The Magsat satellite, successfully launched into a low-altitude polar orbit on October 30, 1979, is expected to reenter the atmosphere in early June. The NASA satellite, designed to map the vector magnetic field near the earth, contains a cesium vapor scalar magnetometer of accuracy ±1 nT and three-axis flux gate magnetometer of accuracy better than 3 nT (the earth's field at the surface ranges from 25,000 to 66,000 nT). The objectives of the mission are to provide a global vector survey of the main geopotential field and low-altitude measurements of crustal anomalies (see ‘Near-Earth Satellite Magnetic Field Measurements: A Prelude to Magsat,’ by R. A. Langel, EOS, 60, 38, 667–668, 1979).

The mission was initially expected to have a 5-month lifetime before reentering the atmosphere, according to a prediction made shortly after launch (see the Figure). This prediction used an atmospheric model appropriate to an average daily solar flux value at 10.7 cm (2800 MHz) of 200 flux units (1 flux unit = 10−22Wm−2 Hz−1). Apogee and perigee heights, however, have declined more slowly than initially predicted, and the observed decay more closely fits a prediction based on 160 flux units. Thus a lifetime of approximately 7 months is now anticipated.