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Mission
control team readies for Venus arrival
27
March 2006
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Artist's
impression of the ESA spacecraft Venus Express in orbit
around Venus, launch date 26 October 2005.
Credits: ESA
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Venus Express mission
controllers at ESA's Space Operations Centre (ESOC) are in
intensive preparation for an 11 April arrival at the spacecraft's
namesake destination.
The critical manoeuvre will
involve a deft combination of basic physics, expert spacecraft
engineering and precise timing.
Next month's Venus Orbit
Insertion (VOI) will mark the arrival of the first ESA mission at
Venus, one of the Solar System's most enigmatic planets.
Orbit
insertion comprises a series of telecommands, engine burns and
manoeuvres designed to slow the spacecraft from a velocity of 29
000 km per hour relative to Venus just before the first burn to
an entry velocity some 15 percent slower, allowing it to be
captured into orbit around the planet.
Pointing
engine for braking
Controllers will
command the spacecraft to 'slew' (rotate) so as to point the
engine nozzle in the direction of motion starting at 08:03 (all
times CEST on Earth) 11 April. Venus Express will perform an
approximately 51-minute main engine burn starting at 09:19.
The spacecraft's solar arrays
will also be positioned so as to reduce the possibility of
excessive mechanical load during engine ignition.
Over the subsequent days, a
series of additional burns will be done to lower the orbit
apocentre (point furthest from the planet) and to control the
pericentre (point closest to the planet). The aim is to end up in
a 24-hour orbit around the 'hothouse' planet early in May.
Critical
manoeuvres require precise timing
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VEX mission
control team in MCR
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All steps must take place
in the correct sequence and the spacecraft must be brought into
the correct configuration in time for the main engine burn, which
itself can only happen at a specific moment. The risk is that, if
any problems occur, the spacecraft could miss its 'window' for
capture, making any recovery extremely challenging.
During the engine burn, the
spacecraft will also enter an occultation, which occurs when
Venus Express travels behind the planet so that the line of site
to Earth is blocked; it will lose radio contact for almost 10
minutes. Controllers will closely watch for reacquisition of
radio contact once the occultation ends at 09:56.
"Venus orbit insertion is
a complex step. The main challenge is that the manoeuvre must
happen at the right time," says Jean-Baptiste Gratadour,
Attitude and Orbital Control Systems Engineer for Venus Express
at ESOC and one of the dozens of engineers and scientists now
readying for arrival at Venus.
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An artist's
impression of the first firing test of the Venus Express
spacecraft main engine, being performed in space during the
night of 16/17 February 2006. The burn started at 01:27 CET
and lasted about three seconds.
Credits: ESA
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Venus Express in great
shape
To prepare for orbit insertion,
the spacecraft has had to pass a series of important tests and
milestones.
"In the night between 16
and 17 February, Venus Express passed its VOI readiness review
after a successful burn of its main engine, providing a thrust of
400 Newtons," said Don McCoy, Venus Express Project Manager.
"This was followed by a minor mid-course correction provided
by the spacecraft's four 10-Newton thrusters on 24 February."
A 400-Newton engine generates
625 kilowatts of power, making a typical automotive engine seem
feeble in comparison.
NASA
to provide direct support
Recent
rehearsals have included establishing the communication link
between the Venus Express control room at ESOC and the 70-metre
deep-space antenna at Madrid, part of NASA's Deep Space Network
(DSN).
This ground station will
support VOI due to its geographic location. For routine
operations, Venus Express communicates using X-band via ESA's new
35-metre deep-space antenna at Cebreros, near Madrid (Spain).
The link was tested by
broadcasting a live signal from the Venus Express low-gain
antenna to Madrid and then into ESOC.
This
test was fundamental because, during orbit insertion, the
low-gain antenna will be used to track the spacecraft's velocity.
The Venus Express high-gain antenna, normally used to communicate
with Earth, will in fact be rotated away and out of line of sight
of ground stations during the operation.
"We are also conducting
continuous tracking of the spacecraft position through different
techniques and using several ground stations, including ESA's
Cebreros station and NASA Deep Space Network stations at
Goldstone (USA) and Canberra (Australia) and Madrid," says
Andrea Accomazzo, Venus Express Spacecraft Operations Manager.
"All our efforts are in fact now concentrated on the
spacecraft navigation to prepare for the big day of arrival at
Venus," he added.
During orbit insertion, the
spacecraft will be 125 000 000 kilometres from Earth and the
round-trip signal time will be 13 minutes and 32 seconds.
Source
/ Credit: ESA
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