Why a probe passing our planet on its way to Jupiter might end a decades-old mystery, and reveal something completely new about gravity.
You may not see it, but sometime today, a spacecraft whizzing 350 miles (560km) above Earth is going to use our planet to slingshot its way towards another one.
In the process, a group of scientists are hoping the craft will help reveal a mystery that has perplexed them for decades.
When Nasa's Juno spacecraft arrives at Jupiter three years from now, it will investigate the planet's origin and evolution by mapping magnetic and gravitational fields.
But before it does that, the probe has an opportunity to contribute to a momentous discovery right here in our celestial backyard, and potentially pave the way for the discovery of entirely new physics.
Spacecraft like Juno routinely use flybys of Earth to gain speed and slingshot themselves towards the outer solar system.
Occasionally, however, passing spacecraft seem to get an extra boost, picking up more speed than calculations suggest they should.
The energy kick is known as the flyby anomaly, and this has happened too many times now to be shrugged off as trivial.
Instead, something unidentified is affecting the velocity of spacecraft at their closest approach to our planet.
When Juno swings by Earth, all eyes will be on the spacecraft to see if it can help figure out what is going on.
The most likely explanation is that it is a miscalculation in the tracking software, but there is a tantalising possibility that navigators are seeing unanticipated physics beyond our present theories of gravity.
If so, it would open up a whole new landscape of understanding about the fundamental nature of the universe.
"If we were to establish that the Earth flyby anomaly really resulted from new physics, that would be of major significance," says Chuck Scott, Juno mission manager at Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California.
It's anyone's guess what any new discovery of this type could yield, but previous breakthroughs in gravitational physics have revolutionised our way of thinking.
In the 17th Century the work of Isaac Newton helped trigger the Age of Enlightenment.
In the20th Century, Albert Einstein's General Relativity led to our discovery of black holes and the big bang.
It also made more down-to-Earth applications possible, such as satellite navigation.
However, Scott is cautious about such heady speculation.
"Extraordinary claims require extraordinary evidence," he says. "Most scientists expect that it will turn out to be a software, or some other minor, miscalculation, rather than new physics."
But only by finding the cause of the surge will we know. Which is why researchers will be examining Juno in as much detail as possible to see what's going on.
Speed mystery
The flyby anomaly was first noticed in 1990, during a close pass by another Jupiter-bound spaceprobe.
When Nasa's Galileo craft shot less than 620 miles (1,000km) above the surface of the Earth, navigators noticed a small discrepancy in the radio signal as it headed off.
It could be understood only if the spacecraft had picked up about 4 millimetres per second more speed than they were expecting.
It may sound minuscule, especially as the spacecraft was travelling at 8.5 miles per second (13.74kps).
But the extra speed was clearly noticeable and defies explanation.
The team examined the navigation software used by Nasa and the basic mathematical models that the flight teams relied on to calculate where their spacecraft were heading.
Yet everything seemed fine.
There was nothing that could account for the extra speed.
So the team hunkered down, waited and prepared.
Galileo would be back in 1992 for a second pass of Earth, and it would be even closer this time.
They made special arrangements with Nasa's Tracking and Data Relay Satellite System to pay particular attention around the time of closest approach.
But it was all in vain.
At its flyby altitude of just 185 miles (300km), the probe encountered the last wisps of Earth's atmosphere, which was enough to slow it down and mask any unexpected increase in velocity.
Source: BBC
