Atoms whizz past Earth from new direction

The solar system moves through a local galactic cloud at a speed of 50,000 miles per hour, driving an interstellar wind of particles, which can be measured by Earth-orbiting or interplanetary spacecraft. (Credit: NASA/Adler/UChicago/Wesleyan)

Neutral interstellar atoms are flowing into the solar system from a different direction than scientists have observed in the past.

The interstellar atoms go past the Earth as the solar system passes through the surrounding interstellar cloud at 23 kilometers per second (50,000 miles per hour).

The latest measurements of the interstellar wind direction from the Interstellar Boundary Explorer (IBEX) were found to differ from those made by the Ulysses spacecraft in the 1990s.

interstellar cloud diagram
(Credit: NASA/Adler/UChicago/Wesleyan)

That difference led the IBEX team to compare the IBEX measurements to data gathered by 11 spacecraft between 1972 and 2011. Statistical testing of the Earth-orbiting and interplanetary spacecraft data showed that, over the past 40 years, the longitude of the interstellar helium wind has changed by four to nine degrees.


“We concluded it’s highly likely that the direction of the interstellar wind has changed over the past 40 years. It’s also highly unlikely that the direction of the interstellar helium wind has remained constant,” says Priscilla Frisch, lead author of the study and a senior scientist in astronomy and astrophysics at the University of Chicago.

“We think the change in wind direction could be explained by turbulence in the interstellar cloud around the sun,” she says.

The spacecraft data used for this study were gathered using three methods to measure the neutral interstellar helium wind direction: IBEX and Ulysses provided direct in situ measurements of the neutral wind.

The earliest measurements from the 1970s used fluorescence of solar extreme ultraviolet radiation of the helium atoms near the Sun. Measurements also were included of the helium flow direction from “pickup ions”—neutral particles in the solar system that become ionized near the sun and join the solar wind.

“This result is really stunning,” says Dave McComas, IBEX principal investigator, assistant vice president of the Space Science and Engineering Division at Southwest Research Institute, and an author of the paper.

“Previously we thought the very local interstellar medium was very constant, but these results show just how dynamic the solar system’s interaction is.”

The paper appears in Science.

IBEX is one of NASA’s series of low-cost, rapidly developed Small Explorer space missions. Southwest Research Institute in San Antonio leads the IBEX mission with teams of national and international partners. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the Explorers Program for NASA’s Science Mission Directorate in Washington, DC.

star diagram
From Earth’s perspective, the interstellar wind flows in from a point just above the constellation Scorpius. Results from 11 spacecraft over 40 years show that the exact direction has changed some 4 to 9 degrees since the 1970s. (Credit: NASA/Goddard Space Flight Center)

Source: U. Chicago via Southwest Research Institute