The intensity of the sun's million-mile-per-hour solar wind has dropped to its lowest levels since accurate records began half a century ago, scientists say.
Measurements of the cosmic blasts of radiation, ejected from the sun's upper atmosphere, were made with the Ulysses spacecraft, a joint mission between NASA and the European Space Agency (ESA).

The solar wind "inflates a protective bubble, or heliosphere, around the solar system," which protects the inner planets against the radiation from other stars, said Dave McComas, Ulysses' solar wind principal investigator and senior executive director at the Southwest Research Institute in San Antonio, Texas.

"With the solar wind at an all-time low, there is an excellent chance the heliosphere will diminish in size and strength," said Ed Smith, NASA's Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, California.

"If that occurs, more galactic cosmic rays will make it into the inner part of our solar system," added Smith.

Scientists say the weakening of solar wind appears to be due to changes in the sun's magnetic field, but the causes of these changes are unknown.