When Europe's comet chaser Rosetta swings by Earth tomorrow for
a critical gravity assist, tracking data will be collected to
precisely measure the satellite's change in orbital energy. The
results could help unravel a cosmic mystery that has stumped
scientists for two decades.
Since 1990, scientists and mission controllers at ESA and NASA
have noticed that their spacecraft sometimes experience a strange
variation in the amount of orbital energy they exchange with Earth
during planetary swingbys. The unexplained variation is noticed as
a tiny difference in speed gained or lost during the swingby when
comparing that predicted by fundamental physics and that actually
measured after the event.
Tiny unexplained speed variations
The unexplained speed variations are extremely small: NASA's
Galileo satellite experienced an increase of just 3.9 mm/s above
the expected value when it swung past Earth in December 1990. The
largest unexpected variation - a boost of 13.0 mm/s - was observed
with NASA's NEAR spacecraft at its Earth swingby in January 1998.
On the other hand, variations seen at the swingbys of NASA's
Cassini in 1999 and Messenger in 2005 were so small that they lay
within the bounds of uncertainty.
ESA's Rosetta has made two Earth swingbys, in 2005 and 2007. It
too, experienced the strange anomaly. Frustratingly, Rosetta sped
up by an unexplained 1.8 mm/s during the first passage, but
experienced no slowing or speeding in 2007. No one knows what will
happen on 13 November 2009 for Rosetta's third and last Earth
swingby: scientists are stumped.
"It's a mystery as to what is happening with these gravity
events. Some studies have looked for answers in new interpretations
of current physics. If this proves correct, it would be absolutely
ground-breaking news," says Trevor Morley, lead flight dynamics
specialist working on Rosetta at ESOC, ESA's European Space
Operations Centre, Darmstadt, Germany.
ESA researchers study Rosetta
Together with ESA colleague and orbital mechanics specialist
Frank Budnik, Morley co-authored a scientific report in 2006 that
studied the Rosetta anomaly during the 2005 swingby and listed
possible causes.
These range from tidal effects peculiar to 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, one of the most fundamental and well-tested
theories of modern physics.
One American research team, led by ex-NASA scientist John
Anderson, is even looking at the possibility that Earth's rotation
may be distorting space-time - the fundamental fabric of our
Universe - more than expected, thus affecting nearby spacecraft.
But there is as yet no explanation how this could happen.
Before even considering such exotic explanations, all the usual
causes of spacecraft speed errors have been thoroughly eliminated
by numerous investigations conducted over the years at both ESA and
NASA. Software bugs, calculation errors, tracking uncertainties and
other, much more mundane, causes have all been systematically
eliminated or accounted for, leaving the speed anomaly maddeningly
unexplained.
NASA's Pionneer 10 & 11 similarly affected
Scientists at a number of universities and research centres in
Europe, the US and Japan have worked on the anomaly problem over
the past years. The Earth swingby anomaly has been compared to
another unexplained anomaly - one experienced by NASA's Pioneer 10
and 11 spacecraft.
As they travel on trajectories that will take them eventually
into interstellar space, both have experienced an unexpected
acceleration directed toward the Sun, which has yet to be
explained.
Watching through the night
At ESOC on 13 November, the mission control and flight dynamics
teams will be watching closely as ESA's 35m New Norcia station in
Australia tracks Rosetta during the closest approach, expected at
08:45:40.0 CET, followed - after a visibility gap of 20 minutes -
by ESA's 15m Maspalomas station.
The tracking activity will generate highly precise data that
will record whether the spacecraft speeds up or slows down more or
less than expected. Deep space ground stations operated by NASA, at
Canberra, Australia, and Goldstone, California, will also observe
the spacecraft before and after closest approach.
"We are using as many ground stations as are available to
maximise the amount of swingby data we record. The more data we
get, the better the chance that we may eventually come up with an
answer," says Morley.
The data is keenly awaited by scientists on a number of
continents, who are hoping to see whether the anomaly is present
and whether an explanation is finally possible.
"As it stands now, no one knows what's behind this - it really
is a mystery. And your prediction as to whether Rosetta will
experience any swingby speed anomaly at all on 13 November is as
good as anyone's," says Morley.
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