After an adventurous 7-year long tour among the planets, the
Cassini-Huygens spacecraft will arrive at Saturn in July 2004.
Once there, Cassini will parachute the Huygens probe to Saturn’s
biggest satellite, Titan. Titan is thought to have an atmosphere
similar to the primitive Earth. However, both the probe and the
Cassini-Huygens team are not in idle state until 2004. They have
plenty of things to keep them busy.

For example, scientists are putting the orbiter instruments
on-board to work, and obtaining new and unexpected science about
the planets encountered. For the first time, Cassini-Huygens
has observed Jupiter’s magnetosphere in action. Jean-Pierre
Lebreton, ESA’s Huygens Project Scientist, talks about it in
this interview.

Jean-Pierre Lebreton’s work has been hectic since the Cassini-
Huygens launch in 1997. The craft will reach its final
destination, Saturn and its satellite Titan, the second largest
moon of the solar system (and the only one with a thick, hazy
and chemically reactive atmosphere) in 2004. However, Lebreton
is enthusiastic about its accomplishments to date.

“Dr. Lebreton, you sound excited about what Cassini-Huygens has
already achieved. Saturn and Titan and their secrets are still
far away. But on the way, the spacecraft has encountered other
planets …”

“The path of Cassini-Huygens so far has ‘touched’ three planets:
Venus, the Earth, and Jupiter. The reason for the fly-bys was
to gain the ‘kick’ from the planets’ gravitational field to
reach Saturn. Naturally, we took our chance to examine every
planet and we collected a lot of new scientific data.”

“Let’s begin with Venus, whose gravity pull you have used twice.
You found something striking in the Venusian sky …”

“Absolutely. We measured some electromagnetic characteristics
of the atmosphere and were surprised to find that Venus does
not have lightning! We think this could be due to the height
of the clouds — more than 40 kilometres above ground: that
does not allow cloud-to-earth lightning, and probably
cloud-to-cloud lightning is too weak to be detectable. We
also checked the night-side maps of the surface that NASA’s
Galileo spacecraft made in 1992 since we needed to confirm
the previous data. These kinds of checks are useful when we
activate our instruments, so that they are ready when we
arrive at Saturn.”

“And what about the Earth fly-by? Was that only to exploit the
gravity pull?”

“No, more than that. We have a magnetometer on-board Cassini-
Huygens that measures magnetic fields. We have used it to
measure Earth’s well-known terrestrial magnetic field to test
the instrument’s accuracy. Once we reach Saturn, we will know
that we are measuring Saturn’s less-familiar magnetic field
accurately.”

“In 2001 Cassini encountered the king of planets, Jupiter. And
something quite special happened.”

“Yes, it was amazing. At Jupiter we performed a really
spectacular experiment. Ten years ago, nobody would have bet
that the Galileo probe would be still operative around Jupiter.
Imagine how thrilled we were that the two crafts could combine
their observations at Jupiter! Using the two probes, the Hubble
Space Telescope, the Chandra X-ray Observatory, and radio
telescopes in California, we made a once-in-a-lifetime set of
observations of Jupiter’s surroundings. We detected an aurora
caused by particles streams coming from the solar wind. This
flow affected Jupiter’s magnetosphere, making it contract.
Although we had suspected this might happen, we had never
observed it before. Without using both probes, it would have
been impossible to prove the magnetosphere can be influenced
like that by the solar wind. It showed that testing our
instruments is more than just routine work. It gives you
results that make you sit up and it keeps your team on the
ball. Now we feel much more ready to take on Saturn.”

“What happens during the flight between planets? Is Cassini-
Huygens simply ‘asleep’?”

“No, far from it.We regularly activate Huygens to check it.
This mission is more complex than any previous mission, so we
need to plan a lot before arrival. In these years, we have
worked very hard to plan the details of the mission, and we
are very close to finishing that. Among the other things,
the checks luckily allowed us to anticipate a potential
telecommunications problem between the probe and the orbiter,
so we could prevent it.”

“You say this mission is very complex. What are the critical
moments? And what is there still to do before arrival?”

“Well, the most critical moments will be when we fire the
engines to intersect Saturn’s orbit. The success of the mission
relies on the outcome of this vital moment and we have to make
sure everything runs smoothly. Before that, in November 2002,
we will do another testing exercise. Early in 2003 we will also
upload new software on the Huygens computers and that requires
testing again to check that it works well. There are always
many things left to do. It’s not like we can take a rest here!”

Project scientist for ESA’s Hugyens Probe, Jean-Pierre
Lebreton’s particular speciality is planetary science and
studying plasma physics. He is also in the team working for
the Rosetta Orbiter, and, in particular, he is involved with
the Plasma Consortium Experiment. He led the divisional
activities on the Tethered Satellite System and is working for
the Venus Express mission. His fields of interests are diverse,
ranging from listening to meteors to observing distant planets.

USEFUL LINKS FOR THIS STORY

* More about Huygens
http://sci.esa.int/huygens/

* More about Cassini-Huygens from NASA
http://saturn.jpl.nasa.gov/cassini/index.shtml

IMGAE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=30362&
ooid=28077]
Jean-Pierre Lebreton, Huygens Project Scientist

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=12&cid=12&oid=30362&
ooid=28545]
This artist’s concept of the Cassini-Huygens orbiter shows the
Huygens probe separating to enter Titan’s atmosphere. After
separation, the probe drifts for about three weeks until
reaching its destination, Titan. Equipped with a variety of
scientific sensors, the Huygens probe will spend 2 – 2.5 hours
descending through Titan’s dense, murky atmosphere of nitrogen
and carbon-based molecules, beaming its findings to the distant
Cassini orbiter overhead. The probe could continue to relay
information for up to 30 minutes after it lands on Titan’s
frigid surface, after which the orbiter passes beneath the
horizon as seen from the probe.