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.
Last week, after analysing the radiometric data gathered by ESA and NASA ground stations that tracked Rosetta during her successful 13 November swingby, ESA's Trevor Morley, the lead flight dynamics specialist on Rosetta, sent us a brief mail update. His report?
"For Rosetta's third and final Earth swingby, there was no anomaly."
(Access more details under 'Full story' below) -- Daniel
First, some background details taken from our earlier web report (see full report in the ESA website here):
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.
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.
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.
ESA/NASA Cassini-Huygens swings by Earth 1999, slowing unexpectedly
"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.
Now, here is the full text of Trevor's email report from last Friday:
For the Earth swingbys where the anomaly has been detected, the main manifestation has been the inability to get anything like a reasonable fit of an orbit to an arc of radiometric data that encompasses both the pre- and post-perigee intervals. This is not the case for Rosetta's Earth swingby one week ago. For those cases when an anomaly has been seen, the determination of the outgoing asymptotic velocity has been very slightly different from the expected value based on processing the pre-perigee data.
In every case, a reasonable data fit could be established only by inserting an artificial velocity change along the direction of the orbital velocity in the vicinity of perigee. In the case of Rosetta's first Earth swingby, this artificial delta-V had a magnitude of 0.67 mm/s, equivalent to an increase in the asymptotic velocity of 1.82 mm/s.
For last week's swingby, one of the additional orbit determination runs made allowed the software to estimate an impulsive manoeuvre at perigee, aligned along the orbital velocity. The <i>a priori</i> value for the manoeuvre magnitude was zero. The resulting estimate was a magnitude of 4 microns/s (retrograde) with a 1-sigma uncertainty of 44 microns/s. The difference in the quality of the data fit was absolutely negligible. For Rosetta's third and final Earth swingby, there was no anomaly.
That's it! The mystery continues, arguably deeper now that we've seen another swingby without an unexplained increase or decrease in velocity.
Anyone have any ideas?