ASIM Technology

From ElectricStorms

Read more about the Spacestation, the Columbus module and the ASIM instruments

Contents 1 The International Space Station 2 Columbus 3 The Optical Cameras 4 The Optical Photometers 5 The X- and gamma-ray detector

The International Space Station

The International Space Station (ISS) orbits the earth at about 400 km altitude. It continues to be expanded with additional modules and facilities, latest with the Columbus module delivered by the European Space Agency (ESA), to be launched in January 2008. Denmark has delivered hardware and software to the ISS and has conducted physiological tests on astronauts to understand the functioning of the human blood pressure under weightless conditions. Denmark has selected three candidates to ESA’s corps of astronauts, to which admission will be opened in 2009.

The ISS is in the lowest, permanently available orbit. The altitude is maintained by the spaceships docking at the station. They give the ISS a boost with their engines, lifting the altitude. If this is not done, the ISS will loose altitude because of air drag and burn up within 2 years.

The orbital plane is 51.6 degrees relative to the equatorial plane. This allows for observations over the main thunderstorm regions of the earth. At the same time, the ISS reaches sufficiently high latitudes to study energetic particle precipitation and aurora, powered by violent storm on the sun.

Columbus

ASIM scientific instruments include 6 cameras, 6 photometers and one X- and gamma-ray detector. The 4 cameras with 4 companion photometers are directed forward towards the horizon (ram, limb). Two cameras, two photometers and the X- and gamma-ray detectors are directed downwards (nadir). The cameras and the photometers constitute the Modular Multispectral Imaging Array (MMIA). Each module includes two cameras and two photometers, such that there are 3 MMIA modules in all, two pointing forward and one downward. The MMIA instruments observe in different optical spectral bands. The two MMIA modules which are directed forward towards the horizon observe thunderstorms from the side, where it is possible directly to identify the effects on the atmosphere as a function of altitude.

The X- and gamma-ray detector is called the Modular X- and Gamma-Ray Sensor (MXGS). X- and gamma-rays are strongly absorbed in the atmosphere. This is why the detector is pointing directly downwards such that a minimum of atmosphere is between the detector and the thunderstorms within its field of view. Most of the atmosphere is below the altitude where giant lightning and terrestrial gamma-ray flashes are generated. Therefore, space is particularly well suited to observe these phenomena in the band reaching from gamma-rays to UV, which is difficult to observe from the ground. ASIM is measuring in these bands (colors).

The Optical Cameras

Field-of-view	Forward: 20x20 degrees; Downward: 80x80 degrees
Pixels	1024x1024
Spatial resolution	Forward: 300-600 m; Downward: 300-400 m
Bits/pixel	12
Time resolution	80 ms
Spectral bands	Forward: 337nm; 391.4nm; 650-800nm;762nm Downward: 337nm; 650-800nm

The cameras are light sensitive without the use of intensifiers. This is made possible by a new CCD, with on-chip amplification. It means that the CCD will not be damaged if the cameras by mistake are viewing the sun or the moon. Because the cameras are light sensitive, they can only observe during the night or at sunrise and sunset seen from the space station.

The Optical Photometers

Photometers are used to measure rapid time variations, which cannot be done by imaging cameras. They view the exact same region but measure only the total photon flux from the region - but with high time resolution.

Field-of-view	Forward: 20x20 degrees; Downward: 80x80 degrees
Time resolution	10 microseconds
Spectral bands	Forward: 337nm; 391.4nm; 650-800nm; 237nm Downward: 337nm; 650-800nm

The X- and gamma-ray detector

The detector plane is made of semiconductor crystals that are sensitive to photons hitting the crystals. The detector measures each photon and determines its energy and time of arrival. This detector principle is relatively new and allows for a simpler design than used in the past.

Energy range	7-500 keV
Energy resolution	<10%
Detector area	1032 cm2