MultiSpectral Imager

The Multispectral Imager (MSI), one of the primary instruments on the NEAR spacecraft, will determine the overall size, shape, and spin characteristics of 433 Eros and will map the morphology and composition of the surface. The imager will also be used for optical navigation and to search for satellites of Eros.

Images taken during approach, flyby, and orbit of Eros will detect surface features as small as 3 meters. The 7-color capability of the instrument will complement the near-infrared and X-ray/Gamma-ray spectrometers in extrapolating variations in surface composition down to these small spatial scales.

MSI data will be critical to understanding the surface processes, composition and compositional variation, and geologic history of Eros.

Specifications:

Mass: Camera head 5.05 k, Electronics, 4.5 kg
Power: Camera head 2.16W, Electronics, 10.86 W
Detector: Si CCD; Thompson-CSF TH7866
Field of view: 2.25° X 2.9°, in 244 X 537 pixels = 3.9 X 5.1 km from 100-km distance
Resolution: 9.5 X 16.1 m from 100-km distance
Exposure time: to 999 ms in 25-ms increments, manual or automatic
Wavelength range: 400-1100 nm
Filter wavelengths: 1 broadband filter, 7 spectral filters at 450, 550, 760, 900, 950, 1000, 1050 nm
Data Character: 12-bit data, with three-tiered compression menu.

Description

MSI consists of a five-element refractive telescope with passively cooled Si CCD and electronics, a filter wheel, and a computer (digital processing unit, or DPU). The telescope is f/3.4 with a 168-mm focal length. The imager provides a field of view of 2.25° X 2.9°, divided into an array of 244 X 537 pixels on the CCD. Pixel angular resolution is 95 X 161 mrad, corresponding to 9.5 X 16.1 m from a distance of 100 km.

Brightnesses are encoded to 12 bits instead of the 8 bits used for similar previous imagers, providing 16 times the brightness resolution of such systems. The Si CCD is sensitive to the wavelength range of 400-1100 nm (visible and short-wavelength near-infrared light). A filter wheel has seven spectral filters designed primarily to discriminate iron-containing silicate minerals, and one broadband filter for low-light imaging and optical navigation.

The DPU contains software that controls the instrument and supports automatic exposure time control, acquisition of image sequences, and a three-tiered compression system offering several modes of lossless compression and seven lookup tables for converting the data from 12 to 8 bits.

Imaging Science at Eros

433 Eros is a member of the "S" class of asteroids, which are composed of iron-containing silicates and metal. At 40 X 14 X 14 km, it is the largest asteroid that approaches close to Earth. Flyby encounters of asteroids 951 Gaspra, 243 Ida, and Ida's moon Dactyl by Galileo provided images that showed for the first time the nature of asteroidal surfaces, but they did not answer some of the most fundamental questions about asteroid origin and evolution. For example, the minerals composing S asteroids are the same as those composing meteorite types with widely divergent histories. The relationship of S asteroids to these materials, and hence the origin of S asteroids, remains unknown.

The main science objectives for NEAR are to determine Eros's surface morphology and the processes that affect its surface, its internal structure, and its composition and relationship to meteorites. MSI is critical to accomplishing each of these goals.

MSI will image the surface at several spatial resolutions, as high as 3-5 meters. These high-resolution images, unprecedented from an orbiting spacecraft, will reveal the distribution and thickness of the asteroid's fragmental surface layer or "regolith," the history of impacts by fragments of other asteroids and comets that is recorded in craters, the character and locations of fractures of the asteroid's body, and the processes that affect the surface layer.

MSI images will also be used to determine the size, shape, and volume of Eros. Unlike previous targets of orbiting spacecraft, Eros's mass and density are unknown. The mass will be measured by radio tracking of the spacecraft as it approaches Eros; shape will be measured from MSI imagery during approach and flyby of the asteroid, as well as by NEAR's laser altimeter. The mass and shape measurements will provide the asteroid's density and density structure, which are necessary both for conducting the later orbital phase of the mission and for evaluating Eros's internal structure.

MSI's seven spectral filters are specifically chosen to distinguish between the spectra of sunlight reflected by the major iron-containing mineral constituents of Eros's surface. MSI has 70 times the spatial resolution of the near-infrared spectrograph, so color imagery from MSI will be used to extrapolate compositional information down to the spatial scale of meters. Correlation of the compositional variations with specific surface features such as fractures or craters will allow discrete rock units to be mapped, and will provide a window into the internal geologic structure exposed by craters.

On approach to Eros, MSI will also carry out a satellite search.

Experiment Profile

The NEAR spacecraft will follow a 2-year DVEGA trajectory beginning with a launch in February 1996.

Sixteen months after launch, en route to Eros, the spacecraft will fly by the 61-km diameter C-class asteroid 253 Mathilde
In January 1998 an Earth swingby will provide a gravity assist, and approach to Eros will begin in early 1999
After flying by Eros the spacecraft will be inserted into orbits of progressively lower radii, culminating in a 35-km low orbit.
On approach to Eros and from the higher orbits MSI will search for satellites and determine the global shape, spin, and color properties of the asteroid.
The first comprehensive map will be obtained from an altitude of about 100 km (resolution about 12 meters).
Most of the mission will be devoted to high-resolution mapping from the 35-km orbit, and stereo coverage of selected areas will be obtained.
MSI will provide the first detailed global geologic map of any asteroid, which will reveal important clues to Eros' evolution and history.

Relative Sizes of Asteroids

MSI Team

Science Team:: Joseph Veverka (Cornell Univ.), Team Leader
James F. Bell, III (Cornell Univ.)
Clark R. Chapman (Planetary Science Institute)
Michael C. Malin (Malin Space Science Systems)
Lucy-Ann A. McFadden (Univ. of Maryland)
Mark S. Robinson (U. S. Geological Survey)
Peter C. Thomas (Cornell Univ.)
Lead Engineer:: S. Edward Hawkins, III (JHU/APL)
Instrument Scientist:: Scott L. Murchie (JHU/APL)
NEAR Payload Manager:: Robert E. Gold (JHU/APL)

Links to the other NEAR instruments:

X-Ray/Gamma-Ray Spectrometer

Near-Infrared Spectrograph

Magnetometer

Laser Rangefinder

Radio Science

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