Space Exploration

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Propulsion

Once in orbit, a spacecraft relies on its own rocket engines to change its orientation (or attitude) in space, the shape or orientation of its orbit, and its altitude. Of these three tasks, changes in orientation require the least energy. Relatively small rockets called thrusters control a spacecraft’s attitude. In a massive spacecraft, the attitude control thrusters may be full-fledged liquid-fuel rockets. Smaller spacecraft often use jets of compressed gas. Depending on which combination of thrusters is fired, the spacecraft turns on one or more of its three principal axes: roll, pitch, and yaw. Roll is a spacecraft’s rotation around its longitudinal axis, the horizontal axis that runs from front to rear. (In the case of the space shuttle orbiter, a roll maneuver resembles the motion of an airplane dipping its wing.) Pitch is rotation around the craft’s lateral axis, the horizontal axis that runs from side to side. (On the shuttle, a pitch maneuver resembles an airplane raising or lowering its nose.) Yaw is a spacecraft’s rotation around a vertical axis. (A space shuttle executing a yaw maneuver would appear to be sitting on a plane that is turning to the left or right.) A change in attitude might be required to point a scientific instrument at a particular target, to prepare a spacecraft for an upcoming maneuver in space, or to line the craft up for docking with another spacecraft.

When an orbiting spacecraft needs to drop out of orbit and descend to the surface, it must slow down to a speed less than orbital velocity. The craft slows down by using retrorockets in a process called a deorbit maneuver. On early piloted spacecraft, retrorockets used solid fuel because solid-fuel rockets were generally more reliable than liquid-fuel rockets. Vehicles such as the Apollo spacecraft and the space shuttle have used liquid-fuel retrorockets. In the deorbit maneuver, the retrorocket acts as a brake by firing into the line of flight. The duration of the firing is carefully controlled, because it will affect the path that the spacecraft takes into the atmosphere. The same technique has been used by Apollo lunar modules and by unpiloted planetary landers to leave orbit and head for a planet’s surface.

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Power Supply

Spacecraft have used a variety of technologies to provide electrical power for running onboard systems. Engineers have used batteries and solar panels since the early days of space exploration. Often, spacecraft use a combination of the two: Solar panels provide power while the spacecraft is in sunlight, and batteries take over during orbital night. The solar panels also recharge the batteries, so the craft has an ongoing source of power. However, solar panels are impractical for many interplanetary spacecraft, which may travel vast distances from the Sun. Many of these craft have relied on thermonuclear electric generators, which create power from the decay of radioactive isotopes and have lifetimes measured in years or even decades. The twin Voyager spacecraft, which explored the outer solar system, used generators such as these. Thermonuclear electric generators are controversial because they carry radioactive substances. The radioactivity poses no danger once the spacecraft reaches space, but some people worry that an accident during launch or during an unplanned reentry into Earth’s atmosphere could release harmful radiation into the atmosphere. Concerned groups protested the 1997 launch of the Cassini spacecraft, which carried its radioactive material in explosion-proof graphite containers.

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Effects of Space Travel on Humans

Space is a hostile environment for humans. Piloted spacecraft must supply oxygen, food, and water for their occupants. For longer flights, a spacecraft must provide a way to dispose of or recycle wastes. For very long flights, spacecraft will eventually have to become almost totally self-sufficient. For healthy spaceflight, the spacecraft must provide far more than just the core physical needs of astronauts. Exercise equipment, comfortable sleeping and recreation areas, and well-designed work areas are some of the amenities that soften spaceflight’s effects on humans.

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Crew Support

The effort to save weight is so inherent to spacecraft design that it even affects the food supply. Much of the food eaten by astronauts is dehydrated to save both weight and space. In space, astronauts use a device like a water gun to rehydrate these items. Many food items are also carried in conventional form, ranging from bread to candy to fruit.

On many spacecraft, including the U.S. space shuttle, drinkable water is produced by fuel cells that also provide electrical power. The reaction between hydrogen and oxygen that creates electricity produces water as a byproduct. A small supply of water for emergency use is also carried in onboard storage tanks.

For very long-duration missions aboard space stations, water is recycled. Drinkable water can be extracted from a combination of waste water, urine, and moisture from the cabin atmosphere. This kind of system was used on the Mir space station and is used on the International Space Station. See also Space Station.

Perhaps the question most frequently asked of astronauts is, “How do you go to the bathroom in space?” The answer has changed over the years. On early missions such as Mercury, Gemini, and Apollo, the bathroom facilities were relatively crude. For urine collection, the astronauts, all of whom were men, used a hose with a condom-like fitting at one end. Urine was then dumped overboard. Feces were collected in plastic bags and brought back to Earth for medical analyses. The Skylab space station featured a toilet that used forced air for suction. Mir used similar toilets, with special fittings for men and women, as does the space shuttle.

Skylab was also the first spacecraft to offer astronauts the chance to bathe in space, by means of a collapsible shower. To prevent globs of water from escaping and floating around inside the cabin, the astronaut sealed the shower once inside. The astronaut used a handheld nozzle to dispense water and a small vacuum to remove it. On the space shuttle astronauts and cosmonauts have had to make do with sponge baths. The International Space Station has a shower in its habitation module.

Most piloted spacecraft have carried oxygen in onboard tanks in liquid form at cryogenic (super-cold) temperatures to save space. Liquid oxygen is about 800 times smaller in volume than gaseous oxygen at everyday temperatures. The Russian Mir space station used an additional source of oxygen: Special generators aboard Mir separated water into oxygen and hydrogen, and the hydrogen was vented overboard.

