August 2005

The Space Shuttle Discovery is launched for the 31st time at the Kennedy Space Center in Florida on the morning of July 26, 2005. The smoky exhaust from Discovery is a mixture of water vapor and aluminum oxide.
Photo credit: NASA/Kennedy Space Center


The space shuttle Discovery blasted off a launch pad at the Kennedy Space Center on the morning of July 26. The seven member crew was heading into orbit to dock at the International Space Station and deliver supplies to that huge orbiting complex of laboratories.

The technology and planning that go into a space shuttle mission is mind-boggling. Chemistry, of course, plays a major role in space exploration, and the chlorine compound ammonium perchlorate, NH4ClO4, helps to launch space shuttles like Discovery by providing the thrust needed for lift-off at a point in time when the shuttle is at its heaviest.

Space Shuttle Basics

Space shuttles consist of three main sections:

(Diagram from
  • Orbiter - the winged vehicle we think of as the space shuttle in which the astronauts live and work.
  • Solid Rocket Boosters - two white pointed tanks on either side of the orbiter at the time of lift-off. Filled with ammonium perchlorate and other solid chemicals, they help thrust the orbiter into the atmosphere.
  • External Tank - the large orange tank attached to the orbiter at the time of lift-off. Filled with liquid hydrogen and oxygen fuels, the tank helps thrust the orbiter into space.

The Role of Solid Rocket Boosters

Escaping the pull of gravity on Earth is no small task. Most of the force needed to push the shuttle skyward in the first two minutes of blast-off comes from the two huge, white solid rocket boosters. These rockets are packed with propellant--a chemical mixture that, when burned, produces a tremendous thrust. Propellants consist of two main ingredients: a fuel and an oxygen source.

Discovery's upward thrust at lift-off came from a solid mixture of ammonium perchlorate and aluminum metal, along with an iron oxide catalyst to speed the burning reaction along, and a "binder" material to hold all the components together. The entire solid mixture has the consistency of a pencil eraser. Ammonium perchlorate is also used by the U.S. military in rockets, explosives, flares and ammunition.

The Chemistry of Lift-Off

The chemistry of the solid rocket booster propellant can be summed up in this reaction:


Once ignited, the fuel-burning reaction cannot be stopped. Oxygen from ammonium perchlorate combines with aluminum metal to produce aluminum oxide--the white solid Al2O3-- and aluminum chloride, AlCl3, water vapor and nitrogen gas. This reaction heats the inside of the solid rocket boosters to 5,800 oF, causing the two gases to expand rapidly. The expanding water vapor and nitrogen lift the rocket boosters with a tremendous force. All the fuel is burned in about two minutes.

The two solid rocket boosters provide about 71 percent of the total upward thrust at lift-off. Meanwhile, the main orbiter engines are also humming, making use of liquid hydrogen and oxygen fuel from the orange external tank:

This reaction also generates extremely high temperatures (approximately 6,000 oF), expanding the water vapor and generating an additional upward thrust.

Propelled into Space with Chemistry

Each solid rocket booster exerts a thrust, or force, of over three million pounds at launch. They provide "lift" for the orbiter to a distance of about 28 miles into the atmosphere. Then the boosters separate from the rocket, but continue on an upward path. Seventy-five seconds after separation the boosters reach their highest altitude (about 41 miles) and begin to fall back to Earth.

The solid rocket boosters parachute into the Atlantic Ocean when their mission to escort the orbiter is over. Solid rocket boosters are equipped with flotation devices, radio signals and lights so they can be located and towed back to Florida by ship for reuse.

Meanwhile, the orange external tank uses up its fuel in just over eight minutes-enough time to help get the orbiter into space and orbiting. The external tank then separates from the orbiter, falls back to Earth and disintegrates as it re-enters the atmosphere at about 62 miles or 100 kilometers. (This does not happen to the solid rocket boosters because they never leave the atmosphere.)

Delivered into orbit by propulsion chemistry, the astronauts are free to carry out their mission, advancing science and space exploration.  

Benefit and Risk: Perchlorate in the Environment

While ammonium perchlorate helps astronauts explore space and aids the military in defending our country, there has been a downside to its use. According to a recent report, perchlorate is present in the environment in 35 states, showing up at low levels in groundwater and in certain public water supplies and food sources. What is the effect of this on public health?

The health effects of perchlorate are difficult to pin-point, so a committee of scientific and public health experts was selected by the National Academy of Sciences to evaluate all the evidence for the health effects of perchlorate. The Academy is called upon often to help sort out complicated scientific and technical issues and to advise the federal government. In this case, the help of the Academy was requested by the U.S. Environmental Protection Agency (EPA), NASA, the Department of Defense and the Department of Energy. The committee wrote a 276-page report that will help the EPA set safe levels of perchlorate in the environment.

NASA works hard to find ways to explore space with the lowest possible degree of risk. Risk is a fascinating topic to explore. Consciously or unconsciously, we all evaluate and compare risks on a daily basis as we go about our activities. Some risks we take voluntarily; we have little or no control over others. For educational materials on risk, see Science Education For Public Understanding and Project Learning Tree.

Follow-up Activities:

  1. If the numbers of atoms of each element are the same on the product side of a chemical reaction as on the reactant side, the chemical reaction is said to be balanced. Use the table below to tally and compare the number of atoms on each side of the chemical reaction of Discovery's propellant.

    Here's your tally sheet with a little help getting started:

  2. List some of the risks and benefits of the space shuttle program. Discuss why we explore space knowing it is risky. What does NASA do to lower the risks of space travel?

For a list of previous "Chlorine Compound of the Month" features, click  here


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