By Ron Cowen
![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2010/09/13224.jpg?resize=300%2C200&ssl=1)
A puzzling knot of atoms at the edge of the solar system is fading, leaving space physicists with an even gnarlier mystery to unravel about the outer boundary of the heliosphere, where the vast bubble in which the solar system resides meets up with interstellar space.
Researchers reported last year that NASA’s Interstellar Boundary Explorer, or IBEX, spacecraft had discovered the knot — the densest part of an equally unexpected narrow ribbon of neutral atoms that makes nearly a complete circle around the fringes of the solar system (SN: 11/21/09, p. 15), about 100 to 125 times Earth’s distance from the sun.