Venus Express Options

[JRehling]

When I worked on Magellan mapping Venus (or in my case, looking at radar telemetry while smart people mapped Venus), it was great fun. I hear that Magellan mapped about 73% of the surface.

From the Magellan page on Wikipedia: <<By the end of its first such eight-month orbital cycle between September 1990 and May 1991, Magellan had sent to Earth detailed images of 84 percent of Venus' surface. The spacecraft then conducted radar mapping on two more eight-month cycles from May 1991 to September 1992. This allowed it to capture detailed maps of 98 percent of the planet's surface.>>

I think your recollection of the low-ish figure around 73% may be based on one of the three distinct imaging campaigns covering (only) that much of the planet. Magellan had unexpected operational issues that cost some of the expected coverage. Eventually those issues were worked out and the orbiter could have been kept running to finish the job had funding been available. As it happens, there was decent "fill" data for the 2% Magellan didn't cover, so no one's lost much sleep over it. The next radar mission to Venus will be about higher resolution coverage of what Magellan did map.

[cndwrld]

VEX Mission Status

Quadrature is the period when the Earth-Venus-Sun angle is less than 90 degrees, which includes inferior conjunction. At the start and end of quadrature, we have to undertake special operations in order to avoid Sun exposure into the VMC instrument which has no shutter. On 16 June, the spacecraft's 10° +Y tilt was removed at the end of the first quadrature transition period.

Removal of the 10° +Y tilt was removed by execution of previously
uploaded commands, just prior to start of the Earth communications pass.
It was done this way so that the spacecraft orientation could be checked
immediately upon the start of communications with Earth, and time for the
science downlink would not be used to observe the orientation change.


Routine observations took place during this reporting period, and
occurred as planned. A full VIRTIS software upload was carried out
including the related testing on DOY 165 and 166.

On DOY 167, the telemetry bitrate changed to 45 kbps.

At the end of DOY 167, Venus Express was orbiting Venus at 97 million km
from the Earth. The one-way signal travel time was 323 seconds.

Payload Activities have been routine. Two VeRA radio science occultation
observations were performed using the ESA New Norcia antenna in Australia
on DOY 164, 166, 168, 170 and 173. For these observations, a highly stable
local oscillator is used to generate the downlink signal to Earth, a ground
antenna locks on the signal, and the signal is measured as the spacecraft
is occulted behind the planet. This is repeated when the spacecraft leaves
occultation and emerges from behind the planet. The spacecraft is re-oriented
during the occulation to account for the diffraction of the atmosphere, to
maintain the link withe the ground station. The changes in the signal are
used to determine properties of a deep slice of the atmosphere. Since this
was in the quadrature period, the smaller high gain antenna was used, greatly
limiting the signal strength.

A VeRA Bistatic Radar observation was also performed, using the 70m Canberra
DSN station on DOY 163. In these experiments, the stable oscillator is used
for the downlink to the big high gain antenna, and the antenna is pointed at
a ground feature and tracks it. The reflected signal is detected by the Earth-
based antenna, which is why the large 70 meter DSN antennas must be used.

For the VIRTIS imaging spectrometer, a full instrument operating software
upload and related testing were carried out on DOY 165 and 166.

On DOY 170, a new on-board control procedure (OBCP) was uploaded for
SPICAV shutter operation. The OBCP is expected to automate the operations of
the SPICAV shutter. The shutter is closed when the instrument is not in use,
because exposure to direct sunlight would damage the instrument optics.
The commands have been inserted into the normal command uploads to the
spacecraft for shutter operations, but in the case of a spacecraft safe
mode the shutters would not have gotten closed and might have exposed the
instrument to hours of exposure in the worst case.

PFS spectrometer tests are being prepared to try to move the instrument's scanner during
a wheel off-loading (WOL). The instrument is non-functional due to a stuck
scanner. It is hoped that operating the scanner motor while the thrusters
kick the spacecraft might free the scanner. Hoped, but not expected.

Today I plotted out the fuel, oxidizer and helium pressurant since the start of the mission.
The state of the consumables is just fine, and VEX life will be limited by money much more than
by fuel. The plot below shows how much was used to get into orbit; since then, we've been using
very little. We've got room for some big orbit changes if it is decided to try something new.

[lyford]

Thanks for the update! I always enjoy reading them even if I don't always comment.

