ACE Spacecraft Nears Completion At Applied Physics Laboratory

August 09, 1996

The Advanced Composition Explorer spacecraft, known as ACE, is undergoing final integration this summer and fall at The Johns Hopkins University Applied Physics Laboratory (APL) in preparation for a mid-1997 launch from Cape Canaveral, Fl.

ACE is a NASA mission to study the nature and origin of energetic particles from the sun and from beyond our solar system. It will also provide advance warning of "space weather" -- powerful geomagnetic storms that can disrupt power grids and communications on Earth and endanger astronauts in orbit.

Among APL responsibilities for ACE are the development, integration, and test of the spacecraft; integration and test of the nine-instrument payload; and development of two of the instruments: the Electron, Proton, and Alpha Particle Monitor (EPAM) and the Ultra-Low Energy Isotope Spectrometer, or ULEIS (with the University of Maryland).

The ACE mission is sponsored by the Office of Space Science Mission and Payload Development Division at NASA Headquarters. Overall project management is by the Explorer Project Office at the NASA/Goddard Space Flight Center (GSFC). Science payload management and the ACE Science Center are the responsibilities of the California Institute of Technology (Caltech) Space Radiation Laboratory.

Current schedule calls for integration of instruments and all subsystems by early fall. Vibration testing will be performed at APL. As early as November 1996, the spacecraft will be shipped to GSFC for full-scale environmental testing. Delivery to the Cape is set for June 1997, with launch on Aug. 21, 1997, aboard a McDonnell Douglas Delta II 7920 rocket from Cape Canaveral Air Station.

To escape the effects of Earth's magnetic field, ACE is targeted for a parking spot at the so-called L1 Lagrangian point, a location of gravitational equilibrium between Earth and the sun about 1.5 million kilometers (1 million miles) from our planet. The spacecraft will coast in a small-radius (158,000 kilometers, or 98,000 miles) "halo" orbit around the L1 point during the two-to-five year mission.

The observatory will measure with a collecting power 10 to 1,000 times greater than past experiments the stream of accelerated particles constantly bombarding Earth from the sun and galactic and interstellar sources. Study of these energetic particles will advance understanding of the formation and evolution of the solar system, as well as the fundamental astrophysical processes involved.

Specific ACE science goals include investigations into the elemental and isotopic composition of matter; the origin of the elements and subsequent evolutionary processing; formation of the solar corona and acceleration of the solar wind; and particle acceleration and transport in nature.

The $50 million ACE (excluding instruments) was designed at APL to be simple, lightweight, and reliable. The spacecraft is octagonal-shaped, 1.6 meters (5.25 feet) across and 1 meter (3.28 feet) high. Weight is 785 kilograms (1,730 pounds), including 189 kilograms (417 pounds) of hydrazine fuel. Four fixed solar panels will provide greater than 440 watts of electrical power at the end of five years.

ACE will spin at 5 rpm, with the spin axis generally pointed along the Earth-sun line. Most of the instruments are located on the top (sunward) deck; the two ACE magnetometers are mounted remotely on separate booms to reduce magnetic effects from the spacecraft and other instruments.

In addition to the APL-built EPAM and ULEIS, the ACE observatory includes the Solar Isotope Spectrometer (SIS); Cosmic Ray Isotope Spectrometer (CRIS); Solar Energetic Particle Ionic Charge Analyzer (SEPICA); Magnetic Field Monitor (MAG); Solar Wind Ion Mass Spectrometer (SWIMS); Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM); and Solar Wind Ion Composition Spectrometer (SWICS).

The ACE Science Team is led by Principal Investigator Edward Stone, Director of the NASA/Jet Propulsion Laboratory (JPL) and a Vice President at Caltech. ACE Science Co-Investigators represent 12 institutions from the U.S. and abroad: APL, Caltech, JPL, GSFC, University of Maryland, Washington University, University of New Hampshire, University of Delaware, University of Chicago, Los Alamos National Laboratory, University of Bern (Switzerland), and the Max Planck Institute (Germany).

Management for the ACE mission includes Donald L. Margolies, ACE Associate Project Manager, Explorer Office, GSFC; John R. Thurber, ACE Observatory Manager, Explorer Office, GSFC; Jonathan F. Ormes, ACE Project Scientist, GSFC; W. Vernon Jones, ACE Program Scientist, NASA Headquarters; and Louis J. Demas, ACE Program Senior Executive, NASA Headquarters.

ACE management at APL includes Mary C. Chiu, ACE Program Manager; Ute I. von Mehlem, ACE Spacecraft Systems Engineer; Cliff E. Willey, ACE Structure Lead Engineer; Elliot H. Rodberg, ACE Spacecraft Ground Test System Lead Engineer; and Joseph P. Staiger, ACE Integration and Test Lead Engineer. Stamatios M. Krimigis is Head of the APL Space Department and Co-Investigator for the EPAM and ULEIS instruments.

Johns Hopkins University

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