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Solar System Exploration
Advanced Compostion Explorer
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ACE Mission to Our Solar System

Mission Type: Orbiter
NASA Center: Goddard Space Flight Center, Johns Hopkins University Applied Physics Laboratory
Spacecraft Mass: 752 kg
Spacecraft Instruments: 1) SWIMS solar wind ion mass spectrometer; 2) SWICS solar wind ion composition spectrometer; 3) ULEIS ultra-low-energy isotope spectrometer; 4) SEPICA solar energetic-particle ionic charge analyzer; 5) SIS solar isotope spectrometer; 6) CRIS cosmic-ray isotope spectrometer; 7) SWEPAM solar wind electron, proton, and alpha monitor; 8) EPAM electron, proton, and alphaparticle monitor; 9) MAG magnetometer and 10) RTSW real-time solar wind experiment
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi

National Space Science Data Center,

Advanced Composition Explorer (ACE) is a spacecraft that collects and analyzes particles of solar, interplanetary, interstellar and galactic origins, spanning the energy range from solar-wind ions to galactic cosmic rays traveling at nearly the speed of light. Studying these particles contributes to our understanding of the Sun and its interaction with Earth, and of the formation and evolution of the solar system.

The spacecraft carries six high-resolution sensors and three monitoring instruments. Measuring a particle's type, charge, mass, energy, direction of travel and time of arrival provides clues to its source and the processes by which it has gained energy.

Different particles tell us different things about the Solar System and the Galaxy. Some are samples of the present-day interstellar medium, the space surrounding our solar system. Fragments of atoms speeding to us from deep in the Galaxy, called "galactic cosmic rays," provide samples of matter that was accelerated millions of years ago. And solar particles represent interstellar matter that has been stored in the Sun for the last 4.6 billion years. Comparing the solar wind and higher energy solar particles with that of meteorites, comets, the Moon, planetary atmospheres and galactic material provides key information on the history of our solar system.

Earth's magnetic field deflects much of what ACE was designed to detect, so the spacecraft flew nearly a million miles (about 1.5 million km) sunward to a location in space called the Earth-Sun Lagrangian Point 1 (or libration point 1, aka L1). Normally, an object closer to the Sun would orbit the Sun faster than at Earth's distance. But at L1, Earth's gravity counterbalances that of the Sun just enough to permit a spacecraft to orbit more leisurely, taking one full Earth year to complete an orbit. So as the spacecraft and the planet orbit the Sun, the spacecraft remains right between the Sun and Earth.

ACE was launched during solar minimum conditions and observed the transition to solar maximum. During this period, the number of solar flares and coronal mass ejections increased dramatically, including some of the largest solar-particle events observed since the dawn of the space age. As part of the fleet of solar-astronomy spacecraft currently operating, ACE helped to make it one of the most enlightening solar maximum periods in history.

ACE also helps us to understand space weather caused by storms on the Sun. It can give us about an hour's warning of solar eruptions headed our way, and what we learn from ACE may help us one day to predict space weather that poses a danger to Earthly electronics and future astronauts.

The ACE mission had a goal of lasting five years, but it has continued operating long past that. To remain stable at L1, the spacecraft orbits the location with the help of thrusters. It has enough hydrazine fuel to enable it to stay there until about 2024.

The Space Science Mission Operations Project Office of Goddard Space Flight Center has overall NASA responsibility for the mission. The California Institute of Technology (Caltech) is the lead scientific institution. The Applied Physics Laboratory of Johns Hopkins University was responsible for building the spacecraft.

Key Dates
25 Aug 1997:  Launch (14:39 UT)
21 Jan 1998:  Spacecraft Operational
Status: Extended Mission in Progress
Fast Facts
ACE Facts ACE analyzes particles blown from the sun and from elsewhere in our galaxy.

The particles ACE detects include galactic cosmic rays, which are fragments of atoms from deep in the galaxy, traveling at nearly the speed of light.

ACE helps us understand the sun, its interaction with Earth, and the Solar System's origin and evolution.

ACE provides about a one-hour warning of space-weather events that could harm electronics and future astronauts. It may help us one day to predict hazardous space weather.

ACE orbits the sun at Lagrangian Point 1 (L1), which is about 930,000 miles (1.5 million km) sunward of Earth.

At L1, Earth's gravity counterbalances that of the sun such that ACE remains directly between Earth and the Sun as the planet and spacecraft orbit the sun.
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Last Updated: 8 Apr 2014