The spacecraft was 3.6 m (12 ft) in diameter, was built of alternating layers of aluminum foil and plastic film, and was covered uniformly with 5.1 cm (2.0 in) white dots for thermal control.

It was designed to yield atmospheric density near perigee as a function of space and time from sequential observations of the sphere's position in orbit.

[6] This experiment was designed to determine nonsystematic changes of the density of the upper atmosphere from studies of the drag on a 3.6 m (12 ft) diameter low-density sphere caused by short-term differences in solar activity.

Density values near perigee were deduced from sequential observations of the spacecraft position, using optical (Baker-Nunn camera network) and radio and/or radar tracking techniques.

[7] This experiment was designed to determine systematic changes of air density as a function of altitude, latitude, and time of day, by measuring the drag on a 3.6 m (12 ft) diameter low-density sphere with ground tracking.