Starship ENTERPRISE

To boldly go where know one has gone before.

Science of Space Exploration

Space is a harsh environment for humans and human-made machines. Radiation from the Sun and other cosmic sources can weaken material and harm the human body. In the vacuum of space, objects become boiling hot when exposed to the Sun and freezing cold when in the shadow of Earth or some other body. Scientists, engineers, and designers must make spacecraft that can withstand these extreme conditions and more.

General Principles of Spacecraft Design

The challenges that spacecraft designers face are daunting. Each component of a spacecraft must be durable enough to withstand the vibrations of launch, and reliable enough to function in space on time spans ranging from days to years. At the same time, the spacecraft must also be as lightweight as possible to reduce the amount of fuel required to boost it into space. Materials such as Mylar (a metal-coated plastic) and graphite epoxy (a construction material that is strong but lightweight) have helped designers and manufactures meet the requirements of durability, reliability, and lightness. Spacecraft designers also conserve space and weight by using miniaturized electronic components; in fact, the space program has fueled many advances in the field of miniaturization.

Since the early 1990s, budgetary restrictions have motivated NASA to plan projects that are better, faster, and cheaper. In this approach, space missions requiring single large, complex, and expensive spacecraft are replaced with more limited missions using smaller, less expensive craft. Although this new approach was successful with spacecraft such as the Mars Pathfinder lander and Mars Global Surveyor 96, budgetary constraints may have contributed to the loss of two other Mars spacecraft, Mars Climate Orbiter and Mars Polar Lander, in 1999. The approach is also difficult to apply to piloted spacecraft, in which the overriding concern is crew safety. However, engineers are always looking for new technologies to make spacecraft lighter and less expensive.

Getting into Space

One of the most difficult parts of any space voyage is the launch. During launch, the craft must attain sufficient speed and altitude to reach Earth orbit or to leave Earth's gravity entirely and embark on a path between planets. Scientists sometimes find it helpful to think of Earth's gravitational field as a deep well, with sides that are steepest near the planet's surface. The task of the launch vehicle or booster rocket is to climb out of this well.

Although some launch vehicles consist of just a single rocket, many are composed of a series of individual rockets, or stages, stacked atop one another. Such multistage launch vehicles are used especially for heavier payloads. With a multistage rocket, each stage fires for a period of time and then falls away when its fuel supply is used up. This lightens the load carried by the remaining stages. In some liquid-fuel boosters, strap-on solid-fuel rockets are used to provide extra thrust during the initial portion of ascent. For example, the Titan III booster has two liquid-fuel core stages and two strap-on solid-fuel motors. The largest example of a successful multistage booster was the Saturn V Moon rocket, which had three liquid-fuel stages and measured 111 m (363 ft), including the Apollo spacecraft, in length.

Despite their utility, most multistage boosters are not reusable, which makes them expensive. Cost-conscious engineers have focused on creating a single-stage-to-orbit (SSTO) vehicle. In an SSTO, the entire spacecraft and booster would be integrated into one fully reusable unit. If successful, this approach would reduce the costs of reaching Earth orbit. However, the technical challenge is enormous: A full 89 percent of an SSTO's total weight must be reserved for fuel, a much higher proportion than any previous launch vehicle. The payload, the crew, and the weight of the vehicle itself must make up only 11 percent of the SSTO's total weight.

Navigation in Space

Spaceflight requires very detailed planning and measurement to get a spacecraft into place or to send it on its proper path. Some of the Apollo spacecraft were able to travel from Earth to the Moon (a distance of almost 390,000 km, or almost 240,000 mi) and land on the lunar surface within a few dozen meters (several dozen feet) of their target. Careful planning allowed the Mars Pathfinder spacecraft to fly from Earth to Mars, traveling more than 500 million km (300 million mi), and land just 19 km (12 mi) from the center of its target area.

Flight Paths

To launch a spacecraft into orbit around Earth, a booster rocket must do two things. First it must raise the spacecraft above the atmosphere—roughly 160 km (100 mi) or more. Second it must accelerate the spacecraft until its forward speed—that is, its speed parallel to Earth's surface—is at least 28,200 km/h (17,500 mph). This is the speed, called orbital velocity, at which the momentum of the spacecraft is strong enough to counteract the force of gravity. Gravity and the spacecraft's momentum balance so that the spacecraft does not fall straight down or move straight ahead—instead it follows a curved path that mimics the curve of the planet itself. The spacecraft is still falling, as any object does when it is released in a gravitational field. But instead of falling toward Earth, it falls around it. See Orbit.

Using its own thrusters, a spacecraft can raise or lower its orbit by adding or removing energy, respectively. To add energy, the spacecraft orients itself and fires its thrusters so that it accelerates in its direction of flight. To subtract energy, the craft fires its engines against the direction of flight. Any change in the height of a spacecraft's orbit also produces a change in its speed and vice versa. The craft moves more slowly in a higher orbit than it does in a lower one. By firing its rockets perpendicular to the plane of its orbit, the craft can change the orientation of its orbit in space.

