Though previously unobserved, scientists long assumed that, just like any other object moving through a medium, the solar system had a tail. Not until NASA's Interstellar Boundary Explorer (IBEX) however, were scientists able to perceive it and map its boundaries, which, it turns out, resembles something like a four-leaf clover.
Published in The Astrophysical Journal, the details of the tail, called the heliotail, were mapped out by combining observations from the first three years of IBEX imagery. The subsequent image portrays a combination of fast and slow moving particles with two lobes of slower particles on the sides, faster particles above and below and the whole thing twisted due to the pushing and pulling of magnetic fields located outside the solar system.
"Many models have suggested the heliotail might be like this or like that, but we've had no observations," said David McComas, lead author on the paper and principal investigator for IBEX at Southwest Research Institute in San Antonio, Texas. "We always drew pictures where the tail of the heliosphere just disappears off the page, since we couldn't even speculate about what it really looked like."
While telescopes have spotted such tails around other stars, the journey to perceiving our own has been a long one, starting with Pioneer 10, which was headed in the right direction after it crossed the orbit of Neptune in 1983 but lost power in 2003 before moving into the tail itself. Meanwhile, watching it from afar is difficult as the particles in the tail, and throughout the heliosphere, don't shine, meaning they cannot be seen in a conventional manner.
IBEX, on the other hand, can map such regions by measuring neutral particles created by collisions at the heliosphere's boundaries - a technique called energetic neutral atom imaging. Because the paths of neutral particles aren't affected by the heliosphere's magnetic fields, they are left to simply travel in a straight line from collision to IBEX, meaning that, by observing where the neutral particles came from, scientists are able to detect what's going on in these distant regions.
"Using neutral atoms, IBEX can observe far away structures, even from Earth's orbit," said Eric Christian, IBEX mission scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "And IBEX scans the entire sky, so it has given us our first data about what the tail of the heliosphere looks like, an important part of understanding our place in and movement through the galaxy."
Going forward, scientists can test their computer simulations of the heliosphere against the new observations and improve their models as needed. Together, data from instruments in space and analyses at labs on the ground will continue to improve scientists' understanding of the comet-like tail.
"The tail is our footprint on the galaxy, and it's exciting that we're starting to understand the structure of it," Christian said. "The next step is to incorporate these observations into our models and start the process of really understanding our heliopshere."
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