|
After a 5-year mission, the Near-Earth Asteroid Rendezvous-Shoemaker (NEAR) spacecraft made a controlled landing 12 February 2001 onto the asteroid, 433 Eros. Onboard the spacecraft, the NEAR Laser Rangefinder (NLR), a laser altimeter, gathered over 11 million measurements throughout 2000 and 2001, providing a spatially dense, high-resolution, topographical map of Eros. This instrument, launched in February 1996, was subjected to a constant, albeit, low radiation background predicted during the mission design phase to be 3 krad, cumulative, from solar protons at a shield depth of 1.8 mm aluminum. Using the onboard NLR calibration capability, and through extended observation of NLR measurement performance, the instrument exceeded requirements for this particular radiation environment. Electronic parts for the altimeter had been reviewed, assessed and screened, as necessary, for space quality and radiation hardness during its development. The NEAR mission included an excursion beyond Mars' orbit during its 4-year transit, followed by a one-year mission orbiting the near-Earth asteroid, 433 Eros, continuously collecting altimetry data. The majority of the data collection occurred during solar maximum and, in particular, operated without interruption through the events on Bastille Day, 14 July 2000 (comparable to the large October 1989 events of the previous solar maximum) and 10 November 2000. At Earth, the July 2000 proton level provided in a few days over half of the expected cumulative radiation, predicted through use of Feynman's model. Based on uneventful operation of the NEAR, including the absence of any degradation in solar array currents due to proton displacement damage and the nominal performance of the altimeter, it appears that the 14 July event did not intersect the NEAR location. The NLR-derived topographic data successfully enabled determination of Eros' shape, mass, and density contributing to the understanding the internal structure and collisional evolution of Eros.
|
