Scientists at the Applied Physics Laboratory at Hopkins have discovered that the solar system is shaped more like a bubble than a comet. The satellite Cassini, currently orbiting Saturn, was used to probe the heliosphere, a bubble of solar wind that separates the solar system from interstellar space.
The solar wind consists of a stream of charged particles ejected out of the upper atmosphere of the sun at speed, forming a bubble around the solar system itself, the heliosphere, which keeps out charged particles from interstellar space.
Scientists previously focused on the collision between interstellar space and the boundary of the heliosphere in modeling their interactions, generating a comet-like model, pointing in the direction the solar system moves.
The heliosphere is relatively little-known, according to Stamatios Krimigis, principal investigator for the Magnetosphere Imaging Instrument (MIMI) project. "The only thing we know about the heliosphere is its first boundary (the termination shock) that was crossed by the two Voyager spacecraft in 2004 and 2007 at distances of about 14 billion kilometers from Earth," said Krimigis. "But we still don't know what lies beyond, and we wanted to get an image around the entire sky, not just where the Voyagers crossed."
The Voyager spacecraft were launched in the 1970s, sent to explore the outer planets of our solar system and beyond.
More recently, the Cassini-Huygens probe was launched in 1997. Named after the astronomers Giovanni Cassini and Christiaan Huygens, it consists of a probe, Huygens, sent to study the largest moon of Saturn, and an orbiter, Cassini, packed with various sensors. Among its instruments, Cassini uses the Ion and Neutral Camera (INCA) to map energetically neutral atoms, which are generated by energetic protons in the heliosphere. These particles interact with the magnetic field of interstellar space, affecting its shape.
"Our results showed that the there is a huge belt of hot particles surrounding the expanding bubble of solar plasma, beyond the termination shock," Krimigis said. "We got this image by detecting fast neutral atoms that take a few months to travel from the edge of the solar system to Saturn, even though they travel at about 300,000 miles per hour. Saturn is 900 million miles from Earth, but still a long way from the edge of the solar system."
Cassini's map of energetically neutral atoms shows that the magnetic interaction between the heliosphere and interstellar space has the most significant impact on the shape of the heliosphere. "Energetic neutral atom imaging has demonstrated its power to reveal the distribution of energetic ions, first in Earth's own magnetosphere, next in the giant magnetosphere of Saturn and now throughout vast structures in space, out to the very edge of our sun's interaction with the interstellar medium," Edmond Roelof, a researcher on the MIMI/INCA projects, said.
"The results mean that our solar system is like a cocoon that travels through the galaxy and its radius is limited only by the pressure of the strength of the galactic magnetic field," said Krimigis.
Now that INCA has been able to map the heliosphere, many new exciting directions in research have opened up.
"We want to know the depth of the belt of hot particles, to determine the details of its shape, to study if it changes with the influence of solar activity, and so on," Krimigis said. "Much work to do, and more observations to make!"
