The Science Flowdown Requirements  The requirements for the Impact boom were based on the scientific needs of the three instruments  mounted to it: the Mag (built at Goddard Space Flight Center); the SWEA, supplied by CESR, Toulouse,  FR; and the STE-D, supplied by UCB - SSL. The magnetometer for this mission is very sensitive: the  magnetic (B) field at 1 Au heliocentric orbit is - 3 to 4 orders of magnitude smaller than near Earth. This  slight field strength was a driver for the EMVEMC design for the spacecraft and the devices near it. To  avoid 'sensing' the spacecraft, the Mag needed to be 3 meters away from it. This requirement set the  minimum boom length. To ensure a low magnetic signature from the boom assembly, no ferritic alloys  were allowed for its construction.  Titanium screws were used to mount the magnetometer to its tray on the 4'h tube element and the tray  itself is made of carbon impregnated PEEK, a high-strength, conductive engineering plastic. To lower  Mag exposure to any eddy currents present in the harness or structure of the boom, the mounting tray  offset the Mag 200 mm from the nearest metal on the tube. Additionally there were not to be any other  instruments closer than 1 m to the Mag. To allow accurate inter-experiment correlation of data, the  angular alignment accuracy and repeatability requirement has an allowable deviation of <0.88' (52.5  arcmin) between the Impact boom mounting feet and the magnetometer  housing from the stowed condition to the deployed state, for the two axes  that form the mounting plane of the Mag.  The second experiment mounted on the Impact boom is the Solar Wind  Electron Analyzer (SWEA, supplied by CESR, Toulouse, FR). The SWEA  has two variably charged hemispheric surfaces that attract electrons into  an anode assembly which counts them as they impinge on to it. The SWEA  would have a limited field of view when mounted directly to the spacecraft  deck, hampering its ability to characterize the electron regime in the  volume around it. Proximity to the spacecraft also causes deflections of the  electrons due to the almost unavoidable static fields that develop near the  spacecraft surface. Since this effect is difficult to model, a better solution  was found. The implementation of the SWEA on the Stereo mission is  extremely good: it is mounted on the extreme end of the boom, allowing a  full 2n radians x 135O field of view (FOV). The demands of the SWEA for  mounting to the boom are not complex: power lines, command lines supplied to it, data return lines from  it. Figure 3- SWEA 1 STE  The final instrument is the STE, a Supra-Thermal Electron detector, mounted on the side of the SWEA  pedestal. It needs a clear 80' x 80° field of view looking along the plane of the ecliptic at a 45' angle  (aligned with the Parker spiral), and to stay at - -4O'C.  The fixed base of the Impact boom is pointed towards the Sun for all science activities, and the only off-  points scheduled are for momentum dumping. The boom deploys away from the Sun, so that there is  minimal solar input to the boom suite, giving very low operational temperatures for the instruments.  Thermal control was a large concern for the instrumenters.  52