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The descent of the Huygens probe on Titan
Imagine a golf player trying to work out the best way to swing a standard golf ball over a fairway of
several million kilometres in size – about ten times the distance between the Earth and Moon. Is this
kind of geometrical proportion of any practical use? Not in golf, of course, but it is what it takes to
navigate interplanetary probes to the new worlds in the Solar System.
On 14 January 2005, a new chapter in the exploration of our Solar System began when the European
planetary probe Huygens reached its destination – a mysterious Saturn moon called Titan. This
achievement, part of the joint NASA-ESA-ASI1 Cassini-Huygens mission, is arguably the most
sophisticated interplanetary endeavour to date. The world-wide team of radio astronomers organised
by the Joint Institute for Very long Baseline interferometry (VLBI) in Europe (JIVE), one of the
RadioNet partners (see project sheet n°4), provided crucial support to this mission by achieving
breathtaking accuracy in measuring the velocity and position of the Huygens probe during its plunge
into Titan’s atmosphere and after reaching the planet’s surface.
Radio astronomers’ encounter with the new world
Conceived originally as a support to one of the Huygens experiments, the Doppler Wind Experiment
(DWE), the radio astronomy segment of the mission completed three mission tasks:
Firstly, the leading radio telescope of the Huygens VLBI tracking network – the R. C. Byrd Green
Bank Telescope of the National Radio Astronomy Observatory equipped with a NASA-supplied Radio
Science Receiver – provided the first confirmation that the Huygens spacecraft was intact after
reaching Titan’s atmosphere, some six hours before deep-space network facilities began receiving
data from the Huygens signal relayed via the Cassini spacecraft. This confirmation was an invaluable
and independent verification of the overall healthy status of the Huygens probe.
Secondly, direct measurements of the parameters of the Huygens signal by Earth-based radio
telescopes helped the scientists achieve a major mission objective – investigating the wind in Titan’s
atmosphere. After the unfortunate failure of one of two communication channels between the
Huygens and Cassini spacecraft, it was up to VLBI tracking to provide the data.
Thirdly, according to the original mission scenario, Huygens was communicating to the Cassini
spacecraft during the first 3½ hours in the atmosphere of Titan. However, as confirmed by Earthbased radio telescopes, in particular, the venerable CSIRO Parkes radio telescope of the Australia
Telescope National Facility (also a RadioNet partner), the Huygens spacecraft continued to transmit
signal from Titan’s surface after completion of the data link with Cassini. The data stored on VLBI
recorders is the only source of unique information collected over the final nearly two hours of the
mission on Titan’s surface.
But what is VLBI-Very long Baseline Interferometry?
VLBI combines, in a single scientific tool, radio telescopes located thousands of kilometres apart, and
often on different continents. The telescopes receive radio signals from a celestial source, mix them
with time marks and highly stable oscillations from a local frequency standard, and record the
resulting data on a reliable storage device, such as a magnetic disk. These disks are then transported
to a data processing centre, where they are ‘played back’ and processed by a special computer,
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National Aeronautics and Space Administration (NASA), European Space Agency (ESA), Agenzia
Spaziale Italiana (ASI)
called a ‘correlator’. The procedure mimics the physical process of building images by a regular
optical system, such as a mirror, lens or even the human eye.
Following this optical analogy, scientists can easily estimate the angular resolution (or the
‘sharpness’) of the interferometer: it should be roughly defined by the ratio of the wavelength to the
distance between the telescopes (baseline). At radio wavelengths of, say, several centimetres, and
baselines of several thousand kilometres, the interferometers angular resolution will be about 3.6
million times smaller than a one degree angle. No other astronomical technique – or indeed any other
available technology – can offer such sharpness of sight.
With this information in hand, the researchers point the telescopes, combined in a VLBI system, at
the planetary probe emitting a radio signal. In principle, leaving aside some manageable technical
complications, the VLBI system should be able to measure the position of the probe’s radio
transmitter with equally high accuracy. Moreover, if the interferometer’s response has sufficiently high
signal-to-noise ratio, the angular position accuracy can be improved by more than an order of
magnitude. At a distance of 1 billion kilometres, the angular scale corresponds to 1 kilometre in linear
measure – so powerful that it would be sharp enough to watch a table tennis game played on the
surface of Moon by spectators sitting next to the reader of this sentence right now!
