The Explorer-class mother spacecraft, ISEE 1 (International
Sun-Earth Explorer 1), was part of the mother / daughter / heliocentric mission
(ISEE 1, ISEE 2, ISEE 3).
The purposes of the mission were:
- to investigate solar-terrestrial relationships at the outermost boundaries of the
Earth's magnetosphere,
- to examine in detail the structure of the solar wind near the Earth and the shock wave
that forms the interface between the solar wind and the Earth's magnetosphere,
- to investigate motions of and mechanisms operating in the plasma sheets, and
- to continue the investigation of cosmic rays and solar flare effects in the
interplanetary region near 1 AU.
The three spacecraft carried a number of complementary instruments for making
measurements of plasmas, energetic particles, waves, and fields. The mission thus extended
the investigations of previous IMP spacecraft. The mother/daughter portion of the mission
consisted of two spacecraft (ISEE 1 and ISEE 2) with
station-keeping capability in the same highly eccentric geocentric orbit with an apogee of
23 Earth radii. During the course of the mission, the ISEE 1 and ISEE 2 orbit parameters underwent short-term and long-term
variations due to solar and lunar perturbations. These two spacecraft maintained a small
separation distance, and made simultaneous coordinated measurements to permit separation
of spatial from temporal irregularities in the near-Earth solar wind, the bow shock, and
inside the magnetosphere. By maneuvering ISEE 2, the
inter-spacecraft separation as measured near the Earth's bow shock was allowed to vary
between 10 km and 5000 km; its value is accurately known as a function of time and orbital
position.
The spacecraft were spin stabilized, with the spin vectors maintained nominally within
1 degree of perpendicular to the ecliptic plane, pointing north. The spin rates were
nominally 19.75 rpm for ISEE 1 and 19.8 rpm for ISEE 2,
so that there was a slow differential rotation between the two spacecraft. The ISEE 1
body-mounted solar array provided approximately 175 Watts initially and 131 Watts after
three years, at 28 volts during normal operation. The ISEE 1 data rate was 4096 bps most
of the time and 16384 bps during one orbit out of every five (with some exceptions).
Both ISEE 1 and ISEE 2 re-entered the Earth's
atmosphere during orbit 1518 on 26 September 1987. Seventeen of 21 on-board experiments
were operational at the end.