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Fact Sheet

 Fast Facts


Launch date: 26-Jan-1978 17:36 UT
Mission end: 30-Sep-1996
Launch vehicle: Delta 2914
Launch mass: 671 kg
Mission phase: Archive
Orbit: Geo-synchronous over Atlantic
Initially 32 050 x 52 254 km; 28.6°; 23.927 h
At mission end 36 360 x 48 003 km; 35.9°
Achievements: World's longest (18.7 years uninterrupted orbital operations) and most productive astronomical space observatory mission. The first general user UV space observatory. The first astronomical satellite in high earth orbit.

IUE, or International Ultraviolet Explorer, was a ESA/NASA-UK venture. It analysed ultraviolet light from the stars, which is blocked by the Earth's ozone layer but often carries storm signals of cosmic upheavals. Launched in 1978, IUE operated until 1996, making it one of the longest-lived astronomical satellites.

Mission Objectives 

  • To obtain high-resolution spectra of stars of all spectral types to determine their physical characteristics
  • To study gas streams in and around binary star system
  • To observe faint stars, galaxies and quasars at low resolution, interpreting these spectra by reference to high-resolution spectra
  • To observe the spectra of planets and comets
  • To make repeated observations of objects with variable spectra
  • To study the modification of starlight caused by interstellar dust and gas


Satellite configuration: 1.45 m-diameter hexagonal-prism bus with telescope assembly along main axis, and fixed solar wings extending from opposing faces. Most of the high-power electronics were mounted in the main equipment bay at the base, near the passive thermal louvers, while the experiment electronics and attitude control elements were on the upper equipment platform.

Attitude/orbit control: 3-axis control with 6 gyroscopes. Spacecraft coarse pointing with three reaction wheels (one spare has never been used in orbit), precision pointing (<1 arcsec. stabilisation over 24 hrs.) with Fine Error Sensors (2-axis star trackers using the telescope optics; 0.27 arcsec resolution in 16 arcmin FOV), fine/coarse Sun sensors and 8×9 N + 4×22 N hydrazine thrusters (27.3 kg hydrazine in 6 tanks) for momentum dumping and orbit adjustment.

Power  system:  424  W BOL/28 Vdc provided by two fixed 3-panel arrays (210 W required)  carrying  4980 2×2 cm Si cells; degraded after 18 years in orbit to 40%;  supported by 2×6 Ah nickel cadmium batteries (17 cells each).

Communications/data: 1.25-40 kbit/s 2.25 GHz 6 W S-band (circular polarisation) downlink; fixed and  reprogrammable formats. 139 MHz VHF for telecommand.


Telescope: 45 cm, f/15 Ritchey-Chretien Cassegrain
Spectrographs: Echelle (115 nm to 198 nm and 180 nm to 320 nm)
Apertures: 3" and 10" by 20" image quality 2"
Resolution: 1.8×104 corresponding to 0.008 nm @140 nm (17 km s-1)
  1.3×104 corresponding to 0.017 nm @260 nm; (20 km s-1)
  27° in low resolution mode @150 nm
  40° in low resolution mode @270 nm
Cameras: SWP (115-197 nm)
Sensitivity: 2×10 -15 ergs s-1 cm-2 Å-1
  LWP (175-330 nm)
Sensitivity: 1×10-15 ergs s-1 cm-2 Å-1
  LWR (175-330 nm)
Sensitivity: 2×10-15 ergs s-1 cm-2 Å-1
Never operational
FES #1 Fine-Error-Sensor #1 has not been used in operations.
FES #2 Fine-Error-Sensor #2 has been used for fine guidance throughout the mission. It has also been important as a photometer to measure the optical brightness of the sources observed. In 1991 scattered light entering the telescope required a revision of guidance procedures and affected its photometric performance.


Last Update: 1 September 2019
19-Oct-2021 04:44 UT

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