To travel from one planet to another, a spacecraft must follow a precise path, or trajectory, through space. The amount of energy that a spacecraft's launch rocket and onboard thrusters must provide varies with the type of trajectory. The trajectory that requires the least amount of energy is called a Hohmann transfer. A Hohmann transfer follows the shape of an ellipse, or a flattened circle, whose sides just touch the orbits of the two planets.

The trajectory must also take into account the motion of the planets around the Sun. For example, a probe traveling from Earth to Mars must aim for where Mars will be at the time of the spacecraft's arrival, not where Mars is at the time of launch.

In many interplanetary missions, a spacecraft flies past a third planet and uses the planet's gravitational field to bend the craft's trajectory and accelerate it toward its target planet. This is known as a gravitational slingshot maneuver. The first spacecraft to use this technique was the Mariner 10 probe (see Mariner), which flew past Venus on its way to Mercury in 1974.

Navigation and Guidance

Most spacecraft depend on a combination of internal automatic systems and commands from ground controllers to keep on the correct path. Normally, ground controllers can communicate with a spacecraft only when it is within sight of an Earth-based receiving station. This poses problems for spacecraft in low Earth orbit—that is, within 2,000 km (1,200 mi) of the planet's surface—as such craft are only within sight of a relatively small portion of the globe at any given moment. One way around this restriction is to place special satellites in orbit to act as relays between the orbiting spacecraft and ground stations, allowing continuous communications. NASA has done this for the U.S. space shuttle with the Tracking and Data Relay Satellite System (TDRSS).

At an altitude of about 35,800 km (about 22,200 mi), a satellite's motion exactly matches the speed of Earth's rotation. As a result, the satellite appears to hover over a specific spot on Earth's surface. This so-called stationary, or geosynchronous, orbit is ideal for communications satellites, whose job is to relay information between widely separated points on the globe.

Spacecraft on interplanetary trajectories may travel millions or even billions of kilometers from Earth. In these cases their radio signals are so weak that giant receiving stations are necessary to detect them. The largest stations have antenna dishes in excess of 70 m (230 ft) across. NASA and the Jet Propulsion Laboratory operate the Deep Space Network, a system of three tracking stations with several antennas each. The stations are in California, Spain, and Australia, providing continuous contact with distant spacecraft as Earth spins on its axis.

Much of the work of ground controllers involves monitoring a spacecraft's health and flight path. Using a process called telemetry, a spacecraft can transmit data about the functioning of its internal components. In addition, engineers can use a spacecraft's radio signals to assess its flight path. This is possible because of the Doppler effect. Because of the Doppler effect, a spacecraft's motion causes tiny shifts in the frequency of its radio signals—just as the motion of a passing car causes the apparent pitch of its horn to go up as the car approaches an observer and down as the car moves away. By analyzing Doppler shifts in a spacecraft's radio signals, controllers can determine the craft's speed and direction. Over time, controllers can combine the Doppler shift data with data on the spacecraft's position in the sky to produce an accurate picture of the craft's path through space.

The guidance system helps control the craft's orientation in space and its flight path. In the early days of spaceflight, guidance was accomplished by means of radio signals from Earth. The Mercury spacecraft and its Atlas booster utilized such radio guidance signals broadcast from ground stations. During launch, for example, the Atlas received steering commands that it used to adjust the direction of its engines. However, Mercury flight controllers found that radio guidance was limited in accuracy because interference with the atmosphere tends to make the signals weaker.

Beginning with Gemini, engineers used a system called inertial guidance to stabilize rockets and spacecraft. This system takes advantage of the tendency of a spinning gyroscope to remain in the same orientation. A gyroscope mounted on a set of gimbals, or a mechanism that allows it to move freely, can maintain its orientation even if the spacecraft's orientation changes. An inertial guidance system contains several gyroscopes, each oriented along a different axis. When the spacecraft rotates along one or more of its axes, measuring devices tell how far it has turned from the gyroscopes' own orientations. In this way, the gyroscopes provide a constant reference by which to judge the craft's orientation in space. Signals from the guidance system are fed into the spacecraft's onboard computer, which uses this information to control the craft's maneuvers.

The Global Positioning System satellites, which enable ships, airplanes, and even hikers to know their positions with extreme accuracy, should play a similar role in spacecraft. The space shuttle Atlantis was equipped with GPS receivers during an upgrade in late 1998.

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.

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.

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.

Crew Support

On Mercury, Gemini, and Apollo, the cabin atmosphere was pure oxygen at about 0.3 kg/sq cm (about 5 lbs/sq in). On the space shuttle a mixture of oxygen and nitrogen provides a pressure of 1.01 kg/sq cm (14.5 lbs/sq in), slightly less than atmospheric pressure on Earth at sea level. Shuttle astronauts who go on space walks 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 lbs/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 micro gravity. 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.

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 micro gravity of space, and working within cumbersome spacesuit's.

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 mid-deck 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.