NASA From New Horizons: “New Horizons Confirms Solar Wind Slows Farther from the Sun”

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From New Horizons

Research could help predict when spacecraft will cross the termination shock

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The SWAP instrument aboard NASA’s New Horizons spacecraft has confirmed that the solar wind slows as it travels farther from the Sun. This schematic of the heliosphere shows the solar wind begins slowing at approximately 4 AU radial distance from the Sun and continues to slow as it moves toward the outer solar system and picks up interstellar material. Current extrapolations reveal the termination shock may currently be closer than found by the Voyager spacecraft. However, increasing solar activity will soon expand the heliosphere and push the termination shock farther out, possibly to the 84-94 AU range encountered by the Voyager spacecraft. (Image credit: Southwest Research Institute; background artist rendering by NASA and Adler Planetarium)

Measurements taken by the Solar Wind Around Pluto (SWAP) instrument aboard NASA’s New Horizons spacecraft are providing important new insights from some of the farthest reaches of space ever explored. In a paper published recently in The Astrophysical Journal, New Horizons scientists show how the solar wind — the supersonic stream of charged particles blown out by the Sun — evolves at increasing distances from the Sun.

“Previously, only the Pioneer 10 and 11 and Voyager 1 and 2 missions have explored the outer solar system and outer heliosphere, but now New Horizons is doing that with more modern scientific instruments,” said Heather Elliott, a staff scientist at the Southwest Research Institute, deputy principal investigator of the SWAP instrument and lead author of the paper. “Our Sun’s influence on the space environment extends well beyond the outer planets, and SWAP is showing us new aspects of how that environment changes with distance.”

The solar wind fills a bubble-like region of space encompassing our solar system, called the heliosphere. From aboard New Horizons, SWAP collects detailed, daily measurements of the solar wind as well as other key components called “interstellar pickup ions” in the outer heliosphere. These interstellar pickup ions are created when neutral material from interstellar space enters the solar system and becomes ionized by light from the Sun or by charge exchange interactions with solar wind ions.

The journey New Horizons is taking through the outer heliosphere contrasts that of Voyager since this solar cycle is mild compared to the very active solar cycle explored during the Voyager passage through the outer heliosphere. In addition to measuring the solar wind, SWAP is extremely sensitive and simultaneously measures the low fluxes of interstellar pickup ions with unprecedented time resolution and extensive spatial coverage. Currently, New Horizons is the only spacecraft in the solar wind beyond Mars and consequently the only spacecraft measuring the interaction between the solar wind and interstellar material in the outer heliosphere.

As the solar wind moves farther from the Sun, it encounters an increasing amount of material from interstellar space. When interstellar material is ionized, the solar wind picks up the material and, researchers theorized, slows and heats in response. SWAP has now detected and confirmed this predicted effect.

The SWAP team compared the New Horizons solar wind speed measurements from 21 to 42 astronomical units to the speeds at 1 AU from both the Advanced Composition Explorer (ACE) and Solar TErrestrial RElations Observatory (STEREO) spacecraft. (One astronomical unit, or AU, is equal to the distance between the Sun and Earth.) By 21 AU, it appeared that SWAP could be detecting the slowing of the solar wind in response to picking up interstellar material. However, when New Horizons traveled beyond Pluto, between 33 and 42 AU, the solar wind measured 6-7% slower than at the 1 AU distance, confirming the effect.

In addition to confirming the slowing of the solar wind at great distances, the change in the solar wind temperature and density could also provide a means to estimate when New Horizons will join the Voyager spacecraft on the other side of the termination shock, the boundary marking where the solar wind slows to less than the sound speed as it approaches the interstellar medium. Voyager 1 crossed the termination shock in 2004 at 94 AU, followed by Voyager 2 in 2007 at 84 AU. Based on current lower levels of solar activity and lower solar wind pressures, the termination shock is expected to have moved closer to the Sun since the Voyager crossings.

Extrapolating current trends in the New Horizons measurements also indicates that the termination shock might now be closer than when it was intersected by Voyager. At the earliest, New Horizons will reach the termination shock in the mid-2020s. As the solar cycle activity increases, the increase in pressure will likely expand the heliosphere. This could push the termination shock to the 84-94 AU range found by the Voyager spacecraft before New Horizons has time to reach it.

“New Horizons has significantly advanced our knowledge of distant planetary objects, and it’s only fitting that it is now also revealing new knowledge about our own Sun and its heliosphere,” said New Horizons Principal Investigator Alan Stern, of SwRI.

New Horizons is the first mission in NASA’s New Frontiers program. The Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, built and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate. SwRI led the payload instrument development and leads the New Horizons science and mission teams from the Tombaugh Science Operations Center located at SwRI facilities in Boulder, Colo. For more information, go to: http://pluto.jhuapl.edu/.

The paper “Slowing of the Solar Wind in the Outer Heliosphere” by Elliott, D.J. McComas, E.J. Zirnstein, B.M. Randol, P.A. Delamere, G. Livadiotis, F. Bagenal, N.P. Barnes, S.A. Stern, L.A. Young, C.B. Olkin, J. Spencer, H.A. Weaver, K. Ennico, G.R. Gladstone, and C.W. Smith, was published November 11 in The Astrophysical Journal.

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The New Horizons mission is helping us understand worlds at the edge of our solar system by making the first reconnaissance of the dwarf planet Pluto and by venturing deeper into the distant, mysterious Kuiper Belt – a relic of solar system formation.

The Journey

New Horizons launched on Jan. 19, 2006; it swung past Jupiter for a gravity boost and scientific studies in February 2007, and conducted a six-month-long reconnaissance flyby study of Pluto and its moons in summer 2015, culminating with Pluto closest approach on July 14, 2015. As part of an extended mission, pending NASA approval, the spacecraft is expected to head farther into the Kuiper Belt to examine another of the ancient, icy mini-worlds in that vast region, at least a billion miles beyond Neptune’s orbit.

Sending a spacecraft on this long journey is helping us to answer basic questions about the surface properties, geology, interior makeup and atmospheres on these bodies.

New Science

The National Academy of Sciences has ranked the exploration of the Kuiper Belt – including Pluto – of the highest priority for solar system exploration. Generally, New Horizons seeks to understand where Pluto and its moons “fit in” with the other objects in the solar system, such as the inner rocky planets (Earth, Mars, Venus and Mercury) and the outer gas giants (Jupiter, Saturn, Uranus and Neptune).

Pluto and its largest moon, Charon, belong to a third category known as “ice dwarfs.” They have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material.

Using Hubble Space Telescope images, New Horizons team members have discovered four previously unknown moons of Pluto: Nix, Hydra, Styx and Kerberos.

A close-up look at these worlds from a spacecraft promises to tell an incredible story about the origins and outskirts of our solar system. New Horizons is exploring – for the first time – how ice dwarf planets like Pluto and Kuiper Belt bodies have evolved over time.

The Need to Explore

The United States has been the first nation to reach every planet from Mercury to Neptune with a space probe. New Horizons is allowing the U.S. to complete the initial reconnaissance of the solar system.

A Team Approach

The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, designed, built, and operates the New Horizons spacecraft and manages the mission for NASA’s Science Mission Directorate.
The National Aeronautics and Space Administration (NASA) is the agency of the United States government that is responsible for the nation’s civilian space program and for aeronautics and aerospace research.