As the New Horizons spacecraft sped past Pluto, it has produced the best-ever pictures of the rocky world, giving new insight into its geology, composition, and atmosphere. It also sent back over three years worth of measurements of solar wind.
The solar wind is made up of the constant flow of solar particles that the sun hurls out into space. The Solar Wind Around Pluto (SWAP) instrument, operated by Southwest Research Institute (SwRI), had collected three years worth of measurements before the July 15 Pluto flyby.
The observations now give unprecedented insight into an almost entirely unexplored part of our space environment, with only a few spacecraft’s every visiting the region. This fills an essential gap between what other missions have observed closer to the sun, and what Voyager spacecraft have seen further out.
Dr. Heather Elliott, lead author and a principal scientist in SwRI’s Space Science and Engineering Division, said that the SWAP instrument was busy even as the rest of New Horizon’s instruments were “hibernating” to save energy on the long, nine-year voyage to Pluto.
“The instrument was only scheduled to power on for annual checkouts after the Jupiter flyby in 2007.
“We came up with a plan to keep the particle instruments on during the cruise phase while the rest of the spacecraft was hibernating. We started observing in 2012.”
The data showed that the tumultuous flow of solar particles, which in the inner solar system is structured by the interaction of fast and slow flows as well as eruptive events on the sun, becomes more uniform by the time the solar wind has traversed the 3 billion miles to Pluto’s orbit, according to Southwest Research Institute.
Eric Christian, a space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement:
“This region is billions of cubic miles, and we have a handful of spacecraft that have passed through every decade or so, we learn more from every one.”
The SWAP data shows particles in the solar wind that have picked up an initial burst of energy, an acceleration boost that kicks them up just past their original speed. These particles may be the seeds of extremely energetic particles called anomalous cosmic rays, NASA wrote in a statement.
If these super-fast, energetic rays travel close to Earth, they can be a radiation hazard to astronauts. However, when they are further away, at lower energies, it is thought the rays play a role in shaping the boundary where the solar wind hits interstellar space (a region of our solar system that Voyager 2 is currently navigating and observing).
Space may be nearly a thousand times emptier than the best laboratory vacuums on Earth, however, it’s not totally lacking of matter. The sun has a continuous discharge of solar wind, filling space with a thin and tenuous flow of particles, fields, and ionized gas (plasma).
It is this solar wind, along with other solar events like giant coronal mass ejections (CMEs), which influences the very nature of space and also interact with the magnetic systems of Earth as well as other planets. It was this space environment that New Horizons was measuring its journey.
Watch this video titled: Simulating space weather at Pluto from NASA.gov:
Because the solar wind comes from the sun, events which happen on it are the main force that shapes the space environment. Shocks in the solar wind, which can produce space weather such as auroras, are produced by either fast, dense clouds of material called CMEs, or by the collision of two different-speed solar wind streams.
These two features are visible in the inner solar system; however, New Horizons had not seen the same level of detail. The data shows the space environment in the outer solar system has less detailed structure than space closer to Earth, since smaller structures tend to be worn down or clump together as they travel outwards, creating fewer — but bigger — features, NASA wrote.
“At this distance, the scale size of discernible structures increases, since smaller structures are worn down or merge together,” said Elliott. “It’s hard to predict if the interaction between smaller structures will create a bigger structure, or if they will flatten out completely.”
It was found that subtler signs of the sun’s influence were also harder to spot in the outer solar system. Characteristics of the solar wind such as the speed, density, and temperature are shaped by the region were it came from.
Patterns are formed as the sun and its different wind-producing regions rotate. New Horizons was unable to see any patterns as defined as they are when observing closer to the sun, however, it did spot some structure, Elliott said:
“Differences in speed and density average together as the solar wind moves out, but the wind is still being heated as it travels and faster wind runs into slower wind, so you see evidence of the sun’s rotation pattern in the temperatures even in the outer solar system.”
New Horizons is presently at about 35 astronomical units, which is around 35 times further than Earth to the sun. It has remained the only operating spacecraft in the outer solar system.
The findings were published in the Astrophysical Journal Supplement.