# Some questions about Chandra's orbit

From: Ahmet Gorgun (agorgun@att.net)
Date: Sun Sep 09 2001 - 15:16:20 EDT

```I was looking at Chandra’s orbit at JTrack 3D. While I was playing with the
orientation of the orbit I noticed that for some angles the orbit looked
circular. It turned out that I was looking at the elliptical orbit from the
short axis and when foreshortened it looked like a circle of diameter with
the minor axis of the ellipse! Thanks to Chandra people for nicely pointing
this out to me:)

But I did some more research on Chandra’s orbit and it seems that the orbit
is actually getting circular. According to Mission Overview Report the
apogee and perigee are drawing closer to each other at a rate of few
thousand kilometers per year and after 5 years the trend will reverse.

According to the Chandra ephemeris in JPL Horizon at present the perigee is
22942.28 km (Sep. 8, 2001, 3:55) and apogee is 138,645.21 km (Sep. 9, 2001,
12:00). This gives a=80,793.74 and eccentricity e=0.7160.

So, in two years the perigee increased by 12942 km or 6471 km per year and
eccentricity decreased from 0.802 to 0.716. But the apogee stayed
essentially the same.

What is the cause of this circularization trend? Does this happen with
every satellite of eccentric orbits? And why the different rate of motion
for perigee and apogee?

I could find only two observations of Chandra in the SeeSat-L archives.
http://www2.satellite.eu.org/sat/seesat/Feb-2000/0066.html
In February 2000 Wayne Hughes observed Chandra near perigee, which showed
up 2.5 minutes earlier than predicted. It took 70 seconds to traverse 1
degree FOV. As it moved further out Chandra was on schedule and covered 1
degree FOV in 126 seconds.

According to JPL Horizon data Chandra should be moving 0.849 degrees per
minute at perigee. I am not sure if the observed speed, given in terms of
FOV, corresponds to the theoretical value? Does it mean that Chandra moved
1 degree in RA in 70 seconds? If so then it seems close to the ephemeris
value.

The other observation was by Ron Lee on 30 Sep. 1999 when chandra was close
to apogee.
http://www2.satellite.eu.org/sat/seesat/Sep-1999/0459.html
In an earlier attempt Ron Lee found that Chandra was late, although I don’t
know how much. He also noted that JPL horizon predicted 10 minutes earlier
passage than Sky Map Pro.

I would like to know if anyone observed chandra more recently, especially
near perigee.

Does it really move 0.84 degrees per minute or with a more circular orbit
than expected, does it move slower?

Unfortunately not much of a possibility of observing chandra here in New
York City. I would appreciate any thoughts and comments.
Thanks
Ahmet Gorgun

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