Mercury is the Greek god with wings on his feet, fog would not slow him down. "The visual journey of the Mercury symbol ends at an equal-armed cross. This is the final stage of solidification of thought. This indicates our thoughts have become manifest in the material world. We could say Mercury is a midwife to thoughts conceived in the ephemeral world," (whatsyoursign.com)

An event horizon is half astrophysics and half science fiction. The idea is that near a black hole, gravity is so strong that not even light can escape, so you could never see what happens past that point.

This poem was commissioned for the Verb radio show on the BBC. That alone indicates that the purpose of the poem was to try to get you thinking about literary stuff. They think they are all brilliant intellectuals on that show.

The thing about a horizon is that you can travel and travel and travel, but you never reach it. It is always ahead of you– (and behind and all around.)That is the theme, the journeying. I don’t think the poem says much about truth though,
References :
http://www.whats-your-sign.com/mercury-symbol.html