Thanks to ion propulsion, the Artemis mission is turning near-defeat into
a success story. Nominal operations could start this summer, with ESA’s
satellite, manufactured by Alenia Spazio as prime contractor (I), playing
a significant role in the pursuit of high technology and advanced
telecommunications.

On 12 July 2001, 30 minutes after lift-off from Europe’s spaceport in
Kourou, French Guiana, it became apparent that the Ariane 5 launcher had
propelled the Artemis satellite into a transfer orbit that was lower than
expected, with the apogee (the most distant point from Earth) at only 17
000 km rather than the nominal 36 000 km.

Under normal circumstances a conventional satellite does not carry enough
fuel to compensate for a shortfall in launcher performance of this
magnitude. But with an innovative application of ion propulsion (*), a
system 10 times more efficient than conventional chemical propulsion,
recovery appeared possible. Embedded in a novel and remarkably flexible
system design, used with outstanding ingenuity, team spirit and
operational skills, Artemis’ propulsion capabilities proved to be the key
to the rescue of an otherwise lost mission.

The initial raising of the orbit, by means of the limited chemical
propulsion available, was carried out in less than 10 days by a team of
Alenia Spazio, Astrium and Telespazio experts supported by ESA engineers
(see ESA press releases 43 and 44/2001). This speed of response was vital
to prevent substantial spacecraft degradation by prolonged exposure to the
radiation levels of the Van Allen belt. The duration of the firings was
calculated to reach a safe parking orbit while retaining some 70 kg of
fuel in the tanks, in order to eventually sustain a nominal on-station
lifetime of 5 to 7 years.

All operations turned out to be extremely successful: with 5 perigee
firings the apogee was lifted to 31 000 km. Subsequently, with 3 nominal
apogee firings, Artemis was able to reach a circular parking orbit at 31
000 km. Considering the exceptional conditions encountered and due to the
perfect performances provided by the spacecraft, both perigee and apogee
firings were performed with high efficiency, consuming about 95% of the
chemical propellant on board.

Preparations for the remaining orbit-raising manoeuvre, using the ion
engines, required a considerable amount of work, mainly due to the need to
shift the spacecraft from its nominal Earth-pointing to an in-flight
pointing orientation. New onboard flight control laws had to be defined
and corresponding new software written, tested and implemented under
pressure of time. New operational procedures had to be established and
hardware configured in a way for which it was not initially conceived.
All this turned out to be a very challenging task, never attempted before
by Europe on a communication spacecraft.

In January 2002 all new software modules were completed and fully tested
by Alenia Spazio and Astrium . Today, Artemis starts spiralling out of its
safe parking orbit to bridge the gap of some 5000 km at a rate of roughly
1 km per hour.

Whereas the initial part of the orbit-raising process, using chemical
thrusters, was completed within a few days, the remaining part is expected
to last more than 200 days, requiring two ion engines to fire almost
continuously. That is because the thrust of these engines is very weak
indeed (15 millinewtons). Their task can be compared to driving an ocean
liner with an outboard motor. Artemis is expected to arrive this summer
at its nominal altitude of 36 000 km.

In parallel with orbit-raising preparations, in-orbit verification of the
communication payloads was performed. The most spectacular events were
communication tests with the CNES (French Space Agency) SPOT-4 Earth
observation satellite (see ESA press release 75/2001), during which image
data from SPOT 4 were transmitted by laser light to Artemis and from there
by radio waves to the Spot Image processing centre in Toulouse. All tests
confirmed that Artemis’ payloads are healthy and ready to support the
technological and operational communication programme.

Now the spacecraft is on course for its nominal slot in geostationary
orbit. An all but lost mission is on its way to full recovery since
Artemis will eventually be able to serve its users from its geostationary
position for at least 5 years of nominal operation.

(*) The principle of any kind of thruster in space is to accelerate
molecules and expel them from the satellite at the highest possible speed.
Conventional thrusters use a chemical reaction between fuel and oxidiser
to heat a gas and eject the molecules at a speed of typically 1 km/sec.
Electrical propellant systems first ionise (i.e. electrically charge) the
molecules of a gas (xenon, for instance). The ionised gas is then
accelerated by electrical fields and ejected from the satellite at a speed
of typically 10 m/sec.

For further information, please contact:

Gotthard Oppenhauser

Artemis Project Manager, ESA/Noordwijk (ESTEC)

Tel: +31(0)71.565.3168

Franco Bonacina

ESA Media Relations Office

Tel:+33(0)1.53.69.7155

Fax:+33(0)1.53.69.7690