Subject: INTEGRAL Orbit Evolution - Preliminary Assessment


Dear Arvind,

we are currently analysing the evolution of the Perigee Passage altitude. The
detailed analysis will take about one to two weeks. Some preliminary results
are as follows.

If we don't implement any orbit correction manoeuvre we will reach an
altitude of about 9000 Km, which was considered as the critical altitude
pre-launch, at the end of 2008. As far as I understand the main problem is
linked to the PROTON belts, which are at around 7000 Km as far as I know.
This altitude will be reached in October 2009. The first analysis shows that
it is advantageous to perform the Perigee raise manoeuvre as late as
possible. The reason is that the decay could be accelerated after a
manoeuvre, which means that the gain is less.

Before having the results of the study it is difficult to define the
strategy. My first proposal is that we prepare the operations (procedures,
etc.) to perform the manoeuvre. Then the manoeuvre could be implemented
quickly. In order to avoid that we lose too much fuel we should perform a
first manoeuvre at a time when we think that the problems on the instruments
start. This is acceptable since the effect on the orbit will be fast. We can
implement further manoeuvres when we believe that there are real problems.

Considering the available information I believe that we are save at least
until early / mid 2009 and that it should be possible to delay the date when
we reach the critical level until about 2010 / 2011. However, this needs to
be confirmed by the analysis.

Besides, it is still important to get a better feeling when, i.e. at which
altitude, we have to expect problems with the instruments.
We (Flight Control Team) are also performing an analysis concerning the
passage through the belts depending on the orbit characteristics because the
altitude is not the only important element but also the path through the
belts plays a role. The fact that we have now an orbit with a high
inclination (currently 83 deg wrt to about 51 deg at Launch) is beneficial as
the Proton belts are worse in the Equator plane. We can see this effect by
looking at the IREM data.
The results of this analysis should also be available within the next two
weeks.


Regards

Michael