A Real-Time Solar Wind Sentinel

NASA’s Advanced Composition Explorer (ACE) launched in 1997 and continues to provide insight into the interplanetary medium while serving as a sentinel for real-time space weather events. Led by Principal Investigator Edward Stone, ACE was designed to investigate the fundamental nature of the matter ejected from the Sun that forms the solar wind, along with its subsequent evolution and acceleration through interplanetary space and its interaction with Earth's magnetosphere. ACE also studies material from outside the solar system, including galactic cosmic rays and anomalous cosmic rays.


Our solar system and our planet are constantly bombarded by particles from the Sun as well as galactic sources. Studying these particles not only contributes to our understanding of the solar system but also helps us understand the space weather phenomena that affect life on Earth as well as our increasing technological presence in near-Earth space and beyond.

ACE is one of the earliest providers of advanced space weather warnings from its sentry orbit upstream of Earth. ACE observes particles of solar, interplanetary, interstellar and galactic origin -- spanning energy ranges from lower-energy solar wind ions to relativistic galactic cosmic ray nuclei -- to better understand the energy and isotopic composition and origin of the solar wind, interplanetary medium and galactic cosmic rays.

Spacecraft and Instruments

Designed and built at APL, the solar-powered ACE spacecraft has proven to be a reliable sentinel in interplanetary space. ACE carries eight science instruments, two of which -- the Electron, Proton, and Alpha-Particle Monitor (EPAM) and Ultra-Low Energy Isotope Spectrometer (ULEIS) -- were also built by APL.

After more than two decades in orbit, ACE and most instruments are still working very well, although the Solar Energetic Particle Ionic Charge Analyzer (known as SEPICA) was turned off permanently in 2011. In fact, a modified maneuver strategy implemented in 2001 conserves enough fuel to allow the spacecraft to continue operating through 2024.

Results and Expectations

Since 1998, the ACE Real Time Solar Wind (RTSW) monitoring capability has been available to the public. The service provides coverage of the solar wind parameters and solar energetic particle intensity. ACE’s position about 1 million miles in front of Earth gives as much as an hour’s warning of coronal mass ejections that can cause geomagnetic storms here at Earth.

More than 4,200 peer-reviewed scientific papers have been written on data provided by ACE, which is a critical part of the NASA Heliophysics System Observatory, as well as the NOAA Space Weather Prediction Center. ACE will also provide coordinated observations with Parker Solar Probe and Solar Orbiter to better understand our heliosphere.


ACE spacecraft orbiting the Sun.
An interactive, 3D rendering of the Advanced Composition Explorer spacecraft. Click on the image and drag to see all angles of the spacecraft.

Mission Facts 

Launch Date
August 25, 1997

Principal Investigator
Edward Stone Jr., California Institute of Technology

Program Manager
W. Vernon Jones, NASA Headquarters

Project Scientist
Jonathan Ormes, NASA Goddard Space Flight Center

APL project manager Mary Chiu (right) and systems engineer Judi von Mehlem with the ACE spacecraft.
In this mid-1990s image, program manager Mary Chiu (right) and spacecraft systems engineer Judy von Mehlem check out a fill valve for the ACE propulsion subsystem before the spacecraft was moved from APL to NASA Goddard Space Flight Center for final prelaunch testing.


Learn More About Instruments on this Mission