When will we know about the PFS?

[cndwrld]

PFS has delivered their inputs for the Scanner test during a wheel off-loading. The tests are to be run over 6 days, during the last three weeks of July.

They'll want to look pretty closely at the data for any signs of movement, and no one is in a hurry to announce bad news. So I wouldn't expect us to get any official word on the result until about mid-August. But that's just my guess.

Of course, if the thing actually moves, we'll hear about it in no time. This test is a Hail Mary pass, so I'm not holding my breath. Still, would be nice if it worked.

[The Messenger]

...so no news is bad news...



The ESA gets kudos for a brave, daring, and resourceful use of a 'hammer'

[cndwrld]

We have a bigger hammer in reserve. This test is during a wheel offloading, where we hold the spacecraft steady with thrusters while dumping the momentum from the reaction wheels. The thruster burns are quite small.

If this hammer doesn't work, they might do the same thing but during an orbit correction maneuver. An OCM is usually a bigger burn, proportional to the amount of delta-V(elocity) that flight dynamics decides we need to get back into our correct orbit.

It still seems unlikely to help. But what the heck. The PFS team is operating their instrument on Mars Express, but it isn't like their too busy to do this test. And the flight control team is probably starting to get bored. Can't hurt to try it.

So we'll kick it. And in the noble and honorable tradition of engineering, if it doesn't do what we want then we'll kick it harder. While calling it a naughty name.

[cndwrld]

PFS Still Non-Operational

The PFS scanner was commanded during recent momentum dumps (wheel off-loadings). Reaction wheels speed up in order to capture unwanted spacecraft momentum and maintain pointing; when the wheels are slowed back down to their zero level (generally once a day), thrusters are fired to hold the spacecraft steady. It was hoped that the physical shock of the thruster firings, performed at the same time the scanner was commanded to move, might jog the scanner lose.

However, kicking the scanner in this way didn't work. No more tests are planned, that I'm aware of. The orbit correction maneuvers, which I thought might be tried, turn out to be smaller than the wheel off-loadings. So trying them is not thought likely to help.

So, we'll have to see if the PFS team can come up with another Hail Mary pass, or if this is the end.

[lyford]

Thanks for the update, though I wish the news was better. I guess the problem with moving parts is that they sometimes aren't moving parts.....

So, we'll have to see if the PFS team can come up with another Hail Mary pass, or if this is the end.

Are ESA missions allowed to use American football sports idioms? But if they come up with a long shot that works, I would gladly allow them to use whatever idioms they deem necessary....

[cndwrld]

I work on an ESA project, but I'm an American. I'm afraid I don't know any European sports idioms.

[lyford]

I was trying to think of what the football (soccer) equivalent of a "Hail Mary" would be myself but couldn't. I think this reflects the poor state of my sports knowledge and of European slang.

Thanks again for all your inside reporting, cndwrld.

[Stu]

I'm afraid I don't know any European sports idioms.


Try Sven Goran Erikson. Oh, sorry, idioms... I thought you said something else...

[nprev]

Based on a casual Google, perhaps ESA might consider launching the aforementioned individual on a purely ballistic intercept trajectory with VEX in order to jar PFS loose...didn't see a whole lotta love there, Stu!

[cndwrld]

Coordinated observations of Venus between VEX and Messenger

The ESA web page for the Messenger fly-by is now up, on the ESA Space Science page, at
http://www.esa.int/esaSC/index.html

If you click on the third story, labeled, "Venusian rendezvous results: chapter one", you go to the dedicated fly-by page at:
http://www.esa.int/esaSC/SEMVN4HYX3F_index_0.html

The fly-by page can also be reached from the dedicated Venus Express page at:
http://www.esa.int/SPECIALS/Venus_Express/index.html

[Rakhir]

500 days at Venus, and the surprises keep coming
http://www.esa.int/esaCP/SEMNOCMPQ5F_index_0.html

...Some of the first detailed analyses are now being completed and will soon be published in acclaimed scientific journals...

[ustrax]

500 days at Venus, and the surprises keep coming
http://www.esa.int/esaCP/SEMNOCMPQ5F_index_0.html

About this anniversary I'm starting today a special issue at spacEurope, today's guest is Pierre Drossart, VIRTIS Co-PI, no shocking announcements but the fact of having people from the team sharing some thoughts is quite revolutionary...and appreciated...