Of the 100 probes that have been sent out into space, only one probe - Voyager 2 in 1989 - has ever sent back any information about the solar system's outer planets. From afar, they seem featureless and devoid of any significant geological activity.

Since then, long-range telescopes have shown that there is much more to Neptune and Uranus than previously thought and that they both warrant closer inspection. Time, however, is of the essence. Deep-space missions are dependent on Jupiter. The slingshot effect it provides to move probes along the way is only available to us every 25 years.

Chris and Maggie look back at some of the stories they have covered in 2021 in the Sky at Night's big review of the year.