Spacecraft glossary
A-G
AOCS
See Attitude and Orbit Control System
Apogee
The apogee is the point in the orbit of a body, such as, the Moon or an artificial satellite that is furthest from the Earth. The path that these bodies take as they orbit the Earth is in the shape of an ellipse, rather than a perfect circle. (Also, see Perigee)
Attitude
Attitude refers to the orientation of the spacecraft in space.
Attitude and Orbit Control System
The Attitude and Orbit Control System (AOCS) monitors and controls the spacecraft's orientation (attitude) and its orbit. It includes: a computer, Star Trackers, Sun Acquisition Sensors and reaction wheels. One function of this system is to make sure that the spacecraft reaches and maintains its planned operational orbit. When a spacecraft is launched the launcher hauls it through Earth's atmosphere and then releases it on the general trajectory it needs to get into orbit. Once separated from the launcher (see separation ring), the AOCS can command the small thrusters onboard the spacecraft to bring it into its operational orbit. This system also ensures the spacecraft is pointing in the right direction, so that the onboard scientific instruments are looking towards the desired astronomical target and that the telecommunication antenna can transmit to ground-stations on Earth. In addition, this system steers, or slews, the spacecraft so that the scientific instruments can view different parts of the sky and keeps it stable during observations. (See Reaction Control System, Reaction wheels, Star Tracker, and Sun Acquisition Sensor).
Coded-mask imaging
Gamma rays and X-rays are extremely difficult to focus using traditional lenses and mirrors, if not impossible in the case of gamma-rays. Therefore, the three instruments that detect high-energy radiation onboard INTEGRAL rely on a technique known as coded-mask imaging. A coded-mask telescope is a pinhole camera with not just one but many pinholes. INTEGRAL's IBIS, SPI and JEM-X don't have a system of lenses and mirrors. Instead, they have a mask that is made up of a pattern of holes. The high-energy radiation can pass through the holes and onto the detector, but is blocked by the opaque parts of the mask. The mask creates a shadow pattern, so when gamma rays pass through the holes from a different direction the shadow pattern falls onto a different place of the detector. These overlapping patterns are then disentangled by computer software that knows the pattern of the mask's shadow, and converted into a map or image of the gamma-ray sources in the sky.
Communications
See telecommunications
Coolers
On Earth, air and water transport heat from one place to another, distributing the temperature evenly. In the harsh environment of space there is no air or water, or any other medium to transport heat. As a spacecraft orbits the Earth it is exposed to extreme differences in temperature depending on whether it is illuminated by the Sun, or Earth or not. The detectors of INTEGRAL's spectrometer work best within a particular low temperature range and therefore need to be protected from the extreme heat or cold. On the INTEGRAL spacecraft this is achieved using electric heaters and coolers (see Heaters, and Thermal control system).
Data handling system
This system is an onboard computer that manages all of the spacecraft's activity and includes a clock and a telemetry system. It interprets and distributes commands sent by the team of Ground Operation Controllers and collects and processes telemetry data ready to send back to Earth (see Telecommand, and Telemetry).
H-R
Heaters
On Earth, air and water transport heat from one place to another, distributing the temperature evenly. In the harsh environment of space there is no air or water, or any other medium to transport heat. As a spacecraft orbits the Earth it is exposed to extreme differences in temperature depending on whether it is illuminated by the Sun or not. The instruments of a spacecraft work best within a particular temperature range and therefore need to be protected from the extreme heat or cold. On the INTEGRAL spacecraft this is achieved using electric heaters and coolers. The heaters are employed for example when the spacecraft is in the Earth's shadow (during a so-called eclipse). (See Coolers, and Thermal control system).
Hold-down
These are small devices that keep parts of the spacecraft such as antennas and solar arrays into a stowed position during launch. Once the spacecraft has been launched small explosive charges release the hold-downs to allow these parts to unfold.
IBIS
See Imager on-Board the Integral Satellite
Imager on-Board the Integral Satellite
The Imager on-Board the Integral Satellite (IBIS) provides high resolution images of many different types of celestial objects. A few examples are, binary star systems where one of the components is a black hole, a white dwarf or a neutron star; gamma-ray bursts - the most powerful explosions known in the Universe; active galactic nuclei – galaxies emitting large amounts of energy from a small, dense source at their centres and clusters of galaxies.
IBIS produces sharper images than any previous gamma-ray instrument.
JEM-X
See Joint European X-Ray Monitor
Joint European X-Ray Monitor
The Joint European X-Ray Monitor (JEM-X) makes observations simultaneously with the two main gamma–ray detectors, SPI and IBIS. JEM-X plays a crucial role in the detection and identification of gamma-ray sources by making observations in the lower X-ray energy band and producing images. As with IBIS and SPI, this instrument works using the coded-mask technique.
MLI
See Multi Layer Insulation
Multi Layer Insulation
Multi Layer Insulation (MLI) is a special foil that is used as a thermal insulation material. A spacecraft is wrapped in this material to protect it from the harsh thermal environment in space. As the name indicates the foil is built up from several layers of different materials, to achieve the desired insulating properties.
