The protective bubble of our solar system is probably not comet-shaped.
Scientists have long positioned this the heliosphere, the enormous bubble of charged particles that the sun blows around itself, has a round leading edge, where the solar system sweeps through space, with a long tail flowing behind it. But the true shape of the heliosphere is rougher and more complex, a recent study suggests – something like a deflated croissant.
It is difficult to map out the heliosphere because the nearest edge is still a whopping 10 billion miles (16 billion kilometers) from Earth. Just two spaceships, NASAs Probe for Voyager 1 and Voyager 2, have directly sampled the boundary, and two data points are far from sufficient to outline the contours of the heliosphere.
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That scientists have done so by other means. They studied measurements, for example galactic cosmic rays, super-energetic charging particles that zoom in from very far in our neighborhood. Researchers have also carefully tracked down “energy neutral atoms” that were interviewed after solar interactions with the interstellar medium, the very cosmic sea that lies outside the heliosphere.
Such tracking has been done by a variety of spacecraft, including NASA’s Interstellar Boundary Explorer and the Probe of Cassini Saturn. Scientists feed this information into computer models, which use it to map the shape of the heliosphere.
The recent study takes a new look at such data and also includes measurements of “pick-up ions” made by NASA. Pluto probe for new Horizons, which is currently more than 4.3 billion miles (6.9 billion km) from Earth.
Pick-up ions are carried by the solar wind, the stream of charged particles flows continuously from the sun. (This current is blocked by the interstellar medium to form the boundary of the heliosphere.) Pick-up ions are much hotter than the particles that make up most of the solar wind, contributing to the early form of the heliosphere, found student team members.
“There are two liquids mixed together. You have one component that is very cold and one component that is much hotter, the pick-up ions,” leads author Merav Opher, a professor of astronomy at Boston University, said in a statement.
“If you have some cold liquid and hot liquid, and you put them in space, they will not mix – they will usually evolve separately,” Opher said. “What we did was separate these two components the sunshine and model the resulting 3D shape of the heliosphere. “
That shape, they determine, is croissant-like: a curved central hump with two jets curling away.
“Because the pick-up ions dominate the thermodynamics, everything is very spherical,” Opher said. “But because they leave the system completely behind the termination shock, the whole heliosphere deflates.”
De termination shock is the boundary region of the heliosphere, in which solar wind particles begin to press into the interstellar medium and slowly to less than the speed of sound.
Opher and her colleagues have gained a better understanding of the shape of the heliosphere. For example, the bubble blocks about 75% of galactic cosmic rays, which can damage space and the DNA of traveling astronauts. Knowing in detail which regions of space are protected can help mission planners. (Life on Earth does not have to worry much about galactic cosmic rays; our planet’s magnetic field and atmosphere provide effective protection.)
The study was published in March 2020 in the journal Nature Astronomy.
Mike Wall is the author of “Out There” (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.
