ACE observes particles of solar, interplanetary, interstellar, and galactic origins, spanning the energy range from solar wind ions to galactic cosmic ray nuclei to better understand the formation and evolution of the solar system. ACE also provides near-real-time solar wind information over short periods of time for space weather forecasting.

Launch Date August 25, 1997
Launch Site Cape Canaveral, Florida, USA
Destination Our Solar System (Lagrange Point No. 1)
Type Orbiter
Status Successful
Nation United States
Alternate Names Explorer 71, Advanced Composition Explorer, 24912, 1997-045A


Advanced Composition Explorer (ACE) 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.


ACE continues to provide space weather reports and warnings of geomagnetic storms that can disrupt communications on Earth and harm astronauts in space. The spacecraft has operated far beyond its expected lifetime.

Key Dates

Aug, 25, 1997: Launch

Jan 21, 1998: Spacecraft Operational

In Depth

The Advanced Composition Explorer (ACE) spacecraft was designed to study spaceborne energetic particles from the L1 Libration Point, about 1.4 million kilometers from Earth. Specifically, the spacecraft was launched to investigate the matter ejected from the Sun to establish the commonality and interaction among the Sun, Earth, and the Milky Way galaxy.

In addition, ACE also provides real-time space weather data and advanced warning of geomagnetic storms. ACE's nine instruments have a collecting power that is 10 to 10,000 times greater than anything previously flown. After launch, the spacecraft's Delta 2 launch vehicle's second stage reignited (for 4 hours) to insert the satellite into a 177 x 1.37-million-kilometer orbit.

After reaching apogee a month after launch, ACE inserted itself into its halo orbit around the L1 point. The spacecraft was declared operational on Jan. 21, 1998 and has operated far beyond its expected lifetime.


Launch Vehicle:

Spacecraft Mass: 752 kilograms

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
  10. RTSW real-time solar wind experiment

Additional Resources

National Space Science Data Center Master Catalog: ACE

NASA Science Missions: ACE

Caltech ACE Site

ACE brochure, 2nd edition

Selected References

Siddiqi, Asif A. Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, NASA, 2002.

Solar System News