 Scheme of the telescope | |||||||||||
| Names | ISO | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Operator | ESA with significant contributions fromISAS andNASA | ||||||||||
| COSPAR ID | 1995-062A | ||||||||||
| SATCATno. | 23715 | ||||||||||
| Website | ISO at ESA science | ||||||||||
| Mission duration | 28 months 22 days | ||||||||||
| Spacecraft properties | |||||||||||
| Manufacturer | Aérospatiale | ||||||||||
| BOL mass | 2498 kg | ||||||||||
| Start of mission | |||||||||||
| Launch date | 01:20, 17 November 1995 (UTC) (1995-11-17T01:20:00Z) | ||||||||||
| Rocket | Ariane 4 4P | ||||||||||
| Launch site | ELA-2 | ||||||||||
| End of mission | |||||||||||
| Deactivated | 16 May 1998 12:00 UTC | ||||||||||
| Orbital parameters | |||||||||||
| Reference system | Geocentric | ||||||||||
| Regime | Highly elliptical | ||||||||||
| Perigee altitude | 1000 km | ||||||||||
| Apogee altitude | 70600 km | ||||||||||
| Period | 24 hr | ||||||||||
| Orbiter | |||||||||||
| Main | |||||||||||
| Type | Ritchey-Chrétien | ||||||||||
| Diameter | 60 cm | ||||||||||
| Focal length | 900 cm,f/15 | ||||||||||
| Wavelengths | 2.4 to 240micrometre (infrared) | ||||||||||
| |||||||||||
 Legacy ESA insignia for theISO mission | |||||||||||
TheInfrared Space Observatory (ISO) was aspace telescope forinfrared light designed and operated by theEuropean Space Agency (ESA), in cooperation with ISAS (now part ofJAXA) andNASA. The ISO was designed to study infrared light atwavelengths of 2.5 to 240micrometres and operated from 1995 to 1998.[1]
The€480.1-million satellite[2][3] was launched on 17 November 1995 from theELA-2launch pad at theGuiana Space Centre nearKourou in French Guiana. Thelaunch vehicle, anAriane 44P rocket, placed ISO successfully into ahighly ellipticalgeocentric orbit, completing one revolution around theEarth every 24 hours. Theprimary mirror of itsRitchey-Chrétien telescope measured 60 cm in diameter and was cooled to 1.7kelvins by means ofsuperfluidhelium. The ISO satellite contained four instruments that allowed for imaging andphotometry from 2.5 to 240micrometres andspectroscopy from 2.5 to 196.8 micrometers.
ESA and theInfrared Processing and Analysis Center made efforts to improve the data pipelines and specialized software analysis tools to yield the best quality calibration and data reduction methods from the mission. IPAC supports ISO observers and data archive users through in-house visits and workshops.
In 1983, the US-Dutch-British IRAS inaugurated space-basedinfrared astronomy by performing the first-ever 'all-sky survey' at infraredwavelengths. The resulting map of the infrared sky pinpointed some 350,000 infrared sources waiting to be explored by IRAS' successors. In 1979, IRAS was in an advanced stage of planning and the expected results from IRAS led to the first proposal for ISO made to ESA in the same year. With the rapid improvements in infrared detector-technology, ISO was to provide detailedobservations for some 30,000 infrared sources with much improvedsensitivity andresolution. ISO was to perform 1000 times better in sensitivity and 100 times better in angular resolution at 12 micrometres compared to IRAS.
A number of follow-up studies resulted in the selection of ISO as the next installment for the ESA Scientific Programme in 1983. Next came aCall for Experiment and Mission Scientist Proposals to the scientific community, resulting in the selection of thescientific instruments in 1985. The four instruments chosen were developed by teams of researchers from France, Germany, the Netherlands and United Kingdom.
Design and development of thesatellite started in 1986 withAérospatiale's space division (currently absorbed intoThales Alenia Space) leading an internationalconsortium of 32 companies responsible formanufacture,integration and testing of the new satellite. Final assembly took place at theCannes Mandelieu Space Center.
The basic design of ISO was strongly influenced by that of its immediate predecessor. Like IRAS, ISO was composed of two major components:
The payload module also held aconical sun shade, to preventstray light from reaching the telescope, and two largestar trackers. The latter were part of the Attitude and Orbit Control Subsystem (AOCS) which provided three-axisstabilisation of ISO with a pointingaccuracy of onearc second. It consisted of Sun and Earth sensors, the before-mentioned star trackers, a quadrant star sensor on the telescope axis,gyroscopes andreaction wheels. A complementaryreaction control system (RCS), usinghydrazinepropellant, was responsible for orbital direction and finetuning shortly afterlaunch. The complete satellite weighed just under 2500 kg, was 5.3 m high, 3.6 m wide and measured 2.3 m in depth.
The service module held all the warmelectronics, the hydrazine propellant tank and provided up to 600watts of electrical power by means ofsolar cells mounted on the sunpointing side of the service module-mounted sunshield. The underside of the service module sported a load-bearing, ring shaped, physical interface for the launch vehicle.
