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[Mr. TOP SECRET]

by Casey Kazan
Daily Galaxy

When scientists send their spacecraft across the universe, they save fuel by performing “slingshot fly-bys”. This is where, rather than firing up the thrusters, the craft changes its trajectory by harnessing the enormous gravitational pull of a planet. However, this trick has had an unexpected side-effect: it seems to produce a change in speed that no one, since it was first discovered in the early 1990's, can account for.

Experts are intrigued by the fact that while the acceleration is tiny and has no significant effect on NASA missions, it holds great interest because no explanation based on conventional physics and understanding has been found. The effect is so persistent that it could indicate some physics not considered in previous attempts to explain the motions of bodies in the universe.

In 1998, for example, NASA's NEAR Shoemaker spacecraft had its speed boosted by an additional 13.5 millimetres per second. There are many examples of this, but no explanation – which raises the tantalizing possibility that it could be a sign that a whole new branch of physics is waiting to be discovered.

Mysteriously, four spacecraft that flew past the Earth have each displayed unexpected anomalies in their motions. These enigmas join the so-called "Pioneer anomaly" as hints that unexplained forces may appear to act on spacecraft. The anomalous acceleration, resulting from a pull about 10 billion times smaller than the acceleration we feel from Earth's gravitational pull, was confirmed through detailed analysis of radio data from Pioneer 10, Pioneer 11 and Ulysses spacecraft as they hurtled out of the solar system, which seemed to experience a tiny but unexplained constant acceleration toward the sun.

Jet Propulsion Laboratory astronomer John Anderson and his colleagues — who originally helped uncover the Pioneer anomaly — have studied five deep-space probes — Galileo to Jupiter, the NEAR mission to the asteroid Eros, the Rosetta probe to a comet, Cassini to Saturn, and the MESSENGER craft to Mercury. Each spacecraft flew past the our planet to either gain or lose orbital energy in their quests to reach their eventual targets. (Galileo made two flybys.) In five of the six flybys, the scientists have confirmed anomalies.

JPL researcher Anderson, first saw the effect in 1980, but waited until he had accumulated data over the next 15 years, before he could easily dismiss it as the result of systematic errors. "Like a lot of problems in astronomy, many years of observation are needed," Anderson said.

Another researcher, Michael Nieto of Los Alamos, has long been interested in the possibility that gravity works differently on antimatter than on the familiar matter that makes up our everyday world. This led him to consider how well we understand gravity's influence on normal matter and whether studies of the motions of comets or spacecraft could be used to identify any deviations from the expected influence of gravity.

"I am feeling both humble and perplexed by this," said Anderson, who is now working as a retiree. "There is something very strange going on with spacecraft motions. We have no convincing explanation for either the Pioneer anomaly or the flyby anomaly."
The one probe that did not confirm an anomaly was  MESSENGER, the spacecraft approached the Earth at about latitude 31 degrees north and receded from the Earth at about latitude 32 degrees south.

The five other flybys involved flights whose incoming and outgoing trajectories were asymmetrical with each other in terms of their orientation with Earth's equator, which "suggests that the anomaly is related to Earth's rotation," Anderson told the Times Online. As to whether these new anomalies are linked with the Pioneer anomaly, "I would be very surprised if we have discovered two independent spacecraft anomalies," Anderson told SPACE.com. "I suspect they are connected, but I really do not know."

"Another thing in common between the Pioneer and these flybys is what you would call an unbound orbit around a central body," Anderson said. "For instance, the Pioneers are flying out of the solar system — they're not bound to their central body, the sun. For the other flybys, the Earth is the central body. These kinds of orbits just don't occur very often in nature — it could be when you get into an unbound orbit around a central body, something goes on that's not in our standard models."

Scientists were hoping to gain more insight into the anomaly when the Rosetta spacecraft swung by Earth on Nov. 13  2009 to pick up a gravitational boost for its journey to rendezvous with a comet in 2014.

However, in a major disappointment – which had deepened the mystery — the Rosetta spacecraft did not experience the flyby anomaly during this swingby of Earth in 2009, even though the same spacecraft did experience the anomaly when it flew by Earth 2005, but didn’t in 2007.

“It’s a mystery as to what is happening with these gravity events,” said Trevor Morley, lead flight dynamics specialist working on Rosetta. “Some studies have looked for answers in new interpretations of current physics. If this proves correct, it would be absolutely ground-breaking news.”

Several ideas have been proposed in an attempt to explain why the anomaly occurs, ranging from tidal effects of the near-Earth environment, atmospheric drag, or the pressure of radiation emitted or reflected by the Earth, to much more extreme possibilities, such as dark matter, dark energy or previously unseen variations in General Relativity.

Anderson and colleagues are even looking at the possibility that Earth’s rotation may be distorting space-time more than expected, and affecting nearby spacecraft. But there is as yet no explanation how this could happen.

Sources:

http://www.space.com/scienceastronomy/080229-spacecraft-anomaly.html

http://www.xs4all.nl/~carlkop/gravnew.html

http://spacefellowship.com/news/art16198/mystery-of-the-flyby-anomaly-endures.html
http://www.dailygalaxy.com/my_weblog/2010/02/the-flyby-mystery-is-there-a-new-physics-waiting-to-be-discovered.html