VEX Science Planning Options

[cndwrld]

Hello-

I wasn't sure how most people would like to see stuff posted. Or if this would interest many people. But in the interests of putting out more information, I created this topic as a place to put information on Venus Express science planning.

If you have any ideas about this, let me know.

Cheers-

Don Merritt

[cndwrld]

VEX Weekly Status

At the end of the last Cebreros ground station (CEB) pass in the reporting period (DOY 069, 18:00z) Venus Express was orbiting Venus at 202 million kilometers from the Earth. The one-way signal travel time was 672 sec.

On 2007-066, after intermittent losses of TM had been observed, the CEB team reported high winds and requested to move the 35 meter antenna to a safe position. Normal operations were resumed 43 minutes later. As a result of this and some minor hardware issues, science data was not correctly received and had to be copied on board to a dedicated packet store for a subsequent dump. No data will be lost.

VIRTIS movie passes are now scheduled for the second week of April (DOY 98-101). This will involve use of a NASA DSN station 70-meter antenna for a higher capacity downlink, and five days of intense VIRTIS imaging spectrometer data taken of the south pole. This should allow excellent imaging with high detail of the southern vortex and the rest of the polar cloud cover. Planning of science operations for this event is now finished.

The testing of operations for quadrature is on-going; the next quadrature phase will start in May 2007. In this phase, the face of the spacecraft with the instruments would normally be pointed directly at the Sun during Earth contact. The spacecraft will be offset 10 degrees in roll during the Earth contact periods, to provide enhanced protection to the instrument suite. Since this operation wasn't foreseen, the Darmstadt operations team has come up with a good solution. When the spacecraft is told to go to Earth pointing, it will automatically go into the correct orientation, even if it is in Safe Mode. However, nothing is ever simple. This Earth-Pointing-with-roll offset exposes other spacecraft faces to the Sun which have limited exposure times due to thermal constraints, so the Earth communications phase which are normally cool (and allow us to cool down from hot science operations) must now be followed by mandated cooling periods. During the cooling periods, observations can only be cool, with the science instruments and spacecraft pointed such that the direct solar exposure is to the two spacecraft faces which are designed to allow for this continuous exposure.

To complicate matters more, in the middle of the quadrature period the operations team will need to flip the spacecraft in order to use our second, smaller High Gain Antenna (HGA). As mentioned, only two spacecraft faces are designed for long durations of direct solar exposure. At about the midpoint of the quadrature season, the use of the large HGA1 for Earth communications would expose senstive faces to the Sun. The spacecraft is designed to be flipped and flown with the smaller HGA2 used for Earth communications, putting the Sun back onto the full exposure faces. The small HGA2 is located directly behind HGA1, and points directly opposite of it. For those interested, you can see the location of the components on the 3-D model which is at the ESA VEX home page:
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=64


Venus is relatively close to the Earth right now, so our data rate is about as good as it gets. When we switch to HGA2, our data rate plummets from 228.531kilobits per second to 28.566 kilobits per second, which of course means we must reduce our science observations accordingly.

And it is in this quadrature period, with the extra operational thermal constraints limiting our science observations, with the switch to the smaller HGA2, and the loss of our data rate, that the NASA Messenger spacecraft does its fly-by of Venus, over a portion of the planet coverd by the portion of the orbit where VEX always dedicates that time to Earth communications instead of science observations. Of course. So the spacecraft team, flight dynamics team and the science operations team have now completed all their planning, and the testing of those plans are underway.

Best regards-

Don