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The Planetary Society BlogBy Emily LakdawallaVenus Express...ExpressOct. 4, 2006 | 09:54 PDT | 16:54 UTC
by Doug Ellison
Truly an pan-European mission, the spacecraft structure was built in Switzerland, the avionics and propulsion outfitted in the UK, the electrical installation in Italy, and the environmental testing in France before being shipped for launch. When it finally arrived at Venus, it conducted a 1251 m/sec burn to enter Venusian orbit, a burn that was just four seconds longer than expected. This put the vehicle into a nine day orbit of 400 x 350,000 kilometres. The final orbit was established by May 7 at 250 x 66,000 kilometres, which, very importantly, is a 24 hour orbit. The Planetary Fourier Spectrometer (PFS) was also mentioned. As some of you will know, the PFS failed before arrival at Venus and is currently turned off. Future efforts to further diagnose and try to fix this are planned, but for now, about 1/3 of the wavelengths covered by PFS are being covered by other instruments onboard.
The floor was then passed briefly to Andrea Accomazzo who talked about the first few months of orbital operations from the engineering perspective. Currently, they are in the nominal science mission which will last two sidereal Venus days. As mentioned each orbit is 24 hours and this is intentional. Science is done mainly during the closest 90 minutes of the orbit -- but also further out -- however for the most distant eight hours of the orbit, Venus is visible from the Ceberos 35-metre European dish in Spain -- so all the data downlink occurs at the same period of time every day, and communications vary between about 19 and 228 kbps resulting in around two Gbits of data per day. Compare this with the unfortunate scientists and engineers who work on Mars time, with blacked out windows and chronic Mars lag as their body clock argues with their job clock. By making VEX stick to a sensible Earth time, they can not only save the sanity of their engineers but also save quite a lot of money by needing only one shift of downlink per day. The other benefit is that the other European deep space facility -- an identical dish in Western Australia is then always visible to VEX at the time when they want to uplink new commands. Command lists of around 800 to 1300 time tagged commands are uplinked daily. There is only storage onboard for a total of 3000 commands, so they uplink every day instead of commanding several days at a time. Solar Conjunction is coming up in a few weeks time, and all the science instruments will be turned off until the 9th of November, and just radio science will be conducted during that period. Nils Muller then presenting a brief look at some science being conducted by the VIRTIS instrument onboard VEX to try to pull data out from the surface. About 95% of the data in the 990-1050 nm range comes from the surface and this reduces to no surface data in the 1250-1320 nm. BUT -- the 990-1050 range still has an input from the clouds. So, using clever maths, Muller basically stacks a few images from the 990-1050 range, and subtracts the influence of the clouds in the longer wavelengths to hopefully get a temperature map of the surface. Due to blurring by the atmosphere, the resolution of this data is only around 100 kilometres per pixel, however he was able to present some images that showed a variety of surface temperatures from around 720 to 740 Kelvin from data taken out of a mosaic of images collected on the 112 orbit. This data actually quite closely matches the altitude maps of Venus collected by the Magellan mission. So, if it matches the Magellan altitude data so well, what's the point in going to all that trouble? The simple answer -- Volcanic activity. Your average Venusian volcanic lava flow is expected to be perhaps 1000-1500 Kelvin and cover many hundreds of square kilometres, so it should be visible in these 100 kilometre resolution thermal maps. Hopefully, if there is any active volcanic activity going on down under the clouds, then this technique should be able to find it as a change of a more than 5 deg Kelvin in his thermal maps. I was really looking forward to the evening lecture by Gerhard H Schwelm, the VEX Mission Manager, but unfortunately new information was a bit thin on the ground. Here are the highlights: ASPERA-4 and the VEX Mag have both begun to measure the material being blown off the upper atmosphere by solar wind and SPICAM has started observing stellar and solar occultations. Details on all three were lacking, but one chart showed a clear dust layer being observed by SPICAM between 60 to 80 km altitude. VIRTIS is mapping the cloud activity from the cloud top down to around 20 km altitude, and they can see very complex cloud activity under the S shaped cloud observed first by Pioneer Venus and recently in images released by the VIRTIS team shortly after VOI. The issue of the failed PFS reared its ugly head once more, and Schwelm said what we have already heard -- they will try to get it working again but for now they are attempting to cover its job with the other instruments. They can't recover the data, but they hope to be able to recover at least some of the science. Questions from the floor were good -- one asking about planetary protection issues for spacecraft. Schwelm said that VEX followed COSPAR regulations, which have no protection rules currently for Venus. Another asked about the distribution of data to the wider scientific community -- Schwelm said that they will follow the ESA rules and distribute the science data after a six month delay via the PSA I mentioned in yesterday's SMART-1 glog entry, and they are working to keep their data compatible with the NASA PDS. The last question asked if there were plans to reuse the platform again. Schwelm explained that when you do this "copy" of a spacecraft design, you have to do it rapidly without a gap. It's been a year since they finished working on VEX and thus the heritage is essentially lost -- the teams have changed and it would not be as cost effective to do the same thing again, and finally, there are other targets with requirements too different to those of VEX and MEX waiting for us to explore, so we will have to develop a different platform for them Not the flood of information I was hoping for, but a trickle is better than a drought.
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