Former President Bill Clinton recently expressed his support for interstellar travel at the 100 Year Spaceship Symposium, an international event advocating for human expansion into other star systems.

Interstellar travel will depend upon extremely precise measurements of every factor involved in the mission. The knowledge of those factors may be improved by the solution a University of Missouri researcher found to a puzzle that has stumped astrophysicists for decades.

"The Pioneer spacecraft, two probes launched into space in the early 70s, seemed to violate the Newtonian law of gravity by decelerating anomalously as they traveled, but there was nothing in physics to explain why this happened," said Sergei Kopeikin, professor of physics and astronomy in MU's College of Arts and Science.

"My study suggests that this so-called Pioneer anomaly was not anything strange. The confusion can be explained by the effect of the expansion of the universe on the movement of photons that make up light and radio waves."

Beams of radio waves were sent to and bounced off the Pioneer spacecraft to measure the probes' movement. The time it took for the photons to complete a round trip was used to calculate the spacecrafts' distance and speed.

Kopeikin's research suggests that the photons move faster than expected from the Newtonian theory thus causing the appearance of deceleration, though the craft were actually traveling at the correct speed predicted by the theory.

The universe is constantly expanding and this alters the Earth-based observations of the photons bouncing off the spacecraft, causing the Pioneer probes to appear to slow down.

"Previous research has focused on mechanical explanations for the Pioneer anomaly, such as the recoil of heat from the craft's electrical generators pushing the craft backwards," Kopeikin said. "However that only explains 15 to 20 percent of the observed deceleration, whereas it is the equation for photons that explains the remaining 80-85 percent."

Physicists must be careful when dealing with propagation of light in the presence of the expansion of space, noted Kopeikin, since it is affected by forces that are irrelevant in other equations. For example, the expansion of the universe affects photons, but doesn't influence the motion of planets and electrons in atoms.

"Having accurate measurements of the physical parameters of the universe help us form a basis to make plans for interstellar exploration," Kopeikin said.

"Discerning the effect of the expansion of the universe on light is important to the fundamental understanding of space and time. The present study is part of a larger on-going research project that may influence the future of physics."

The study "Celestial ephemerides in an expanding universe" was published in the journal Physical Review D.