New Horizons' Solar Wind Around Pluto (SWAP) instrument is designed to measure the interactions of Pluto and Charon with the solar wind, the high-speed stream of charged particles flowing out from the Sun. Understanding these interactions will expand our knowledge of the astrophysical processes affecting these bodies and that part of the solar system.
Researchers understand the extremes of solar wind interactions (called the bounding states) with planets, comets and other bodies, but no one knows what kind of interaction is present at Pluto. A flyby observation of comet Borrelly in 2001 recorded a "strong" solar wind interaction with a body that was evolving lots of material into space. In this strong bounding state, the solar wind is reduced to a very low speed near the nucleus of the comet because of strong mass-loading of the wind. Cometary neutrals become ionized (electrically charged) and picked up. The charged cometary material becomes entrained in the flow, resulting in what is commonly seen as a comet's ion tail.
"As the solar wind flows through the region surrounding a comet, the material coming off it adds mass to the local solar wind," says David McComas, SWAP principal investigator and a senior executive director at Southwest Research Institute (SwRI). "The principals of conservation of energy and momentum come into effect, causing the solar wind to slow down and cometary material to speed up - similar to what happens when one vehicle rear-ends another."
The opposite of a comet's strong interaction is the weak bounding state, or a Venus-like interaction. In this extreme, a planet's relatively strong gravity keeps the atmosphere close to the planet. Because the solar wind can't easily penetrate the layer of ionized gas that surrounds Venus at the top of its atmosphere, this creates a different type of tail behind the planet - one that is not mass loaded, but induced.
"It's a very different sort of configuration - like when you drag a fork through a serving of spaghetti," McComas says. "The spaghetti doesn't pass between the tines, but rather drapes around the fork, creating a 'tail' behind it."
Between the Extremes
McComas expects solar wind interactions at Pluto to lie somewhere between the strong and weak extremes.
After taking measurements at Pluto, researchers hope to use the data to define basic parameters about the system. For example, once researchers know how such material comes off Pluto, they can then estimate the amount of Pluto's atmosphere that escapes into space. This will reveal insights into the structure and destiny of the atmosphere itself.
SWAP would go on to take similar measurements at Charon and at least one Kuiper Belt object; however, the team expects those interactions to be much weaker simply because the atmospheres of these objects are likely to be less extensive and not likely to emit much material. Another of the many Pluto mysteries is where the interactions of the solar wind will occur around the planet, so New Horizons science plans call for SWAP to take continuous measurements as it nears and passes Pluto.
"We know when and where to use some of the instruments to take an image or measurement at Pluto," says McComas. "Solar wind interactions, however, present quite a challenge because we're trying to measure this invisible thing surrounding Pluto at an uncertain distance from it."
"The science SWAP to perform is impossible to accomplish without actually going to Pluto-Charon and directly sampling its environment. That capability is something that NASA pioneered and which, to this day, only the United States can do," says Alan Stern, principal investigator of New Horizons and an executive director at SwRI.
The incredible distance of Pluto from the Sun required that the SWAP team build the largest aperture instrument ever used to measure the solar wind. It would allow SWAP to make measurements even when the solar wind is very tenuous. The instrument also combines a retarding potential analyzer (RPA) with an electrostatic analyzer (ESA) to enable extremely fine, accurate energy measurements of the solar wind.
"Should the interaction between Pluto and the solar wind turn out to be very small, the RPA and ESA combination will allow us to measure minute changes in solar wind speed," says Scott Weidner, the SWAP instrument manager and an SwRI principal scientist.
Picking Out Particles
The various instruments aboard New Horizons were designed and are being built independently, yet they are expected to work together to reveal significant new insights about Pluto, Charon and their Kuiper Belt neighbors. SWAP measures low energy interactions, such as those caused by the solar wind. Its complement, the Pluto Energetic Particle Spectrometer Science Investigation, or PEPSSI, looks at higher energy particles, such as pickup ions. The top of SWAP's energy range can measure some pickup ions, and PEPSSI picks up where SWAP leaves off to see the highest energy interactions.
The Sun and solar wind affect the entire solar system and create interesting science opportunities for SWAP throughout its planned nine-year voyage to Pluto. SWAP will operate for more than a month each year and will sample heliospheric pickup ions - ions that originate in interstellar space and are ionized when they come near the Sun. Other pickup ions come from material inside the solar system. Researchers have shown that even collisions between Kuiper Belt objects result in tiny grains that drift toward the Sun, evaporate and become ionized.
The Cassini spacecraft, when it reaches Saturn this July, will allow researchers to observe these so-called "outer source" pickup ions to 10 astronomical units, the region where pickup ions from the outer source are believed to begin. (1 astronomical unit [AU] is the distance from the Earth to the Sun.)
"We'll be out to 30 AU before New Horizons even reaches Pluto. While we're targeting a Kuiper Belt object, we could be anywhere from 30 to 50 AU, where the influence of heliospheric pickup ions becomes greater and greater in the solar wind," says McComas. "On the journey out to Pluto, we'll be able to validate or disprove the outer source theory, which is an exciting warm up to reaching Pluto itself."
Last Updated: 2 March 2011