Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: The captivating motion of various satellites orbiting our planet has been revealed in a fascinating Nasa video. Created by the agency’s Goddard Space Flight Centre in Maryland, it shows their entire Earth-observation fleet and the paths they take around Earth. While the ISS sweeps across ‘diagonally’, in the video at least, others like the Landsat series are seen travelling from pole to pole. Scroll down for video Nasa scientists in Maryland, US have created a satellite visualisation (shown). It shows dozens of Earth-observing satellites currently orbiting our planet. They include Landsat 7 and 8, part of Nasa's long-serving series. And the ISS can also be seen sweeping diagonally across EarthNasa scientists in Maryland, US have created a satellite visualisationIt shows dozens of Earth-observing satellites currently orbiting our planetThey include Landsat 7 and 8, part of Nasa's long-serving seriesAnd the ISS can also be seen sweeping diagonally across EarthMany Earth-observing satellites orbit pole to pole to see the whole planet 
 Question:‘Together, they provide a picture of the _ as a system,’ said Nasa.

Earth