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T

Telemetry

Data and commands sent from the spacecraft to ground stations.

Telescope

Instrument used to focus electromagnetic radiation (light, X-rays...) into an image.

Telescope (Wolter)

German physicist Hans Wolter (1911-1978) conceived a series of designs for grazing-incidence telescopes, requiring photons to undergo two successive reflections from combinations of paraboloid-hyperboloid (particularly suited for extended field of view imaging) or paraboloid-ellipsoid surfaces (for spectroscopy requiring smaller field of view but higher spatial resolution).

Temperature

Physical parameter characterising the thermal state of a body. Measured in units of degrees Celsius (oC), Fahrenheit (oF) or Kelvin (K).

Terrestrial planets

The four innermost planets in the Solar System, which have solid rocky surfaces.

Testing (spacecraft)

All the procedures, including the simulation of space environment, to ascertain that a spacecraft is ready for launch.

Thermal gradient

The rate at which the temperature changes with position.

Thrusters

Small reaction engines on a spacecraft that can provide thrust used to control its orbit, orientation and attitude.

Titan

The largest and most intriguing moon of Saturn. Titan is the second largest moon in the Solar System after Ganymede (one of Jupiter's moons), and is larger than the planet Mercury. Titan was discovered by the Dutch astronomer Christiaan Huygens on March 25 1655.

Tracking

Ground facilities employed to follow the progress and to communicate with a satellite.

Tube

Cylindrical or conical structure in a telescope between its optics and its focal plane.

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