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The Royal Observatory, Greenwich and the Quest for Longitude

EPIC's first view of a full Earth from the Sun-Earth L1 point

Air Date: 07/28/2015
Run Time: 43:09

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Topics: New Horizons, Pluto, history, Charon, mission status, astronomy, Planetary Radio, explaining technology, Earth, explaining science, Plutonium-238

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Come with us on a visit to the home of the prime meridian for a conversation with the curator of the Royal Observatory, Greenwich about the race to create a practical means for determining longitude. It’s a dramatic story of precision astronomy and clock making, told by the Curator of the Observatory to Mat Kaplan as they examined the timepieces that helped revolutionize navigation. Bill Nye is on vacation, but we’re joined by Emily Lakdawalla with hazy news from Pluto, and Bruce Betts' explanation of the plutonium that just passed its namesake planet.

John Harrison's H1 timepiece

National Maritime Museum Collections

John Harrison's H1 timepiece
John Harrison's H4 timepiece

National Maritime Museum Collections

John Harrison's H4 timepiece

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Trivia Contest

This week's prizes are a fabulous Planetary Radio t-shirt and a beautiful new LightSail patch.

This week's question:

If you were a car company trying to market the Apollo mission Lunar Roving Vehicle, what would you name it, and what would your slogan be? There is no right answer. Answers will be judged based on humor and/or effectiveness or whatever other criteria we feel like.

To submit your answer:

Complete the contest entry form at or write to us at no later than Tuesday, August 4th at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

As of July 14th, what was the unofficial name given by the New Horizons team to the dark feature that some have also unofficially called “The Whale?”


The answer will be revealed next week.

Question from the week before:

What was the mass of the plutonium flown on New Horizons?


You may think there are 10.9 kilograms of plutonium-238 dioxide on New Horizons, but the actual mass is about a kilo less than that. Anyway, it seems to have been enough.


No trivia contest spoilers please!

John: 07/28/2015 07:00 CDT

So, starting with 9.75kg PuO2, then one Pu238 and 2 O16 makes the molecular weight 270 and 238/270 is 88% plutonium. 88% of 9.75 is 8.6kg of plutonium. But wait, there's less. With an 88 year half life (and approximating the exponential decay as linear) we'd only have 4.3kg left after 88 years. Since New Horizons has only been running for about 10% of that half-life only about 10% of the decay has occurred - losing .4kg. Starting with 8.6kg of Pu and fissioning .4 leaves around 8.2kg today.

Chris C.: 08/06/2015 10:31 CDT

Hi guys ... I have corrected the Wikipedia article! I also currently have the New Horizons reference book by the NH team (published by Springer) and it confirms the 9.75 kg number. John, are you sure that radioactive decay actually directly reduces the MASS of the plutonium as you describe?

John: 08/11/2015 09:59 CDT

School was a long time ago, but Wikipedia says Pu238 decays into an alpha (helium nucleus) and Uranium 234. So, total mass change from the decay is small (but is the source of the released energy) but. However, after the decay there is less Plutonium since it's now Uranium and a bouncing alpha particle. Every one of these questions leads further down the rabbit hole ...

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