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]

Venus Express Support Of NASA's Messenger Spacecraft Fly-By of Venus

In about 82 days, NASA's Messenger spacecraft will fly by Venus on its way to its final destination of Mercury.

The Venus Express team is planning special observations ov Venus during the fly-by, to coordinate their observations with the data taken by the Messenger instruments.

The first complication was the location, and its effect on planning. The VEX orbit is rigidly controlled to maximize savings on the ground. The orbit period is maintained so that the downlink to Earth can be done during normal working hours at the Cebreros station near Madrid, minimizing mission cost. Unfortunately, the location of the Messenger closest approach to Venus is during our nominal downlink period.

Since VEX is re-using as much as possible of the Mars Express design, a bus designed for a cold planet is operating at a hot one, which causes us to have a lot of restrictive thermal constraints. If certain panels of the spacecraft are exposed to the Sun, a cooling period in an approved cooling attitude is required.

The solution we're using is to skip three downlink opportunities. In orbit 409, the downlink is skipped so that we can stay within our rigid thermal constraints for instrument operation in orbit 410. In orbit 410, on 05 June 2007, orbservations will be done to capture the cloud data "before" images for the pass prior to Messenger closest approach. The time of the observations, in our descending branch, is during the time we would normally be communicating with Earth, so that Earth communication window is skipped.

At the point of Messenger closest approach, at about 00:05 UTC on 06 June, VEX is in our orbit 411 but not within sight of the area. We will be on the other side of the planet, in our ascending branch. In the descending branch, we will observe the area that Messenger saw previously, making our observations between about 09:30 and 11:30 UTC. This requires skipping this Earth comm pass, too.

For orbits 409 - 411, all the data taken by the instruments must be stored on board within the spacecraft memory. It is not usual for an instrument to take more in one orbit that can be downlinked in the next pass, we just need to keep track of what is expected to be used, so that the circular buffer does not get overwritten by new data. The amount of data that is taken in support of the Messenger fly-by, over three orbits, must fit within the on-board memory allocated to each instrument, so what they take must be limited. Since our first post-Messenger downlink is in orbit 412, and the instruments also want to take their normal observations in orbit 412 ascending branch and pericenter, there are actually four observation possibilities which must be made to fit into their memory allocations. Each instrument team has to make the trade-offs of what they want to take, how much they want to take, and how long they expect it to take to download it.

The Magnetometer will not be making any attempts to change their normal observations. And radio science will not be done in this period. The ASPERA, SPICAV, VMC and VIRTIS instruments are all making special observations to support the fly-by. The data for all but VIRTIS will be on the ground no later than 16 June, and all of the VIRTIS data should be down no later than 18 June.

There's information on Messenger at: http://messenger.jhuapl.edu/. VEX information is at:
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=64.

Cheers-

Don Merritt