Friday, June 07, 2013

Bringing the ATV to the ISS: a stepped process

The ATV-4 Albert Einstein, currently on it's way to the International Space Station (see images of passes over Leiden here), does not ascend in a linear fashion. The ascend towards the ISS is in phases.

Below is a diagram showing you how the orbital altitude of the ATV-4 spacecraft has developed over the past two days and will develop, if everything goes according to plan, over the coming few days untill it docks to the ISS at the 15th. As the orbit of the spacecraft is not neatly circular but somewhat an ellipse, values for the highest point (apogee) and lowest point (perigee) of the orbit are given.

click diagram to enlarge

It might surprise you, but over the past two days the orbital altitude of ATV-4 has gradually come down, not up. You can see this in the diagram above. It concerns a slight orbital altitude decrease of just a few kilometers, no more. This is because the ATV  is currently just moving in a parking orbit around earth without major manoeuvering. As a result, the orbit currently slightly decays (it is slowly pulled in by Earth as a result of gravity and atmospheric drag): it is very slightly coming down!

A major manoeuvre to counter this will occur on June 10th, lifting the spacecraft up from approximately 250 km to approximately 290 km altitude. The orbit will then slightly decay to slightly lower altitudes again, as the ATV will again be drifting. More manoeuvres raising the orbit slightly to counter this orbital decay will occur on June 12th, but the major manoeuvres will be on the 14th and 15th. These latter are the manoevres which will bring the ATV up close to the orbital altitude of the ISS (see diagram above), first bringing it up to approximately 380 km altitude, and then to about 400 to 410 km, and close to the ISS. It will be somewhat in front of the ISS initially, and move closer to the ISS as the orbit is gently raised. A complicated set of manoeuvres then will bring the ATV to the ISS for docking.

click diagram to enlarge

Note: diagrams constructed using orbital data for the ATV-4 kindly provided by ESA here.

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