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Encountering Jupiter

Nearly eight years from its Pluto target, the New Horizons SWAP instrument observes solar wind interactions near Jupiter

As it rounded Jupiter for a gravity-assist that will speed its journey to the edge of the solar system, the New Horizons spacecraft began testing its science payload and making scientific observations. The Solar Wind Around Pluto (SWAP) instrument, built by Southwest Research Institute, generated data that will help resolve puzzling questions about the interactions between the solar wind, the million-mile-per-hour stream of ionized gas flowing out from the Sun, and Jupiter’s magnetosphere, the magnetic bubble that surrounds the planet and encloses ionized gas.

From a distance of about 0.4 astronomical unit (one AU is the distance from the Earth to the Sun, or about 100 million miles) from Jupiter, SWAP observed an immense structure of compressed, dense, hot ionized gas that forms in the solar wind, called a co-rotating interaction region. These structures form when solar wind streams that are both fast and slow come out of the Sun, and flow out in different directions in response to the rotation of the Sun. The fast layers try to overtake the slow layers yet are unable to flow through them, instead compressing the slow material like a snow plow and bunching up solar wind to create the co-rotating interaction region. These regions contain significantly higher densities and pressures that eventually expand and form discontinuities, or shocks, in the solar wind, which spread out and away from the high pressure regions.

“These solar wind structures collide with the magnetospheres of planets and cause major variations in their size and structure,” said Dr. David McComas, SWAP principal investigator and senior executive director of the SwRI Space Science and Engineering Division. “Because it has the largest magnetosphere in the solar system, understanding the effects of the solar wind at Jupiter could have significant implications for all the planets.”

Studies of these interactions at Jupiter could help determine how much of the Jovian magnetosphere and aurora are driven by external processes, such as the solar wind, versus internal processes, such as planetary rotation.

“There’s an active debate about how much solar wind variability affects what magnetospheric responses we will see at Jupiter,” McComas continued. “We’ve never had the opportunity to simultaneously measure the interactions upstream of Jupiter as we’re observing its aurora, but the New Horizons encounter is changing that.”

The team is making collaborative studies that combine SWAP data with imaging and spectroscopic observations of Jupiter’s aurora using the Hubble Space Telescope. The fusion of these data provides the first simultaneous upstream observations of solar wind interactions at Jupiter as the aurora builds and subsides.

The Jupiter encounter is also enabling SWAP to take measurements inside Jupiter’s magnetosphere, on an orbit that has never before been traveled. That orbit is carrying the spacecraft deep down the magnetotail, the portion of the magnetosphere that is pushed away from the Sun by the flowing solar wind. This route provides the first close look at Jupiter’s more distant magnetotail.

Built to evaluate the solar wind’s interaction with Pluto at about 30 AU from the Sun, SWAP’s sensitivity was successfully decreased at Jupiter’s relatively close distance of about 5 AU from the Sun to generate these early results.

New Horizons is the first mission in NASA’s New Frontiers program. The Johns Hopkins University Applied Physics Laboratory manages the mission and will operate the spacecraft for the NASA Science Mission Directorate.

Comments about this article? Contact McComas at (210) 522-5983 or

Published in the Spring 2007 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

Spring 2007 Technology Today
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