But how could the scientists pinpoint the position of the Huygens probe – which at the time of the
Titan encounter was about 1.2 billion kilometres away from Earth – within kilometre accuracy? The
task was made more difficult by Huygens’ weak signal (its transmitter emits about as much power as
a standard mobile handset). The Huygens probe’s radio system was designed to communicate to the
relatively nearby Cassini spacecraft over a distance of about 100 000km. For the Earth-based radio
telescopes, ‘eavesdropping’ on the radio conversation between Huygens and Cassini meant dealing
with a signal a hundred-million times weaker.
To handle such a weak signal, the team used a modification of the VLBI technique which makes it
possible to detect very weak signals from a target source (the spacecraft transmitter on board
Huygens in this case) and measure its position with respect to background celestial reference
sources. To complicate the situation further, Huygens was to enter the Titan atmosphere at the time
when Saturn culminates over the eastern Pacific, and thus was observable only using radio
telescopes in the United States (except eastern-most areas), Australia and Eastern Asia (China,
Japan), a combinations of radiotelescopes which is not normally used for astronomical observations.
The radio astronomers, however, won the day in the end.
On the mission day back in January, 17 radio telescopes from the USA, Australia, China and Japan
then carried out a once-in-a-life observation of the signal from the Huygens probe. Important data
translation tasks were also performed by the Metsähovi Radio Astronomy Observatory of the Helsinki
University of Technology (FI). Two additional radio telescopes, in Wettzell (DE) and Onsala (SE),
were recruited on very short notice to try to record the Huygens signal several hours after the
projected completion of the mission. All VLBI data recordings (total of 27 Terabytes, an equivalent of
about 6000 90-minute-long movies on DVD) came to the European VLBI Data Processor at JIVE for
correlation and post-correlation data processing.
Short sections of VLBI data acquisition were quickly analysed at JIVE immediately after the mission.
The high-quality findings were delivered to the Huygens Mission Control within hours of the
observation, as solid proof that the scientific goals of the mission had been achieved and even
exceeded.
What’s next?
At the time of writing, massive data processing of Huygens VLBI observations continues. Early results
obtained to date indicate:
•
Independent confirmation of radial velocity measurements conducted at the Green Bank and
Parkes radio telescopes by the NASA JPL team. The data was used by the Doppler Wind
Experiment team to reconstruct the wind profile. The winds on Titan flow in the same direction
as the planet’s rotation at nearly all altitudes, the scientists concluded. There are large
unexplained wind speed variations at altitudes between 60 and 100km. Because of
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•
•
remarkable similarities to some landscapes on Earth, these results can help planetologists
understand the role of atmosphere dynamics on the planet’s surface.
Highly accurate measurements of the velocity oscillations of the probe under the parachute
believed to be related to its pendulum and/or spin motion.
There is now every indication that the ultimate km-scale accuracy of determination of the
probe’s position will be achieved.
The Huygens mission is a historic scientific milestone, opening up new horizons in the exploration of
the Solar System. This exploration required ultra-precise navigation. The VLBI technique was up to
the challenge. It is not unthinkable to claim that, in the coming decade, VLBI navigation support to
interplanetary missions anywhere in the Solar System will be provided with GPS accuracy, just like
onboard trip navigators in high-end cars.
Partners
The Joint Institute for VLBI in Europe is funded by the national research councils, national facilities
and institutes of the Netherlands (NWO and ASTRON), the United Kingdom (PPARC), Italy (CNR),
Sweden (Onsala Space Observatory, National Facility), Spain (IGN) and Germany (MPIfR). The
European VLBI Network is a joint facility of European, Chinese, South African and other radio
astronomy institutes funded by their national research councils. The National Radio Astronomy
Observatory is operated by Associated Universities, Inc., under a co-operative agreement with the
National Science Foundation. The Australia Telescope is funded by the Commonwealth of Australia
for operation as a National Facility managed by CSIRO.
Contact
Leonid Gurvits, Joint Institute
E-mail: [email protected]
Project website: http://www.jive.nl/
for
VLBI
in
Europe,
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Dwingeloo,
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Netherlands