http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=40580
25 Jan 2007 09:48
Report for Period 14 January to 20 January 2007
Sorry for that stupid question, but what Earth is occultating seen from VE?
Or is venus occultating VE from Earth?
Report isn't very explanatory...
THe orbit was previously designed, as I understand it, so that Earth was always visible from the spacecraft - it didn't go 'behind' Venus from our perspective...but now it does which gives an opportunity for radio science during ingress and egress
Doug
The VEX orbit is rigidly fixed in inertial space. As the planet rotates and revolves, the orbit doesn't change in relation to, say, galactic coordinates. The spacecraft is in contact with the Earth every day for at least 8 hours, but normally has to point out of the orbit plane. In Earth occulation phase, the spacecraft antenna stays aligned with the orbit and sees Earth. About every three months, lasting for about two months, the orbit will be aligned with the direction to Earth. As the spacecraft ascends from apocenter to pericenter, the line between the spacecraft and the Earth will cross Venus. If the spacecraft is pointed directly at Earth (instead of, say, pointing at nadir to take data) and the spacecraft transmitter is turned on, the signal received on Earth will vary as the beam cuts through the atmosphere. Once can do this at ingress (beam goes from outside atmosphere, into atmostphere) and at egress (beam comes from behind planet, then towards outer atmosphere).
There are also phases when the orbit is aligned with the Sun; the ascending branch of the orbit will see the Sun go behind the planet, and then will see the Sun come out from behind the planet. Instead of taking radio data at this point, the Spicav SOIR instrument can measure the solar radiation as the spacecraft goes through ingress and egress, getting a lot of data on atmosperic composition.
A picture would make it all clear. But I hope the explanation helps.
Cheers-
Don Merritt
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