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Sun-Earth Day Presents: Ancient Observatories, Timeless Knowledge

Technology Through Time: Issue #19, ACE

The solar wind rushes past Earth on its journey to Pluto and beyond. Satellites like ACE are the scientist's space buoys, alerting them to incoming storms from the sun that can cause blackouts, and interruptions in our communication technology. To give us a more complete picture of the nature of particles showering Earth from space, ACE also samples matter from the local interstellar medium and from the Milky Way galaxy beyond our solar system.


ACE = 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. The elliptical orbit affords ACE a prime view of the Sun and the galactic regions beyond. The spacecraft has enough propellant on board to maintain an orbit at L1 until ~2019.


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 195 kg of hydrazine fuel for orbit insertion and maintenance. The solar arrays generate about 443 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.

ACE launched on a McDonnell-Douglas Delta II 7920 launch vehicle on August 25, 1997 from Cape Canaveral in Florida.

The spacecraft carries six high-resolution sensors and three monitoring instruments. They sample low-energy particles of solar origin and high-energy galactic particles with a collecting power 10 to 1000 times greater than past or planned experiments. The 10 instruments are designed to measure the magnetic field of the solar wind, and the composition of cosmic rays and other particles that flow by the spacecraft. The measurements include composition, speed and energy of the various atomic constituents.


From a vantage point approximately 1/100 of the distance from the Earth to the Sun ACE performs measurements over a wide range of energy and nuclear mass, under all solar wind flow conditions and during both large and small particle events including solar flares. ACE provides near-real-time solar wind information within 60-minutes of its occurrence. When reporting space weather ACE can provide an advance warning (about one hour) of geomagnetic storms that can overload power grids, disrupt communications on Earth, and present a hazard to astronauts.



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Photo:Advanced Composition Explorer

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