The
Infrared Space Observatory (ISO)
is one of the projects of ESA's Scientific Programme. Both NASA and ISAS
(Japan) contribute to the operational phase of the project. ISO carries
out observations of astronomical objects in the 2.5 -240 µm range
of the spectrum.
This part of the spectrum is of great scientific interest, not only because it is here where cool objects (15-300 K) radiate the bulk of their energy, but also because of its rich variety of atomic, ionic, molecular and solid state spectral features.
Measurements at these wavelengths permit determination of many physical parameters, for example energy balance, temperatures, abundances, densities and velocities. Owing to the much-reduced extinction, infrared observations are particularly well-suited to probing the properties of objects shrouded by dust clouds and thus obscured at visible wavelengths. ISO provides high sensitivity and sophisticated observing facilities for a relatively unknown part of the spectrum. Thus it is expected that the scientific programme of ISO will touch upon virtually every field of astronomy, ranging from solar system studies to cosmology.
The ISO spacecraft is 5.3m high, 2.3m wide and weighed approximately 2400Kg at launch. It consists of a payload module, which carries the conical sun shade, the two star trackers and a service module, which provides the basic spacecraft functions. These include the structure, the solar array mounted on the sun shield, and sub-systems for thermal control, data handling, power conditioning, telemetry and telecommand (using two antennas) and attitude and orbit control. The last item provides the three-axis stabilisation to an accuracy of less than 1 arcsecond, and also the raster pointing facilities needed for the mission. It consists of sun and earth sensors, star trackers, a quadrant star sensor on the telescope axis, gyros and reaction wheels. A hydrazine reaction-control system is used. The downlink bit rate is 32 Kbit/s, of which about 24 Kbit/s are dedicated to the scientific instruments.
The payload module is essentially a large cryostat with a toroidal tank which contained at launch, over 2000 litres of super fluid helium. This is being depleted at a rate of about 5mg/sec. The lifetime will be over 18 months. Some of the infrared detectors are directly coupled to the helium tank and are at a temperature of around 2K. All other units are cooled by means of the cold boil-off gas from the liquid helium. Suspended in the middle of the tank is the telescope, which is a Ritchey-Chrétien configuration with an effective aperture of 60 cm.
The light (IR) gathered by the telescope is directed to the four instruments, only one of which is normally operated.
Observations are continuously made during the 16 hour science period. The satellite is slewed immediately to a new object after an observation has been completed. More than 50 slews per orbit are not uncommon.
After many steps in the processing of observational data, individual observers/institutes receive their data on CD ROM's.
The control of ISO was transferred from ESOC to the SCC in Villafranca on 21 November 1995 and the Satellite Commissioning Phase began immediately with the first instrument checkout (ISOPHOT).
The next milestone was the perigee longitude drift stop manoeuvre on 24 November 1995. The telescope was opened to space on 27 November 1995 with the ejection of the telescope cover. Thereafter followed an intensive campaign to commission the instruments and perform detailed performance measurements on the AOCS.
The satellite Commissioning Phase was completed on 9 December 1995. The Observatory Performance Validation Phase started on 9 December 1995 and was completed on 3 February 1996. The Routine Phase started immediately thereafter.
Fot those interested in ISO´s scientific results up to date, please visit ISO Results page of ESA´s Center in Netherlands (ESTEC)
The spacecrafts are approximately 2 m wide and 2.5 m high, with a 2 m L-Band dish antenna. The solar arrays extend between 14 m from tip to tip. Spacecraft mass at launch was 1015 Kg. The two satellites are positioned in a geostationary orbit.
The Shore to Ship link (Forward Link) through the satellites is established
at C-Band for the up-path (6420.25-6425 MHz) and at L-Band for the down-path
(1537.75 - 1542.5 MHz).
The Ship to Shore Link (Return Link) is performed at L-Band for the
up-path (1638.6 - 1644.5 MHz) and at C-Band for the down-path (4194.6 -
4200.5 MHz).
The Flight Control Centre is located at Redu and the TT&C ground station working at C-Band at Villafranca. A back-up VHF TT&C station and telemetry reception in C-Band is located at Redu.
In 1986 INMARSAT requested to relocate the two MARECS satellites. Thus MARECS-B2 was moved from the Pacific Ocean Region (POR) to the Atlantic Ocean Region (AOR), whilst MARECS-A was moved to the POR to be used as in stand-by.
In 1991 MARECS-A was removed from INMARSAT services and relocated to 22.5° East due to an extended degradation of solar array sections and has since then been successfully used by ESA for land mobile experimental communications with the ground based PRODAT terminal being located at Villafranca and later at Lario (Italy).
Following a request of INMARSAT, MARECS-B2 was moved in 1990 to 55.5° West to cover a new region (AOR-West) with prime maritime communication services. The new relocation was preceded by an extensive campaign of 28 manoeuvres compressing effectively three years of North-South station keeping into three months. 38 Kg hydrazine were expended in 33 hours of thruster firings, to impart a delta V of 132 m/sec.
INMARSAT requested again in 1992 to relocate MARECS-B2 to 15° West (AOR-East) where it stayed till December 1996 when MARECS leasing to INMARSAT finished.
MARECS-B2 is being used by FUGRO Co. since May 1997 to disseminate differential GPS data to South America and Africa FUGRO users.
As a consequence of a further loss on solar array section MARECS-A was not able anymore to support communications service in eclipse. Therefore, the decision was made to terminate the mission. MARECS-A was de-orbited on 22 August 1996. The future prospects of MARECS-B2 mission is primarily limited by the extended degradation of its solar array sections.
