Venus Express Options

[tedstryk]

If all goes well, Venus Express will be a major topic for discussion in this forum a year from now. Does anyone know how good the surface coverage will be from VIRTIS and VMC? My understanding is that VIRTIS will obtain low resolution multispectral maps, and that VMC will, in addition to cloud monitoring, have one channel that can see the surface, but I don't know at what resolution or at what quality. It will be nice to have some non-radar images of Venus' surface besides the Venera snapshots and the shadowy images from Earth and Galileo's NIMS.

Ted

[RNeuhaus]

According to the VE's pdf document, VE has greater challenge to orbit around Venus than Mars Express since it has two great concern that must be adjusted continuously:

1) The thrusters has to adjust to correct the altitude of orbit's pericentre approximately once every day. In fact, due to the gravitational pull of the Sun while the spacecraft is further away from the planet, the pericentre naturally drifts upwards at a rate of about 1.5 kilometres per day.

2) For a spacecraft in orbit around Venus, it is not always possible to point a single
antenna dish at Earth while always keeping the cold face of the spacecraft, hosting
delicate instruments, away from the Sun.To overcome this pointing constraint,Venus
Express has two high-gain antennas mounted on different spacecraft faces.The
main high-gain antenna, used for most of the communications with Earth, is a 1.3
metre-diameter dish.The second, smaller high-gain antenna (30 centimetres
diameter) is used when the spacecraft is in the part of its orbit closest to Earth (less
than 0.78 AU* away).

On the other hand:

I have a doubt about the VE's trajectory to Venus. Will the Soyus-Fraget travel in the opposite way to the Earth's rotation before going to Venus?

I tought it since the Earth position is on the apehelion comparing to the Venus position as perihelion. Hence, the trajectory from Earth to Venus is of inward bound. To launch a spacecraft from Earth to an inner planet such as Venus using least propellant, its existing solar orbit (as it sits on the launch pad) must be adjusted so that it will take it to Venus. In other words, the spacecraft's aphelion is already the distance of Earth's orbit, and the perihelion will be on the orbit of Venus.

This time, the task is to decrease the periapsis (perihelion) of the spacecraft's present solar orbit. A spacecraft's periapsis altitude can be lowered by decreasing the spacecraft's energy at apoapsis. To achieve this, the spacecraft lifts off of the launch pad, rises above Earth's atmosphere, and uses its rocket to accelerate opposite the direction of Earth's revolution around the sun, thereby decreasing its orbital energy while here at apoapsis (aphelion) to the extent that its new orbit will have a perihelion equal to the distance of Venus's orbit. Of course the spacecraft will continue going in the same direction as Earth orbits the sun, but a little slower now. To get to Venus, rather than just to its orbit, again requires that the spacecraft be inserted into its interplanetary trajectory at the correct time so it will arrive at the Venusian orbit when Venus is there. Venus launch opportunities occur about every 19 months.



Will do VE follow the trajectory of least energy orbit as mentioned above?

Rodolfo