About Capsules

Capsules are intended to land people or instruments on the surface of Earth or another planet.

What does a space capsule contain?
Space capsules are the compartments designed to support humans during their journey through space. They must contain the basic elements that astronauts need to live — air to breathe, water to drink, and food to eat. They also have to protect the astronauts from the cold of space and space radiation. Capsules are well insulated and contain systems to adjust the internal temperature. There must be a way for the astronauts to secure themselves so they don't get jostled around during launch or re-entry; for this there are seats with strap systems. They also may need to strap themselves in a seat to work or bed to sleep when they are in space because they will be weightless. Capsules also have to be equipped with a way to communicate with mission control.

What are the forces acting on a capsule when it is approaching a planet's surface?
Gravity and drag are the most significant forces a capsule and its contents — delicate instruments and humans — experience.

Gravity is a property of matter and is one of the universal forces of nature that attracts one object to another . The gravitational force between two objects depends on their masses; the greater the object's mass, the greater its gravitational force. Consequently, it is easiest to see gravity in action when at least one of the objects is very large, such as the Earth. When rockets leave Earth to get to another planet, they are working agai nst the strong pull of gravity. When capsules return to Earth, gravity pulls them and speeds them up. For example, 3 seconds of free fall toward Earth increases the speed of the capsule by 29.4 meters per second (about 65 miles per hour). Different planets, moons, and other large bodies in our Solar System have different masses and, therefore, exert different gravitational attractions. M artian gravity is about one-third of Earth's gravity.

Drag is the capsule's resistance to motion caused by the rocket's movement through air. Air is a mixture of atoms and molecules. An object falling through an atmosphere hits these particles, causing it to slow. The drag experienced by a capsule depends on several factors, including the density of the air, the shape of the rocket, the amount of surface area, and the roughness of the rocket's surface. The actual speed that a spacecraft experiences is the combined effect of between the gravitational pull of the planet (which speeds up the capsule) and drag on the capsule (which slows it down). Capsules entering Earth's atmosphere are slowed significantly by the thick atmosphere.

The collisions also produce friction, creating heat. A shooting star is a vivid example of what happens to an object falling through an atmosphere. Typically, these objects are sand-grain-sized meteorites. They collide with air particles, creating so much heat that the air around the meteorite glows white hot. Small meteorites burn up before reaching the surface.

Why don't space capsules continue to accelerate as they fall?
The rate at which capsules fall is called terminal velocity. This rate is the speed at which drag ( air resistance on a capsule) matches the pull of gravity, resulting in a constant fall rate. Typical terminal velocity is 200 to 300 kilometers per hour (about 125 to 185 miles per hour). Terminal velocity is greatly affected by the shape of the object facing the direction it is moving (a rocket point has less resistance than the wide base of a capsule). Once an object reaches terminal velocity, it does not accelerate unless the driving or resistive forces change.

What factors have to be considered when designing a space capsule?
Engineers need to be concerned with building a capsule that is strong enough to slow down quickly, can survive high temperatures, and can withstand quite a jolt when it lands.

When a space capsule approaches a planet's surface, it needs to slow down. If it slows down — decelerates — too fast, the capsule and its contents can be crushed. Shape is important in controlling how the spacecraft slows. Engineers design capsules with a blunt shape rather than a streamlined one; this causes more resistance and helps the spacecraft to slow, but it also causes the capsule to heat up as it passes through an atmosphere. Parachutes may also be used to create more drag and slow the capsule's descent.

As the capsule slows, much of the energy from movement is converted to heat (friction from air molecules striking the capsule's surface). The mechanisms that engineers use to help slow the capsule's descent by increasing drag actually present the engineers with another problem — high temperatures! The surface of the Space Shuttle can reach temperatures of 1480 degrees C (~2700 F) as it descends through Earth's atmosphere. This heat has to be dissipated. Early space capsules, such as the Apollo capsules, were coated with material that melted and vaporized. While this may seem counterintuitive, the vaporization actually carried heat away from the capsule. The Space Shuttle is protected by silica tiles; silica is a strong insulator. These tiles are specially designed to be highly porous (light weight) and have very low conductivity for heat. They keep the heat of reentry from reaching the interior of the Space Shuttle.

Capsules also have to be able to survive impact at high speed when they reach Earth's surface. The earliest capsule landings took place in the water. These capsules were not self powered and the astronauts could not maneuver the craft; they essentially fell through the atmosphere. Today's Space Shuttle is similar to a plane in its design. Upon re-entry into Earth's atmosphere, a computer guides the Shuttle through a series of maneuvers that allow it to gradually slow. As it approaches the runway, the Shuttle commander and pilot take control and fly the Shuttle in for landing. Regardless of the type of capsule, the body must be light weight, very strong and heat resistant. Materials often are designed in unique ways, such as the Apollo's honey-combed structure of aluminum. Aluminum is light weight; the structure adds strength to the capsule. The early spacecraft were coated in glass cloth that had been embedded with synthetic resin and subjected to high temperatures. New materials, such as carbon fiber reinforced plastics and ceramic, constantly are under development for use in space exploration.