Thecryostat of the payload module surrounded the telescope and science instrument with a largedewar containing atoroidal tank loaded with 2268litres of superfluid helium.Cooling by slowevaporation of the helium kept thetemperature of the telescope below 3.4 K and the science instruments below 1.9 K. These very low temperatures were required for the scientific instruments to be sensitive enough to detect the small amount of infrared radiation from cosmic sources. Without this extreme cooling, the telescope and instruments would see only their own intense infraredemissions rather than the faint ones from afar.
The ISO telescope was mounted on thecenter line of the dewar, near the bottom-side of the toroidal helium tank. It was of theRitchey-Chrétien type with an effectiveentrance pupil of 60 cm, afocal length ratio of 15 and a resulting focal length of 900 cm. Very strict control over straylight, particularly that from bright infrared sources outside the telescope'sfield of view, was necessary to ensure the guaranteed sensitivity of the scientific instruments. A combination of light-tight shields, baffles inside the telescope and the sunshade on top of the cryostat accomplished full protection against straylight. Furthermore, ISO was constrained from observing too close to the Sun, Earth and Moon; all major sources of infrared radiation. ISO always pointed between 60 and 120 degrees away from the Sun and it never pointed closer than 77 degrees to Earth, 24 degrees to theMoon or closer than 7 degrees toJupiter. These restrictions meant that at any given time only about 15percent of the sky was available to ISO.
Apyramid-shaped mirror behind theprimary mirror of the telescope distributed the infrared light to the four instruments, providing each of them with a 3 arc-minute section of the 20 arc-minute field of view of the telescope. Thus, pointing of a different instrument to the same cosmic object meant repointing the entire ISO satellite.
ISO carried an array of four scientific instruments for observations in the infrared:
All four instruments were mounted directly behind the primary mirror of the telescope, in a circular arrangement, with each instrument taking up an 80degree segment of the cylindrical space. The field of view for each instrument was offset to the central axis of the telescope's field of view. This means that every instrument 'saw' a different portion of the sky at a given moment. In standard operational mode one instrument was in primary operation.
After a very successful development and integration phase ISO was finally launched into orbit on 17 November 1995, on board an Ariane-44P launch vehicle. Performance of the launch vehicle was very good with the apogee only 43 km lower than expected.ESA's Space Operations Centre inDarmstadt in Germany had full control over ISO in the first four days of flight. After early commissioning primary control over ISO was handed over to the Spacecraft Control Centre (SCC) atVillanueva de la Cañada in Spain (VILSPA) for the remainder of the mission.In the first three weeks after launch theorbit wasfine-tuned and all satellite systems were activated and tested. Cool-down of the cryostat proved to be more efficient than previously calculated, so the anticipated mission length was extended to 24 months. Between 21 and 26 November all four science instruments were switched on and thoroughly checked out. Between 9 December 1995 and 3 February 1996 the 'Performance Verification Phase' took place, dedicated to commissioning all instruments and fixing problems. Routine observations started from 4 February 1996, and lasted until the last helium coolant depleted on 8 April 1998.
The perigee of ISO's orbit lay well inside theVan Allen radiation belt, forcing the science instruments to be shut down for seven hours during each pass through the radiation belt. Thus, 17 hours in each orbit remained for scientific observation. A typical 24-hour orbit of ISO can be broken down into six phases:
Contrary to IRAS, no science data was recorded on-board ISO for later transmission to the ground. All data, both science data and housekeeping data were transmitted to the ground in real-time. The perigee point of ISO's orbit was below theradio horizon of the mission control centers at both VILSPA and Goldstone, thus forcing the science instruments to be switched off at perigee.
At 07:00 UTC on 8 April 1998flight controllers at VILSPA noticed a rise in temperature of the telescope. This was a clear sign that the load of superfluid helium coolant had depleted. At 23:07 UTC the same day, the temperature of the science instruments had risen above 4.2 K and science observations were ceased. A few detectors in the SWS instrument were capable of making observations at higher temperatures and remained in use for another 150 hours to make detailed measurements of an additional 300stars. In the month following depletion of coolant the 'Technology Test Phase' (TTP) was initiated to test several elements of the satellite in off-nominal conditions. After completion of TTP, the perigee of ISO's orbit was lowered sufficiently enough to ensure ISO will burn up in Earth's atmosphere in 20 to 30 years after shutdown. ISO was then permanently switched off on 16 May 1998 at 12:00 UTC.
On average, ISO performed 45 observations in each 24-hour orbit. Throughout itslifetime of over 900 orbits ISO performed more than 26,000 successful scientific observations. The huge amounts of scientific data generated by ISO was subject to extensivearchiving activities up to 2006. The full data-set has been available to the scientific community since 1998 and many discoveries have been made, with probably many more still to come:
| Operating |
| ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Planned | |||||||||||||
| Proposed | |||||||||||||
| Retired |
| ||||||||||||
| Hibernating (Mission completed) | |||||||||||||
| Lost/Failed | |||||||||||||
| Cancelled | |||||||||||||
| Related |
| ||||||||||||
| |||||||||||||||||||||||||||
Future missions initalics | |||||||||||||||||||||||||||
| International | |
|---|---|
| National | |
| Other | |
