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Insights from members of the New Horizons Science Team

July 8, 2015 Science Shorts: A space physicist’s view of Pluto

Matt Hill

Matt Hill
New Horizons Co-Investigator

Although I’m privileged to have been involved in exciting space events (like the first interstellar boundary crossing), this month’s Pluto encounter is my only experience with a first-time reconnaissance of a world. Eight of the nine traditional planets were explored either before I was born or before I was an adult; one opportunity remained, but I never dreamed  I would be one of the scientists to help make it happen, to round out humankind’s initial exploration of the solar system. This period of human history can only happen once, as Carl Sagan pointed out, and we are the only ones who will ever experience the period in history when we became a spacefaring species.

Now let’s get into what kind of science I do. Unlike astronomy’s centuries-old history, space physics dates back only to the 20th century, and it’s less well known than its cousins astronomy, astrophysics and nuclear physics, and is not as obviously linked with Pluto, so a little background is in order.

Solids, liquids and gasses surround us all. Then there is the fourth state of matter, plasma (hot, electrically conductive “gas”), which is actually the most common form of matter in the universe, even though it is not common on Earth—nor on Pluto.  Plasma differs fundamentally from neutral gas because the electric and magnetic fields and matter in plasma push and shove one another, forming waves and shocks and other collective interactions. The main plasma I’m interested in is the solar wind, which streams from the sun at around 450 kilometers per second – or about a million miles per hour. Traveling through the solar wind is an even faster population of atomic and subatomic particles known as energetic particles. The most energetic of these particles, cosmic rays, were first definitively discovered with experiments on hot air balloons.

The energetic particles in plasma are sort of my thing. I don’t only focus on the quantum-scale objects themselves, but largely on the invisible, astronomic-scale structures they form and interact with. Consider: the distance from the Sun to the inner edge of the interstellar medium is about 1013 meters and the diameter of a proton is about 10-15 meters, a factor of 10,000,000,000,000,000,000,000,000,000 difference (that’s 10 octillion —I can’t pass up a chance to use the word “octillion”). That’s a lot of ground to cover! Fortunately, we can ignore most of what’s in between, which is basically any distance even remotely close to the distances with which people have experience or intuition.

So I study invisible structures with mind-blowing spatial scales, based on particles that travel at terrific speeds and are even more exotic than plasma—the mostly unearthly fourth state of matter. Add to this the fact the experiments we use to make observations take significant fractions of a lifetime to travel to interesting places, and you realize how patient we must be.  For example, New Horizons has taken just under a decade to reach Pluto, and will take longer to reach a Kuiper Belt Object, and should continue returning measurements until the 2030s. That’s a long time be studying invisible stuff.

The four states of matter are on display in this icy scene from Earth.  The floating ice sheets and the snow dusted land are solid, the chill water of the river is a liquid, the unseen air that ripples the river’s surface and carries the clouds is a gas, and the sun is a ball of plasma so hot it can sear your skin from nearly 100 million miles away. (Credit: U.S. Fish and Wildlife Service)

In particular, I (patiently) get excited about energetic particles propagating through plasmas near planets and near solar wind boundaries, like shocks. These energetic particles are particularly interesting because, due to their high speeds, they are excellent tracers of the global structures through which they must propagate to reach our experiments.

Yes, but why would a space physicist study Pluto?

After all, at Pluto we don’t have much to go on. We don’t expect a significant magnetic field, since we don’t believe there is a molten metal core, but mounting evidence demonstrates that the atmosphere of Pluto is large (if tenuous by Earthly standards) and releasing a significant, steady rate of material.  These neutrally charged, Plutonian gas particles from its atmosphere are expected to interact with the impinging, highly charged solar wind and solar photons resulting in freshly charged “pickup ions” that, depending on their kinetic energy, are detectable by the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) time-of-flight mass spectrometer on board New Horizons.

These pickup ions can help tell us about the rate at which Pluto loses its atmosphere, but also Pluto’s production of these pickup ions provides us with the opportunity to study the pickup ions themselves at the moment of their creation. The physics of how they are accelerated and transported is not well understood (although there are many theories). So when New Horizons races through Pluto’s atmosphere and PEPSSI detects the ions there, I will be looking out for pickup ions and hoping to learn how the pickup ions get their energy and how they exit the Pluto system. Only a little more patience is needed—it all happens next month!

Matthew E. Hill is the New Horizons PEPSSI instrument scientist from the Johns Hopkins University Applied Physics Laboratory, where he also works on the Voyager, Cassini and Solar Probe Plus missions.