I think he got onto Icarus Two whilst the crew were looking around Icarus One, then he detached the two ships somehow which is why they had to fire themselves back across the gap. Wasn't he the Captain of Icarus One?
To be honest, the whole film turned into a confusing mess by the end and I lost interest. The film was also totally inacurate, as the Sun won't change for the next two billion or so years when it will expand, swallowing the whole Solar System, before exploding it's outer shell and evetually shrinking into a White Dwarf. It wouldn't just go out like a coal fire.