How do spacecraft land on a planet's or moon's surface without being destroyed?
Most space capsules use parachutes to slow their descent, reduce their acceleration, and aid in a soft landing. Some capsules, such as the Russian Soyuz spacecraft, use both parachutes and jets that fire immediately before impact to help reduce the force of the impact. Three recent robotic spacecraft to Mars (Mars Pathfinder in 1997 and two Mars Exploration Rover missions in 2004) used a combination of parachutes and large airbags, which cushioned the spacecraft, to bounce along the surface before coming to rest. Spacecraft like the shuttle are designed to fly like gliders and land aerodynamically.

Landing on the Moon provided more challenges for slowing spacecraft. The Moon has no atmosphere, therefore there are no atoms or molecules for the spacecraft to pass through. In one way, this is good – no collisions with particles means no heat from friction. However, no atmosphere also means that there is no drag on the capsule to slow its descent. Parachutes will not help; parachutes work because of air resistance — drag. Lunar landing vehicles were equipped with rocket engines that were fired by the pilot to provide lift — thrust in the opposite direction of descent - during the rapid descent to the Moon's surface.

Humans in Space
Prior to human exploration of space, test flights involved animals, including dogs, monkeys, and mice. In 1957, Russian scientists sent the first dog into space to allow them to investigate the effects of space flight on a living organism This was followed by other missions involving animals, leading up to the successful 108 minute Earth orbit by Cosmonaut Yuri Gagarin on 12 April 1961. Astronaut Alan Shepard was the first American astronaut to orbit Earth; he was aboard the Mercury capsule. The Gemini capsule carried the second generation of astronauts into Earth orbit for longer periods of time. The Apollo capsule took astronauts to the Moon, and the Lunar Module landed astronauts on the surface. Dozens of Russian cosmonauts have orbited Earth in the Russian Soyuz capsule. The Space Shuttle today serves as a means of transport and support for astronauts as they move between Earth and the International Space Station. Unlike earlier capsules, the Space Shuttle is designed to be used for many flights.

Space capsules have not always been successful and the price for exploration is high when counted in human lives; one early Soyuz capsule lost three cosmonauts when it depressurized upon re-entry. Two Space Shuttles, the Challenger and the Columbia, and their crews, were tragically destroyed due to malfunctions. The Challenger failed during lift-off, when a seal malfunctioned in the solid-rocket booster, causing the craft to explode 73 seconds after launch. The Columbia was destroyed on February 1, 2003, during re-entry, when a catastrophic failure occurred due to damage caused by foam that fell and struck the panels on the underside of the wing during launch.

Unmanned Space Capsules
Unmanned space probes have landed on Venus, the Moon, Mars, and Jupiter, as well as on asteroid Eros. They have sampled solar wind, flown through the tail of a comet, and landed on Titan, one of Saturn's moons. In the 1970's the Russians sent eight Venera spacecraft to Venus to explore this planet with extreme surface temperatures and pressures – and an atmosphere with carbon dioxide and sulfur. Under these conditions, none of the probes was expected to last long – and none did! However, the probes returned images and atmospheric data, just as planned. Early U.S. and Russian Moon landers (the Surveyor and Luna programs, respectively) brought back images of the Moon before humans landed there. The Viking 1 and 2 Mars landers in 1976 sent back images and data from the martian surface. In 1997 the Mars Pathfinder bounced over the surface of Mars, landing right side up and opening to deliver the Sojourner rover to the surface to transmit images and data back to Earth. Two more rovers successfully followed in 2003 and 2004 when Spirit and Opportunity were delivered to the surface of Mars to extend our understanding of the history of water on Mars. These rovers dropped toward the surface in capsules that used parachutes to slow their descent. The capsules were surrounded by protective balloons that deployed close to the surface, allowing the rovers to bounce across the surface — over 25 times — before coming to rest. NASA has turned its focus to having humans living and working on the Moon and Mars in the next few decades. In order to accomplish this, it will be necessary to gather information about these two places using orbiters, probes, and sample return missions — all before the next stages of direct human exploration.

How is our egg-drop capsule like a real spacecraft?
As an object falls toward a planet or moon, gravity pulls it, causing it to accelerate until it impacts the surface. Planets with atmospheres create friction with the spacecraft, which slows the re-entry. Children can borrow ideas used in spacecraft. For example, they can take advantage of air resistance by creating a wide base and using parachutes, wings, and propellers. They can calculate their egg's speed at impact and estimate the force generated, which would help them determine how much padding they need. Children can design padded capsules, capsules with parachutes, capsules with airbags, or virtually any type of capsule using the materials on hand.

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Last updated
January 21, 2005