OMC
See Optical Monitoring Camera
Optical Monitoring Camera
The Optical Monitoring Camera (OMC) uses a CCD (charge-coupled device) to take images of the visible light coming from the sources that INTEGRAL's high-energy instruments observe at the same time in the gamma-ray and X-ray parts of the spectrum. These images also provide data for accurately calculating the pointing of the spacecraft's scientific instruments. The OMC tells INTEGRAL astronomers exactly where the SPI or IBIS instruments are looking in the sky. This is important when looking at high-energy emission from celestial sources because this emission can change very quickly.
Payload Module
The payload module (PLM) carries INTEGRAL'S four scientific instruments and is mounted on top of the service module (see SVM). These modules form the two main parts of the INTEGRAL spacecraft.
Perigee
The perigee is the point in the orbit of a body, such as, the Moon or an artificial satellite that is closest to the Earth. The path that these bodies take as they orbit the Earth is in the shape of an ellipse, rather than a perfect circle. (Also, see Apogee)
PLM
See Payload Module
Radiator
A radiator is a passive cooler that removes excess heat from the spacecraft via a surface that radiates heat out into space.
Reaction wheels
Reaction wheels (sometimes called momentum wheels) are small spinning wheels that are used to control the spacecraft's attitude. If the Star Tracker or Sun Acquisition Sensor detect that the spacecraft is drifting from its desired orientation, the spinning reaction wheels speed up or slow down to correct the position. A spacecraft or satellite that uses reaction wheels is known as a 'three-axis stabilised' satellite. (See Attitude and Orbit Control System).
S-Z
SAS
See Sun Acquisition Sensor
Scientific instruments
The INTEGRAL gamma-ray observatory has four scientific instruments; SPI, IBIS, JEM-X and OMC, that make observations in order for the mission's science objectives to be achieved. INTEGRAL's mission objectives involve providing new insight into some of the most violent and exotic objects in the Universe.
Separation Ring
The separation ring is the connecting element between a spacecraft and the launcher. It is an important mechanism that releases, or separates, the spacecraft from the launcher at just the right moment once they have reached space.
Service Module
The service module (SVM) is the part of a spacecraft that holds all the utilities and services required to operate the spacecraft and the scientific instruments that are housed in the payload module (see PLM). These essential systems look after the spacecraft's electrical power (via the Solar Array), attitude, orbit, temperature (thermal control), data handling and telecommunications.
Solar Array
A spacecraft's Solar Array is made up of a large number of solar cells that convert light from the Sun into electric energy for operating the spacecraft and the instruments on board. Solar arrays can be placed flat on the hull of the spacecraft, or attached as wings that extend outward. Solar Array wings can also be given the ability to turn and orient to catch the maximum amount of sunlight available at any position (see Solar Array Drive Mechanism). For fixed wings this is achieved instead by the orientation of the entire spacecraft. When a spacecraft orbits a Solar System body (like a planet or moon) there are times when the spacecraft is in the shadow of the object that it is orbiting and the Solar Arrays are not illuminated by the Sun. During these periods, called eclipses, the spacecraft runs on power stored in batteries.
Spectrometer
The Spectrometer on Integral (SPI) performs high resolution spectrography. It measures the energy of gamma rays that are detected. These gamma rays are produced by some of the most energetic phenomena in the Universe, such as neutron stars, black holes and supernovae.
SPI
See Spectrometer
Star Tracker
The Star Tracker is the primary compass of a spacecraft. It compares the image of stars in its field of view with a star map of the sky. As a result the spacecraft navigation system receives information about the orientation and attitude of the spacecraft in its orbit. This is a critical function and therefore spacecraft usually are equipped with two Star Trackers just in case one is no longer able to function. (See Attitude and Orbit Control System)
STR
See Star Tracker
Sun Acquisition Sensor
The Sun Acquisition Sensor (sometimes shortened to Sun Sensor) is similar to the Star Tracker. It is a device to determine the orientation of the spacecraft that uses the Sun rather than the stars. Sun Sensors play an important role should there be an emergency on the spacecraft. For example, if for an unknown reason the spacecraft stops operating normally and the spacecraft does not report back to ground as it routinely should, the spacecraft's onboard computer will declare an emergency. The spacecraft then shuts down all but the essential systems and is put into a slow spin, during which the Sun Sensor searches for the Sun. Once the Sun has been found the Solar Arrays are orientated towards the Sun to charge the batteries. One by one other systems onboard the spacecraft, such as the Star Tracker, begin to operate. The spacecraft then turns the communications antenna to point towards the Earth so that it can transmit its status and await new instructions from the Ground Control Operators at the European Space Agency's Operation Centre (ESOC). (See Attitude and Orbit Control System)
SVM
See service Module
Telecommand
Telecommands are the commands sent from Earth to the spacecraft to operate it remotely. Typically these include instructions for the spacecraft pointing, changing system or instrument settings, and performing observations.
Telecommunications system
This system consists of antennas and radio equipment that are used to transmit and receive radio signals to and from Earth (see Telecommand and Telemetry).
Telemetry
Telemetry is the data that are sent from the spacecraft down to Earth. Telemetry contains information about the health of the spacecraft and its instruments and the data gathered from observations made by the payload of scientific instruments.
Thermal control system
This system monitors the temperature and keeps all parts of the spacecraft within specified temperature limits so that all the equipment doesn't get too hot or too cold (see Coolers, Heaters and Radiator).