On Mercury, Gemini, and Apollo, the cabin atmosphere was pure oxygen at about 0.3 kg/sq cm (about 5 lb/sq in). On the space shuttle a mixture of oxygen and nitrogen provides a pressure of 1.01 kg/sq cm (14.5 lb/sq in), slightly less than atmospheric pressure on Earth at sea level. Shuttle astronauts who go on spacewalks must pre-breathe pure oxygen to purge nitrogen from their bloodstream. This eliminates the risk of decompression sickness, called the bends, because the shuttle space suit operates at a lower pressure (0.30 kg/sq cm, or 4.3 lb/sq in) than inside the cabin. Sudden decompression can cause nitrogen bubbles to form in blood and tissues, a painful and potentially lethal condition. The International Space Station has an oxygen-nitrogen atmosphere at a pressure similar to that in the shuttle.

In the past, astronauts on missions of a few days or less have often worked long hours. Some found that their need for sleep was reduced because of the minimal exertion required to move around in microgravity. However, the intense concentration required to complete busy flight plans can be tiring. On longer missions, proper rest is essential to the crew’s performance. Even on the Moon, astronauts on extended exploration missions—with surface stay times of three days—knew that they could not afford to go without a good night’s sleep. Redesigned space suits, which were easier to take off and put on, and hammocks that were strung across the lunar module cabin helped the Moon explorers get their rest.

On the Skylab space station, each astronaut had a small sleeping compartment with a sleeping restraint attached to the wall. On Mir, cosmonauts and astronauts sometimes took their sleeping bags and moved them to favorite locations inside one module or another. The International Space Station, like Skylab, has private sleeping quarters, and these will be expanded in the future to accommodate a greater number of people.

Recreation is also essential on long missions, and it takes many forms. Weightlessness provides an ongoing source of fascination and enjoyment, offering the opportunity for acrobatics, experimentation, and games. Looking out the window is perhaps the most popular pastime for astronauts orbiting Earth, providing ever-changing vistas of their home planet. On some flights, astronauts and cosmonauts read books, play musical instruments, watch videos, and engage in two-way conversations with family members on the ground.

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Work in Space

Humans face many challenges when working in space. These challenges include communicating with Earth and other spacecraft, creating suitable environments for scientific experiments and other tasks, moving around in the microgravity of space, and working within cumbersome spacesuits.

Spacecraft in orbit around Earth cannot communicate continuously with the ground unless special relay satellites provide a link between the spacecraft and ground receiving stations. This problem disappears when astronauts leave Earth orbit. As Apollo astronauts traveled to the Moon, they were in constant touch with mission control. However, when they entered lunar orbit, communications were interrupted whenever the spacecraft flew over the far side of the Moon, because the Moon stood between the spacecraft and Earth. Lunar landing sites were on the near side of the Moon, so Earth was always overhead and the astronauts could maintain continuous contact with mission control. For astronauts who venture to other planets, the primary difficulty in communications will be one of distance. For example, radio signals from Mars will take as long as 20 minutes to reach Earth, making ordinary conversations impossible. For this reason, planetary explorers will have to be able to solve many problems on their own, without help from mission control.

The design of spacecraft interiors has changed as more powerful booster rockets have become available. Powerful boosters allow bigger spacecraft with roomier cabins. In Mercury and Gemini, for example, astronauts could not even stretch their legs completely. Their cockpits resembled those of jet fighters. The Apollo command module offered a bit of room in which to move around, and included a lower equipment bay with navigation equipment, a food pantry, and storage areas. The Soviet Vostoks had enough room for their sole occupant to float around, and Soyuz includes both a fairly cramped reentry module and a roomier orbital module. The orbital module is jettisoned prior to the cosmonauts’ return to Earth. The space shuttle has two floors—a flight deck with seats, controls, and windows and a middeck with storage lockers and space to perform experiments.

For the Skylab space station, designers had the luxury of creating several different kinds of environments for different purposes. For example, Skylab had its own wardroom, bathroom, and sleeping quarters. Designers have tried several different approaches to work spaces on spacecraft. Most rooms on Skylab were designed like rooms on Earth with a definite floor and ceiling. However, Skylab’s multiple docking adaptor had instrument panels on each wall, and each had its own frame of reference. Thanks to weightlessness, this was not a problem: Astronauts reported that they were able to shift their own sense of up and down to match their surroundings. When necessary, ceiling became floor and vice versa. On Salyut and Mir, the ceilings and floors were painted different colors to aid cosmonauts in orienting themselves. Because simulators on Earth were given the same color scheme, the cosmonauts were accustomed to it when they lifted off.

To help astronauts anchor themselves while they work in weightlessness, designers have equipped spacecraft with a variety of devices, including handholds, harnesses, and foot restraints. Foot restraints have taken a number of forms. Skylab crews used special shoes that could lock into a grid-like floor. Apollo astronauts used shoes equipped with strips of Velcro that stuck to Velcro strips on the capsule floor. Space shuttle astronauts have even used strips of tape on the floor as temporary foot restraints.

Astronauts and cosmonauts who perform spacewalks use a variety of devices to aid in mobility and in anchoring the body in weightlessness. Any surface along which astronauts move is fitted with handholds, which the astronauts use to pull themselves along. Foot restraints allow astronauts to remain anchored in one spot, something that is often essential for tasks requiring the use of both hands. During many spacewalks, astronauts use tethers to keep themselves from drifting away from the spacecraft. Sometimes, however, astronauts fly freely as they work by wearing backpacks with thrusters to control their direction and movement.

Astronauts who have conducted spacewalks report that the most difficult tasks are those that involve using their gloved hands to grip or manipulate tools and other gear. Because the suit—including its gloves—is pressurized, closing the hand around an object requires constant effort, like squeezing a tennis ball. After a few hours of this work, forearms and hands become fatigued. The astronauts must also keep careful track of tools and parts to prevent them from floating away. In general, designers of space hardware strive to make any kind of assembly or repair work in space as simple as possible.

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