TD-1A
Encyclopedia
TD-1A, or Thor-Delta 1A, was a Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

an astrophysical
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...

 research satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 which was launched in 1972. Operated by the European Space Research Organisation
ESRO
The European Space Research Organization was an international organisation founded by 10 European nations with the intention of jointly pursuing scientific research in space. It was founded in 1964...

, TD-1A made astronomical surveys primarily in the ultraviolet, but also using x-ray
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...

 and gamma ray
Gamma-ray astronomy
Gamma-ray astronomy is the astronomical study of the cosmos with gamma rays. Gamma-rays are the most energetic form of "light" that travel across the universe, and gamma-rays thus have the smallest wavelength of any wave in the electromagnetic spectrum.Gamma-rays are created by celestial events...

 detectors.

Spacecraft

TD-1A was named after the Thor-Delta series of rockets, a derivative of which was used to launch it. It was a 473 kilograms (1,042.8 lb) satellite which measured 100 centimetres (39.4 in) by 90 centimetres (35.4 in) by 220 centimetres (86.6 in). The spacecraft was three-axis stabilised
Three-axis stabilisation
Three-axis stabilisation involves the use of three gyroscopes—one for each axis —to keep satellites correctly oriented in space.The same technology is used to 'steer' guided missiles, but it involves only 2 gyroscopes....

, with sun sensors used to maintain a constant attitude with respect to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

.

Instruments

Seven instruments were carried aboard TD-1A, with a combined mass of 120 kilograms (264.6 lb).

The Stellar UV Radiation Experiment, operated by University College London
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...

 and the University of Liège
University of Liège
The University of Liège , in Liège, Wallonia, Belgium, is a major public university in the French Community of Belgium. Its official language is French.-History:...

, consisted of a 1.4 metres (4.6 ft) ultraviolet telescope, attached to a spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...

. It was used to study extinction
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...

 and to produce a star catalogue
Star catalogue
A star catalogue, or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some...

 using ultraviolet observations.

The UV Stellar Spectrometer was operated by the Astronomical Institute of Utrecht University
Utrecht University
Utrecht University is a university in Utrecht, Netherlands. It is one of the oldest universities in the Netherlands and one of the largest in Europe. Established March 26, 1636, it had an enrollment of 29,082 students in 2008, and employed 8,614 faculty and staff, 570 of which are full professors....

. It consisted of a grating spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...

 attached to a 26 centimetres (10.2 in) Cassegrain reflector
Cassegrain reflector
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas....

 telescope. Spectrometry of Primary Charged Particles was an experiment conducted by the Saclay Nuclear Research Centre
Saclay Nuclear Research Centre
The Saclay Nuclear Research Centre in the Essonne department of northern France is one of 9 laboratories belonging to the CEA. It is the administrative headquarters of the CEA and also hosts a laboratory....

, which studied cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s, and measured their charge spectra.

Spectrometry of Celestial X-Rays was another payload operated by the Saclay Nuclear Research Centre. It consisted of two slot collimator
Collimator
A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.- Optical collimators :In optics, a collimator may...

s and a proportional counter
Proportional counter
A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...

, which were used to study the spectra of incident x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s. Due to an encoder malfunction, the experiment was not operational for most of the first survey; it was deactivated a few months after launch and remained inactive until 1 July 1973; at the start of the second survey.

Solar Gamma-Rays in the 50 to 500 MeV Energy Range was an experiment operated by the University of Milan
University of Milan
The University of Milan is a higher education institution in Milan, Italy. It is one of the largest universities in Europe, with about 62,801 students, a teaching and research staff of 2,455 and a non-teaching staff of 2,200....

 which was used to measure the flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

 of solar gamma ray emissions. It consisted of scintillator
Scintillator
A scintillator is a special material, which exhibits scintillation—the property of luminescence when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate, i.e., reemit the absorbed energy in the form of light...

s and photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...

s, which measured the radiation. The Solar X-Ray Monitor was operated by the Utrecht University. It used a scintillation counter to detect hard x-rays emitted by the Sun. The Saclay Nuclear Research Centre's Gamma-Ray Measurement experiment used a spark chamber to detect gamma rays during the sky survey.

Launch

A Delta N
Delta N
The Delta N or Thor-Delta N was an American expendable launch system used for nine orbital launches between 1968 and 1972. It was a member of the Delta family of rockets, and the last Delta to be given an alphabetical designation - subsequent rockets were designated using a four digit numerical...

, also known as the Thor-Delta N, was used to place it into orbit. The launch occurred at 01:55:08 UTC, from Space Launch Complex 2E
Vandenberg AFB Space Launch Complex 2
Space Launch Complex 2 is an active rocket launch site at Vandenberg Air Force Base, in California, USA. It consists of two launch pads. The East pad , which has been demolished, was used for Delta, Thor-Agena and Thorad launches between 1966 and 1972...

 at Vandenberg Air Force Base
Vandenberg Air Force Base
Vandenberg Air Force Base is a United States Air Force Base, located approximately northwest of Lompoc, California. It is under the jurisdiction of the 30th Space Wing, Air Force Space Command ....

. The launch of TD-1A marked the final flight of the Delta N, and the last launch from Space Launch Complex 2E.

TD-1A was placed into a low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

; on 11 April 1974 it was tracked in an orbit with a perigee
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 of 525 kilometres (326.2 mi), an apogee
Apsis
An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 of 544 kilometres (338 mi), 97.5 degrees of inclination, and a period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

 of 95.32 minutes.

Operation

TD-1A operated for twenty six months, conducting two complete sky surveys, and completing approximately half of a third scan, in all mapping 95 percent of the sky. By maintaining a constant orientation to the sun, TD-1A was able to scan a thin band of the sky on each orbit, and Earth's movement around the sun enabled it to scan the entire sky over a period of six months.

TD-1A was affected by reliability issues with an onboard tape recorder, used to store experimental data for transmission to Earth, however the fault was intermittent, and still enabled the completion of the spacecraft's mission. An encoder problem led to the Spectrometry of Celestial X-rays experiment not being run during the first survey, however this was activated in time for the second survey, and was still completed successfully.

In May 1974 TD-1A ceased operations when its attitude control system depleted its last remaining propellant, leaving it unable to maintain its orientation. It decayed from orbit
Orbital decay
Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit.This can be due to drag produced by an atmosphere due to frequent collisions between the satellite and surrounding air molecules. The drag experienced by the object is larger in the case of increased solar...

 and reentered the atmosphere
Atmospheric reentry
Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

on 9 January 1980.
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