This is a story describing events that happened in the past, but it is written in a narrative style called Historic Present, where the present tense is used to describe past events.
Here is part of a more typical story that is written in the past:
I told him very gently, and afterwards he was very ashamed. Excalibur: a novel of Arthur
Two things happen in this sentence: first, "I told him", and then "he was very ashamed". Afterwards indicates that the second event occurred after the first event. Note that both events occurred in the past, and so are in simple past.
If we adopt the narrative style of the passage you quoted, where present tense is used to describe past events, it will look like this:
I tell him very gently, and afterwards he is very ashamed.
Both events still occurred in the past, but are now described in present tense, and afterwards places the second event firmly after the first event.
In your passage, the second event is "It then sends out a powerful radio signal, aimed exactly at Saturn." The first event is not described in the passage that you quoted, but is probably in a preceding paragraph.