Photo of TD-1A

* Mission Overview

The Thor-Delta rocket system became known as the TD satellites. TD-1A was successfully launched on 11 March 1972 from Vandenberg Air Force Base (12 March in Europe). It was put in a nearly circular polar sun-synchronous orbit, with apogee 545 km, perogee 533 km, and inclination 97.6 degrees. It was Europe's first 3-axis stabilized satellite, with one axis pointing to the Sun to within +/- 5 degrees. The optical axis was maintained perpendicular to the solar pointing axis and to the orbital plane. It scanned the entire celestial sphere every 6 months, with a great circle being scanned every satellite revolution. After about 2 months of operation, both of the satellite's tape recorders failed. A network of ground stations was put together so that real-time telemetry from the satellite was recorded for about 60% of the time. After 6 months in orbit, the satellite entered a period of regular eclipses as the satellite passed behind the Earth -- cutting off sunlight to the solar panels. The satellite was put into hibernation for 4 months, until the eclipse period passed, after which systems were turned back on and another 6 months of observations were made. TD-1A was primarily a UV mission however it carried both a cosmic X-ray and a gamma-ray detector.

* Instrumentation

The X-ray detector was a 100 sq-cm proportional counter covering the energy range 3-30 keV. When switched on, the experiment caused abnormal readouts in the satellite's telemetry. Alas, it was switched off and remained that way.

Also on board was a small spark chamber experiment called MIMOSA for observations of gamma-rays from 70-300 MeV. The instrument was equipped with a stereoscopic TV system viewing through the chamber portholes to record the particle tracks. It operated from March-October 1972. There was a significant particle-induced background, despite an anti-coincidence system. Numerous gamma-rays were detected, but no gamma-ray point sources could be identified.

There were two stellar ultraviolet experiments on board. The Utrecht Orbiting Ultraviolet Stellar Spectrometer S59 used a small telescope and a three-slit scanner covering three bands between approximately 2100 and 2800 Å. About 200 bright stars were observed by the instrument and were later published in a catalog. The S2/68 spectrophotometric sky survey telescope was developed jointly by the UK and Belgium. It operated in the range 1350 and 2550 Å. This experiment used an off-axis reflecting telescope to focus radiation onto a set of entrance slits. These in turn fed a photometer and a three-channel spectrophotometer. The light falling on the spectrophotometer entrance slit was reflected onto a diffraction grating, and the dispersed light then passed through one of three slits and then onto individual photomultipliers. The orbital motion of the satellite caused the dispersed beam to scan across the exit slits.

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