ACE

Spacecraft nº:

US spacecraft nº: 5

Launch data: Designation 24912 / 97045A Launch date 25 Aug 1997 Launch site CC, LC17A Launch vehicle Delta-2 (7920-8 D247) Mission Scientific Operator NASA JPL Perigee/Apogee 177/1367769 km Inclination 28.7° Period 95,1 min

Specifications Prime contractor JHU/APL, Laurel, MD Platform   Mass at launch 785 kg (includes 189 kg of hydrazine fuel for orbit insertion and maintenance) Dry Mass appr. 560 kg Basic shape  two octagonal decks Dimension  1.6 m across and 1 m high (not including the four solar arrays and the magnetometer booms attached to two of the solar panels) Solar array  four deployable solar panels (each 86.4 cm x 149.9 cm); wing span about 8.3 m Stabilization  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 Equipment  two deployable magnetometer booms (magnetometer sensors on ends of boom) DC power  about 500 W; 443 W (EOL) Design lifetime In Solar orbit

Credit: NASA

Description:

• The ACE spacecraft was designed and built at JHU/APL, Laurel, MD. The S/C structure (decks and panels: honeycomb with aluminum alloy facesheet) has two octagonal decks, 1.6 m across and 1 m high; the overall wing span is about 8.3 m. The S/C is spin stabilized with the spin axis Earth/sun pointing (star and sun sensors). Attitude is measured by a star tracker (CT-632 solid-state scanner of BATC) and sun sensors, attitude knowledge is within ± 0.7º (goal of ± 0.5º for the magnetometer). The nominal spin rate is 5 rpm, the S/C is oriented in the Earth/sun direction.

Mission details:

• Orbit: Initial circular orbit parking orbit of 185 km with an inclination of 28.7º. Then transfer trajectory insertion toward the sun (with re-ignition of 2nd stage). Final halo orbit (Lissajous) about the Lagrangian (or Earth-sun libration) point L1 (250 Earth radii toward the sun, or about 1.5 million km from Earth toward the sun).
• ACE was placed in a halo orbit around the first Lagrangian point (L1) of the Earth-Sun system, about 1.4 million km from Earth. It carries six high-resolution sensors and three monitoring instruments for sampling low-energy particles of solar origin and high-energy galactic particles with a collecting power 10-1,000 times greater than previous experiments. The spacecraft can give advance warning, of about one hour, of geomagnetic storms that might overload power grids, disrupt communications, and pose a hazard to astronauts.
• The spacecraft has enough propellant on board to maintain an orbit at L1 until about 2019. Lately, a fuel use strategy has been implemented that will allow continued operations through the year 2022.
• In 2002 the SEPICA instrument experienced problems with the gas flow regulation of its proportional counters and with a high-voltage power supply. Two thirds of the instrument is non-functional, but the third counter is returning good science data. Data of SEPICA is only available until Feb. 4, 2005 when further problems arose resulting in a de facto retirement of the instrument.
• Three types of maneuvers (attitude, orbit and spin) have been used since July 2001 to control ACE. Orbit maneuvers use ~1.5 kg/year of fuel per year and keep the spacecraft bound to the L1 libration point. Attitude maneuvers use ~ 3 kg/year and are required to maintain the HGA antenna constraint. With this strategy, fuel use is 4.5 kg/year total, and the 70 kg of fuel remaining as of October 2007 will be consumed by 2024.
  
  

  Schematic view of the halo orbit of ACE at L1, the point of equilibrium between the Earth and sun's gravitational fields (image credit: NASA, Caltech)

  
  

External links:

• NASA: Satellite description
• EOportal.org Satellite description