Cassini reshapes view of solar system

JOHNS HOPKINS (US)—It turns out that the solar system may look more like a basketball than a comet. Images from one of the sensors on NASA’s Cassini spacecraft suggest that the heliosphere—the region of our sun’s influence—may not have the comet-like shape predicted by existing models.

In a paper and images published Oct. 15 in Science, researchers from the Johns Hopkins University Applied Physics Laboratory present a new, spherical view of the heliosphere.

“These images have revolutionized what we thought we knew for the past 50 years. The sun travels through the galaxy not like a comet but more like a big, round bubble,” says Stamatios Krimigis, principal investigator for the Magnetospheric Imaging Instrument aboard Cassini, which is orbiting Saturn. “It’s amazing how a single new observation can change an entire concept that most scientists had taken as true for nearly 50 years.”

As solar wind flows from the sun, it carves out a space in the interstellar medium, the gas and dust that exists between solar systems within our galaxy. Models of the boundary region between the heliosphere and interstellar medium have been based on the assumption that the relative flow of the interstellar medium and its collision with the solar wind dominate the interaction. This would create a foreshortened “nose” in the direction of the solar system’s motion and an elongated “tail” in the opposite direction.


View the movie: This animation starts with our sun and pulls out to show us the heliosphere (gray) and the heliosheath (yellow) of our solar system. As the animation zooms away from the sun, it shows an artist’s concept of the interstellar medium (in black arrows) flowing past the heliosheath. The interstellar magnetic field (smoky gray vertical stripes) parts and slides around the bubble of hot, high pressure particles. The interstellar medium contains the bubble and holds it in a more spherical configuration. The colors on the heliosheath represent the intensity of the hot high pressure particles, with red being the most intense, highest pressure.

Images from the Ion and Neutral Camera, one of the sensors on MIMI, suggest instead that the solar wind’s interaction with the interstellar medium is more significantly controlled by particle pressure and magnetic field energy density.

“The map we’ve created from INCA’s images suggests that pressure from a hot population of charged particles and interaction with the interstellar medium’s magnetic field strongly influence the shape of the heliosphere,” says Don Mitchell, MIMI/INCA co-investigator at APL.

Since Cassini entered into orbit around Saturn in July 2004, INCA has been mapping energetic neutral atoms near the planet as well as their dispersal across the entire sky. The energetic neutral atoms are produced by energetic protons; they are responsible for the outward pressure of the heliosphere beyond the interface where the solar wind collides with the interstellar medium and they interact with the magnetic field of the interstellar medium.

“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,” says Edmond Roelof, MIMI/INCA coinvestigator at APL.

Researchers from University of Arizona, Tucson, Southwest Research Institute, and the University of Texas at San Antonio contributed to the Science article. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency.

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