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March 27, 2015

New Horizons Sampling ‘Space Weather’ on Approach to Pluto

Technicians install and work on the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI, top) and Solar Wind Around Pluto (SWAP) instruments on New Horizons in 2005. PEPSSI was provided by the Johns Hopkins University Applied Physics Laboratory; Ralph McNutt of APL leads the investigation. SWAP was provided by Southwest Research Institute; Dave McComas of SwRI leads the investigation.

As NASA’s New Horizons spacecraft approaches the Pluto system, its space plasma (also called charged particle) instruments —SWAP and PEPSSI — have already been taking measurements and assessing the space weather environment in the Kuiper Belt near Pluto.

“Results from those measurements are being radioed to the ground and our team is already learning new things about the distant environment near Pluto’s orbit, 3 billion miles from Earth,” says New Horizons Principal Investigator Alan Stern, from Southwest Research Institute, Boulder, Colo.

The solar wind consists mainly of protons and electrons, but also has trace amounts of ions of helium and oxygen that New Horizons detects. Typical solar wind speeds at Pluto’s orbit distance range from 350-500 kilometers per second (about 750,000-1 million miles per hour); typical densities of the solar wind near Pluto’s orbit are about 6,000 particles per cubic meter – about a 1,000 times less than the solar wind at Earth. For comparison, the Earth’s atmosphere at its surface is one hundred billion-billion times denser.

“For some unknown reason, the Sun has been blowing less hard over the past decade and a half, and we are seeing the weakest solar wind of the space age,” says New Horizons co-investigator Dave McComas, also of SwRI, who leads the SWAP instrument team. “A weaker solar wind means that the size of the region where the solar wind interacts with the planet’s escaping atmosphere is expanded beyond our earlier predictions.”

Added New Horizons co-investigator Fran Bagenal, from University of Colorado, Boulder: “This means that New Horizons may cross the interaction boundary between the solar wind and Pluto’s atmosphere up to dozens of Pluto radii [and several hours] before its closest approach on July 14, creating a scientific bonanza for studies of the composition and escape rate of Pluto’s atmosphere!”

How’s the Space Weather Out There?

A forecast for the New Horizons flyby of Pluto on July 14, from our space weather meteorologists: “Expect typical gusts of solar protons and electrons with speeds up to 1 million miles per hour, varying by maybe 10 degrees or so radially from the Sun, that will slow down as they interact with Pluto’s escaping atmosphere, picking up energetic ions created from nitrogen, carbon monoxide and methane along the way.”

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