So we had to know the velocity of Rosetta much better than one centimeter per second, and its location in space better than 100 meters at 500 million kilometers from Earth.
因此我们得将罗塞塔的速度误差 精确到小于1厘米每秒， 而它在太空中远离地球5亿公里的位置， 误差要精确到100米以内。