The outer boundary of the solar system is more dynamic and complex than ever imagined, astronomers said.

The Interstellar Boundary Explorer satellite, launched two years ago, is studying the heliosphere, the invisible bubble far beyond the planetary orbits where the solar wind meets the particles and radiation that fill interstellar space, researchers told the Los Angeles Times.

The heliosphere, which protects the solar system from 90 percent of the cosmic rays outside it, is changing much faster than scientists expected, according to data published Thursday in the Journal of Geophysical Research.

The sun emits a steady stream of particles traveling outward in all directions about 1 million mph. When they have traveled about 100 times farther than the distance between Earth and the sun the particles collide with the interstellar medium. They deflect most cosmic rays back into space and produce uncharged particles that stream back into the inner solar system.

Over the last two decades, the solar wind has weakened and the heliosphere has shrunk, letting more cosmic radiation enter. Increased cosmic radiation could be very dangerous to future interstellar space travelers, said astronomer David McComas of the Southwest Research Institute in San Antonio.