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Astronomers this week have been warming up for an encounter as far from the Sun as ever attempted. It's the finale of the New Horizons mission which successfully passed Pluto in 2015 and is now on its way to Ultima Thule - a Kuiper belt object on the edge of the solar system. Marc Buie is just back from Senegal where he and a team of fellow astronomers have been observing this ancient rock to get a final look at its size and shape, before the momentous flyby on Jan 1st 2019. He explains why the encounter will be so valuable in unlocking key secrets in the formation of our solar system.

It's the 150th birthday of the discovery of helium, which, after hydrogen is the second most abundant element in the universe. It's surprisingly rare on Earth, but it makes up much of the content of the gas giants in our local neighbourhood, Jupiter and Saturn. Adam Rutherford hears from particle physicist and Science Museum curator Harry Cliff on how it was first discovered through a telescope rather than in a lab, and Jessica Spake of Exeter University who after an 18-year search has used similar techniques to discover helium around an exoplanet 200 light-years away.

We hear from scientist and author Dan Davis from the University of Manchester, the next in our preview of authors shortlisted for this year's Royal Society book prize. The Beautiful Cure, is the rollicking story of how the intricate immune system came to be understood.

And there's an update from physicist Helen Czerski. She's part of a 40-strong team of field scientists on board the Oden, a Swedish ice breaker and research ship. They're set to find a suitable iceberg, and moor to get to grips with the factors that guide the arctic weather patterns.

Producer: Adrian Washbourne.