecraft breaks up as it enters
the Martian atmosphere due to confusion among its constructors between metric and old English measuring units.
October
28, 1999 - A Russian Proton rocket carrying a communications satellite crashes shortly after take-off from Baikonur.
December
3, 1999 - NASA's Mars Polar lander loses contact with earth after reaching the Red Planet. The $165 million mission is a write-off.
August
15, 2002 - NASA's $159 million Contour space probe, launched on July 3 and designed to chase comets, breaks up on leaving
Earth's atmosphere.
December 11, 2002 - An upgraded European Space Agency Ariane-5 rocket explodes soon after blast-off
from Kourou, French Guiana, sending two satellites worth about $600 million plunging into the Atlantic Ocean.
February
1, 2003 - The space shuttle Columbia, carrying seven astronauts including the first from Israel, breaks up over Texas on re-entering
atmosphere at end of 16-day flight.
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