OSIRIS: Optical, Spectroscopic, and Infrared Remote Imaging System
OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) is the scientific imaging system on the orbiter of ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko.
OSIRIS comprises two cameras: a high resolution Narrow Angle Camera (NAC) and a Wide Angle Camera (WAC). With them, OSIRIS observes the cometary rotation, and studies the physical and chemical processes that occur in, on, and near the cometary nucleus. It also maps the cometary morphology, which will help Rosetta’s lander (Philae) to find a suitable spot for setting down in the comet’s surface.
The strength of OSIRIS is the coverage of the whole nucleus and its immediate environment with excellent spatial and temporal resolution and the spectral sensitivity across the whole reflected solar continuum up to the onset of thermal emission. This provides a context for the interpretation of the results from Philae.
After the launch of Rosetta (2nd March 2004), OSIRIS was activated on several occasions before the arrival to the main target, comet 67P/Churyumov-Gerasimenko. It was commissioned in seven slots between March 2004 and June 2005, and it performed several important scientific observations:
- A monitoring campaign of comet 9P/Tempel 1 around the Deep Impact event on 4 July 2005
- A swing-by manoeuvre at Mars on 26 February 2007
- The fly-by of asteroid 2867 Steins on 5 September 2008
- Two Earth swing-bys in Nov. 2007 and Nov. 2009
- The observation of the remnant of a collision between two main-belt asteroids in February 2010 - The fly-by of asteroid 21 Lutetia on 10 July 2010
- Early observation of the comet from more than 1AU distance in March 2011.
The OSIRIS cameras were provided by a consortium of 9 institutes from 5 European countries and from ESA, under the leadership of the the Max-Planck-Institute for Solar System Research (MPS) (Principal Investigator: Holger Sierks). The participating institutes of the consortium are: MPS (Katlenburg-Lindau, Germany), LAM (Marseille, France), UPD (Padova, Italy), IAA (Granada, Spain), University of Uppsala (Sweden), ESTEC (ESA, Noordwijk, The Netherlands), UPM (Madrid, Spain), INTA (Madrid, Spain), IDA (Braunschweig, Germany)
The main objective of OSIRIS is to study the physical and chemical processes that occur in, on, and near the cometary nucleus. In particular, OSIRIS allows researchers to determine the outflow of gas and dust from different regions of the cometary nucleus, and to compare active areas with deviations in surface mineralogy, in topography, and in local insolation. The main science objectives of OSIRIS are:
- characterise the size, shape, and density of the nucleus of comet 67P/Churyumov-Gerasimenko
- determine the rotational properties of the comet
- monitor the nucleus activity and evolution over many months at various scales
- study the chemical composition of the nucleus surface
- find a suitable landing spot for Philae, the Rosetta lander
The Instrument in detail:
OSIRIS comprises two cameras: a high resolution Narrow Angle Camera (NAC) and a Wide Angle Camera (WAC). Both are unobstructed off-axis mirror systems, equipped with two filter wheels containing 8 positions each, and with backside illuminated CCD detectors comprising 2048 x 2048 pixels with a pixel size of 13.5 µm.
NAC (Narrow Angle Camera)
The NAC (Narrow Angle Camera) is designed to obtain high resolution images of the surface of comet 67P/Churyumov-Gerasimenko at distances from more than 500 000 km down to 1 km (resulting in a resolution of ~2 cm per pixel). The camera should be able to detect small ejected particles close to the cometary nucleus (brightness ratio = 1/1000). The 12 filters of the NAC will characterise the reflectivity spectrum of the nucleus surface over a wide spectral range from 250 to 1000 nm. The NAC has a square field of view (FOV) of 2.2 degrees, with an instantaneous field of view (IFOV) of 18.6 µ rad (3.8 arcsec) per pixel, and is a moderately fast system (f/8). The system has a 717 mm focal length. A flat-field, three anastigmatic mirror system is adopted. It has a mass of 13.2 kg.
WAC (Wide Angle Camera)
The WAC (Wide Angle Camera) The WAC is optimized to provide images of the near nucleus environment. The principal objective of this camera is to study the intensity of gas emissions and dust-scattered sunlight as functions of position and viewing angle in the vicinity of the nucleus. The WAC is accomplished by 14 filters (240 to 720 nm.). Seven of the narrow band filters isolate gas emissions from the cometary coma; the other filters measure the dust continuum at wavelengths close to that of the gas emissions. The WAC has a FOV of 12x12 degrees, with an angular resolution of 101 µ rad (20.5 arcsec) per pixel, and is a system with a fast focal ratio of f/5.6. The system has a 140 (sag)/131 (tan) mm focal length. A two aspherical mirror system is adopted. It weighs 9.5 kg.
Basic parameters of the NAC and WAC systems
|Optical design||3-mirror off-axis||2-mirror off-axis|
|Angular resolution [μrad px-1]||18.6||101|
|Focal length [mm]||717.4||140 (sag)/131 (tan)|
|Field of view [°]||2.20 - 2.22||11.35 - 12.11|
|Spatial scale from 100 km [m px-1]||1.86||10.1|
|Typical filter bandpass [nm]||40||5|
|Wavelength range [nm]||250 - 1000||240 - 720|
|Number of filters||12||14|