For the longest time, spacecraft operators were puzzled by the Pioneer anomaly, the strange
thrust that was slowing spacecraft down as they journeyed through the Solar System.
That anomaly was finally solved just a few years ago, but there’s still another glitch
in our understanding of spaceflight, and it happens every time a spacecraft does a slingshot
around a planet to get a speed boost or course correction.
It’s called the flyby anomaly.
Before we get into the flyby anomaly, let’s talk about the Pioneer anomaly.
You might have heard of it, but not realized that it was recently solved.
The Pioneer anomaly was first discovered after NASA’s twin Pioneer spacecraft had reached
20 astronomical units from the Sun, about the distance to Uranus.
When controllers on Earth communicate with the spacecraft, they know precisely how quickly
they’re moving thanks to the Doppler effect.
Even a slight difference in speed, like even a few millimeters per second, is detectable.
They know exactly how fast they’re going.
Which makes it surprising that some mysterious force was decelerating the spacecraft.
They weren’t as far out into the Solar System as the laws of physics would predict.
Researchers first noticed the anomaly in 1980, but didn’t start looking into it seriously
until 1994.
Scientists proposed several ideas to explain the anomaly, including dark objects in the
Solar System that might be tugging on the probes.
Of course, if that was the case they would be detected by their interaction with other
asteroids and Kuiper Belt Objects.
Or perhaps the spacecraft were leaking gas unexpectedly, although, it was strange for
them to be having the same deceleration.
Of course, some researchers wondered if they were seeing entirely new physics, like a fundamental
misunderstanding of how gravity works in the Solar System, or changing definitions of time.
It got pretty weird for a while there.
Researchers came together to form the Pioneer Explorer Collaboration, and met several times
to try and get to the bottom of it.
In the end, the answer turned out to be heat.
In 2012, Slava Turyshev from NASA’s Jet Propulsion System produced a detailed study
that showed how electrical current flowing through the spacecraft generated heat.
Because the spacecraft were both pointing their main antennae back at Earth, they were
experiencing a deceleration in the same direction.
Just photons emanating from the Pioneer spacecraft could explain the deceleration.
It’s like walking at night with a flashlight, and getting slowed down by the force of the
photons pushing you backwards.
Very subtle and a fascinating discovery.
Got that?
Okay great.
Well, the flyby anomaly on the other hand, is totally unsolved.
But researchers have some ideas, and again, some of them are pretty strange.
Researchers first realized there was something going on with NASA’s Galileo mission.
This Jupiter-bound mission was carried into space on board the space shuttle Atlantis
in 1989.
This was shortly after the Challenger disaster, and for safety reasons, NASA decided to change
the upper stage that would boost the spacecraft out to Jupiter.
It used a lower powered booster known as the Inertial Upper Stage which would take it on
a series of flybys to get the trajectory it needed to reach Jupiter.
When Galileo did a slingshot past Earth in 1990, researchers detected that the spacecraft’s
velocity had increased by 4 mm/s.
Now, that doesn’t sound like much, but it’s enough that it was detectable.
Here’s where it gets weirder.
On its next flyby of Earth in 1992, no anomaly was detected.
Same spacecraft, same planet, no anomaly.
When NASA’s Near Earth Asteroid Rendezvous spacecraft flew past Earth in 1998, its velocity
changed by a much larger 13 mm/s.
Cassini gained a fraction of a millimeter per second, and Rosetta gained almost 2 mm/s.
Oh, and NASA’s MESSENGER spacecraft had no anomalies during its flyby.
And guess what, it happened again with NASA’s Juno spacecraft on its way out to Jupiter.
In a paper published in November 2017, a group of Spanish researchers calculated that Juno
is off its expected velocity by about 1 mm/s depending on the point of its orbit around
Jupiter.
So, the flyby anomaly comes and goes, without rhyme or reason, and this is really puzzling
to astronomers.
And when astronomers get this puzzled, the exotic theories come out to play.
And we’ll talk about what could be causing the flyby in a second, but first I’d like
to thank:
Howard Amos Bjørn Karlsen
Epyx911
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All right, let’s get into some of the ideas that could be causing the flyby anomaly.
The first idea comes from the fact that the Earth isn’t a perfect sphere, it’s actually
an oblate spheroid, a squashed sphere, flattened by its rapid rotation.
Furthermore, the Earth has different densities, mountains, oceans, ice caps, etc.
As the spacecraft fly past the Earth, they could be experiencing a different amount of
gravity depending on exactly which features they’re flying past.
The problem is that there are spacecraft orbiting Earth all the time, and you would expect them
to be experiencing the flyby anomaly as well.
Perhaps it’s the configuration of the Moon or other planets in the Solar System during
the flyby, or the positions of the ocean and the state of the tides during the exact moment
of flyby.
Although the Earth’s atmosphere is tenuous at the flyby altitude, there could still be
enough atmospheric particles to affect the trajectory.
Maybe the Sun is causing it, hitting the spacecraft with different levels of solar winds during
each flyby.
Or even just different levels of photons.
Perhaps photons of heat coming from Earth, are bouncing off the spacecraft and giving
it a slight course correction.
Some even more extreme ideas think that it could come down to relativistic effects.
Maybe the rotation of the Earth is causing a distortion of the spacetime around it that
changes the trajectory.
In order to study the predictions of relativity made by Einstein, scientists are proposing
the launch of a new spacecraft called STEP, or the Satellite Test of the Equivalence Principle.
STEP was selected a mission to study by the European Space Agency back in 2012, and if
finally selected would fly to space by 2024.
It’s only job would be to study gravity.
The most sensitive measurement of gravity ever made.
The spacecraft would orbit the Earth with an incredibly accurate clock on board.
Shielded from the Universe’s radiation it would be free to watch gravity.
It could be able to sense both the gravity from the Earth and the gravity of the Sun
to a sensitivity of 1 x 10^-7.
It would also be able to test a prediction made by Einstein called the weak equivalence
principle.
He said that any local non-gravitational experiment in a freely falling laboratory is independent
of the velocity of the laboratory and its location in spacetime.
STEP will be the most freely falling laboratory every built, and should be able to test this
equivalence to an accuracy of 10^-15.
It’ll follow an orbit that carries it through the various flyby distances that spacecraft
experienced the anomaly.
If the cause is gravity, STEP should be able to show exactly how the magnitude of the effect
changes over distance from the Earth.
So, STEP will be a boon to the study of gravity, but as a side effect, it should be able to
figure out what’s going on with the flyby anomaly.
Let’s hope it eventually launches.
ESA’s BepiColumbo spacecraft will be launching for Mercury in 2018, and will do a flyby of
Earth in 2020.
This will be another chance to study the flyby in detail.
This time, astronomers will be ready and making very careful observations.
Perhaps they’ll finally figure out what’s causing the flyby anomaly, and maybe the mystery
will continue on for decades to come.
Well, have you got any ideas?
This is still an unsolved problem, so go ahead and suggest your out-of-the-box ideas.
Let me know your thoughts in the comments.
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