Electronic Design

IR Device To Detect Vegetation On Mars

An infrared spectrophotometer, suitable for space missions, has been designed to detect absorption bands characteristic of certain hydrocarbons found in vegetation. The instrument is being evaluated by the Jet Propulsion Laboratory for use in a Mariner vehicle shot to Mars, tentatively scheduled for 1964. Perkin-Elmer Corp., Stamford, Conn., built the prototype instrument.

Spectral resolution of better than 330 Å over the 2- to 4-micron band is provided with a 1-mm slit and an oscillating grating. Detection is accomplished with a lead-selenide cell, radiation-cooled to −78°C. An umbrella cuts off the sunlight on one side of this detector and leaves the other side exposed to deep space temperatures. Chopping is accomplished with a darkened flag on a resonant reed, vibrating at 300 cps.

To confirm the presence of certain types of vegetation, an IR spectrophotometer must determine the precise wavelengths of peak absorption (a 0.02-micron shift could be sufficient to invalidate a particular life-process theory), and both the width and amplitude of the absorption band.

The information rate for transmission will be time-expanded some 100 to 200:1, because of the expected 60-million mile distance from Mars to earth. The instrument will scan perpendicular to the direction of motion across the Mars disk, generating about 300 picture elements. At an expected 9500-mile altitude, the surface projection will cover an area about 40 by 400 miles on the Martian surface. The spectrophotometer weighs less than 20 lb and dissipates less than 6 W. (Electronic Design, April 12, 1961, p. 6)

Because of the great advances made in digital ICs and computers over the past 40 years, we might overlook the sophisticated scientific instrumentation that existed back in the '60s.

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