Nasa’s ACE Spacecraft
Advanced Composition Explorer
ACE orbits the L1 libration point, which is a point of Earth-Sun gravitational equilibrium about 1.5 million km from Earth and 148.5 million km from the Sun. From its location at L1, ACE has a prime view of the solar wind, interplanetary magnetic field and higher energy particles accelerated by the Sun, as well as particles accelerated in the heliosphere and the galactic regions beyond.
ACE also provides near-real-time 24/7 continuous coverage of solar wind parameters and solar energetic particle intensities (space weather). When reporting space weather, ACE provides an advance warning (about one hour) of geomagnetic storms that can overload power grids, disrupt communications on Earth, and present a hazard to astronauts.
Mission and Spacecraft Characteristics
The spacecraft is 1.6 meters across and 1 meter high, not including the four solar arrays and the magnetometer booms attached to two of the solar panels. At launch, it weighed 785 kg, which includes 189 kg of hydrazine fuel for orbit insertion and maintenance. The solar arrays generate about 500 watts of power. The spacecraft spins at 5 rpm, with the spin axis generally pointed along the Earth-sun line and most of the scientific instruments on the top (sunward) deck.
In order to get away from the effects of the Earth’s magnetic field, the ACE spacecraft has travelled almost a million miles (1.5 million km) from the Earth to the Earth-sun libration point (L1). By orbiting the L1 point, ACE stays in a relatively constant position with respect to the Earth as the Earth revolves around the sun.
Communication Subsystem: Primary Mission, S-band, DSN;
Communication Subsystem: NOAA – S-band @ NOAA Receiving Stations, TBD
Total Onboard Data Storage in Two Solid State Recorders — 2 Gigabits
Primary Mission Downlink Rates: 78 & 6.9 kbps and 434 bps
NOAA Real Time Solar Wind Mission: 434 bps
ACE Orbit and Fuel Use Strategy
Three types of maneuvers (attitude, orbit and spin) have been used since July 2001 to control ACE. Orbit maneuvers use 3lbm/year of fuel per year and keep the spacecraft bound to the L1 libration point. Attitude maneuvers use 6lbm/year and are required to maintain the HGA antenna constraint. With this strategy, fuel use is 9 lbm/y total, and the 154 lbm of fuel remaining as of October 2007 will be consumed by year 2024.
Initially, two Z-axis maneuvers using 16 lbm/y were used to prevent the Sun-Earth-Spacecraft (SES) angle from dropping below 4.75° due to natural evolution of the Lissajous orbit (see figure at right). This conservative Solar Exclusion Zone (SEZ) was designed to avoid solar radio interference with the downlink. As a result of discontinuing the Z-axis maneuvers after July 2001, ACE transited the SEZ with an SES angle of <2° every 3 months from September 2003 thru May 2005. Although the solar radio flux reduced the telemetry link margin to <3 dB during some of these transits, no science data were lost, as the ACE solid-state recorder has 86-hour capacity. Spacecraft commanding continued uninterrupted. Neither NASA nor NOAA has lost any data during SEZ transits, even in May 2004 when the minimum SES angle was 0.2° during a 21-hour crossing of the solar disk as viewed from Earth.
History and Background
ACE was conceived at a meeting on June 19, 1983 at the University of Maryland. The meeting was hosted by George Gloecker and Glen Mason. The participants were Drs. L. F. Burlaga, S. M. Krimigis, R. A. Mewaldt, and E. C. Stone. This meeting had been preceded by preliminary documentation from the Johns Hopkins University Applied Physics Laboratory (APL) and the University of Maryland under the proposal name of Cosmic Composition Explorer. An unsolicited proposal was put together and forwarded to the NASA Explorer Program Office later that year, but was not acted upon.
The proposal was resurrected at the instigation of Dr. Vernon Jones and officially resubmitted to NASA in 1986 as part of the Explorer Concept Study Program. In 1988, the ACE mission was selected for a one-year “Phase A” (concept) Study. This study was a collaborative effort between spacecraft design and science teams.
The ACE Mission officially began on 22 April 1991 when the contract between NASA/GSFC and the California Institute of Technology was signed. APL, designer and builder of the ACE spacecraft, was involved in planning for Phase B (definition). The early ACE Spacecraft effort (April to July 1991) was primarily for ACE mission support, spacecraft system specification and ACE instrument support and interface definition. Phase B of the ACE mission officially began in August 1992.