Tracking Spacefaring Objects

The methods used to track objects orbiting the Earth include optical, radio and radar techniques. Optical techniques range from observations with the naked eye through to photographic methods. Radio tracking is any process that receives energy transmitted by the object being tracked. Radar bounces energy, from a source, usually on the ground, off the satellite, and back to a receiver. There are limitations to each method, and so combinations of them all are used to track satellites and space...

Pre Spaceflight

Rockets have been used for ceremonial and warfare purposes since the ancient Chinese first created them, probably in about ad 600. The first written record of the use of rockets as powered artillery was during the siege of Kai-Fung-Fu in ad 1232 , when the Chinese used rocket fire-arrows to repel the Mongol invaders. Through the 13th century other Asian armies developed gunpowder-propelled fire-arrows, and the use of these weapons quickly spread throughout Asia, Europe and the Arab countries....

Ground Tracks

The point on the Earth's surface directly below the satellite is known as the subsatellite point, and the path on the ground that it follows during its orbit is called the ground track. The subsatellite point is where an imaginary line taken from the centre of the satellite to the centre of the Earth hits the Earth's surface. If only the altitude of the satellite's orbit is changed, the ground track remains the same. The position of the ground track is useful for many reasons, including weather...

Tracking Telemetry and Command

The information about and the control of a satellite is called the tracking, telemetry and command or TT& C. The tracking element of the TT& C system determines the position of the spacecraft in space. This uses information such as the angle between the spacecraft and the receiver on the ground and the range rate, or the speed or rate at which the range or distance between the receiver and the spacecraft changes. The data from the onboard instruments that monitor the health and position...

Living in Space

The terms zero gravity, weightlessness, freefall and micrograv-ity are all used in connection with space, some of which are misleading. There is no such thing as no gravity, as might be assumed from the term zero gravity. In space, just as on the Earth, the force of gravity is always there, be it from the Earth, Moon, Sun or any other body. In a spacecraft, to all appearances there is no gravity, as everything floats. This is why it has been called zero gravity or zero G. However, everything in...

Launch System

A rocket launch site must meet many criteria. Unless a perfect site or location is available, a compromise must be made between the mission requirements, geography, climate and logistics. The most important requirement is safety. This has led to most launch sites being built in remote, unpopulated areas, such as deserts or coastal areas, as an accident involving burning fuels in an urban area could be disastrous. The climate of the site makes a lot of safe areas such as islands and deserts...

Chemical Rocket Propulsion

Solid Rocket Motor Design

Chemical rocket propulsion is the only method we currently have to escape from the surface of the Earth. No other type of propulsion system can produce enough thrust to overcome the gravity on the Earth's surface. Launch vehicle propulsion systems must have a high thrust and therefore produce a lot of force, but they are usually only required to run or burn for a short time such as a few minutes. Chemical rockets not only work in space, but they also work in the atmosphere. However, until the...

Amateur Tracking

If the sky is clear, a number of satellites should be visible in the early morning just before dawn or in the early evening just after dusk. It is pleasant to just watch the satellites cross the sky and daydream about what they may be, but it is easily possible to look for a specific satellite. Since the 1990s satellite predictions or tracking programs have become readily available on the Internet and provide information in various forms and are available both for viewing satellites and also...

Attitude Control and Movement

The attitude, or direction the spacecraft is pointing, must be controlled and kept stable once in space. This is so that the communications antenna can be accurately pointed towards the Earth and any onboard instruments can obtain data from known directions. Also, propulsive manoeuvres used to change the orbit must be conducted when the satellite is orientated in the correct direction, otherwise the new orbit will be incorrect. Some instruments or equipment, such as solar panels and optical...

Catalogues of Objects in Space

Of the 10,000 items in our solar system that are currently tracked, only about 700 are operational satellites, the rest is junk and other space debris. It is important to know the location of all working satellites, so that information, commands and other data can be transferred between them and the Earth. There would be no point broadcasting into space a set of commands for a specific satellite if that satellite was over the opposite side of the world. However, it also important to know where...

Emergency Systems

Manned spacecraft require an emergency escape system from the launch vehicle in case of, for example, a launch pad emergency such as a fire. There are different designs, depending on the type of launch vehicle and how many crew are onboard. A Launch Escape System (LES) uses rockets to separate and remove the crew module from the launch rocket quickly. The system must be able to take the crew to a height sufficient that when the parachutes are deployed, their module can then be slowed and land...

Changing Orbits

Changing Orbital Speed

The orbit of most spacecraft will have to be changed at sometime in its life. This could be from a parking orbit to the final orbit or trajectory, to avoid space debris or micrometeors, as protection from solar storms or to correct an orbit that has changed due to perturbations. Also, at the end of the spacecraft's life, it may be transferred to a graveyard orbit. The main influence on an orbit is usually the gravity of the body being orbited. However, other, usually much smaller, influences...

Urine Transfer System

Figure 8.7 Apollo Urine Receptacle Assembly. Image courtesy NASA sticky tape. The bag had a finger pocket in the centre of one side, which was like a glove, so that a finger could be inserted but remain protected. After defecation, the finger was used to separate any remaining matter from the anus, and push the faeces down into the bag. After using tissue wipes, which were also put into the bag, the crewmember then had to seal the bag and knead it, so that it mixed with a germicide that...

Launch and Reentry

The human body can only tolerate a certain amount of physical stress before permanent damage occurs. Manned spacecraft must be designed with these limitations in mind. The physical stresses of most concern during the launch are acceleration, noise and vibration. The force felt as a result of acceleration cannot be blocked, unlike sound that can be diminished by wearing ear defenders. Therefore an astronaut must endure the force from the acceleration at launch and the maximum acceleration of the...