ISEE 1 (Explorer 56)

ISEE 1 [NASA]

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.