Dancing with Eros
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Dancing with Eros

October 23, 2009
Because of Eros' irregular shape, it is a challenge to navigate the NEAR Shoemaker spacecraft accurately through its sequence of low orbits. To help, the mission team plans and acquires sequences of images that meet objectives both for scientific mapping of the asteroid and for tracking the spacecraft's position.

Several times daily, NEAR Shoemaker's imager returns navigation images like the one shown at top, taken March 4, 2000, from a range of 203 kilometers (126 miles). The mission's Navigation Team analyzes the images to track directions to familiar landmarks like hills or craters, and thus finds the spacecraft's position relative to the asteroid. This requires very accurate knowledge of where the imager is pointed. Pointing is measured by taking images of stars, which are fixed reference points in space. Based on analysis of hundreds of star field images like the one at bottom, taken March 6, 2000, the imager's pointing is known to within about eight thousandths of a degree - approximately the size of a golf ball two hundred yards away. The combination of techniques used to navigate the spacecraft provides a high degree of accuracy in predicting future spacecraft location, so that detailed imaging sequences can be planned weeks in advance.

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