AstroSat Picture of the Month of Jan, 2019

AstroSat Picture of the Month of Jan, 2019

The 97 minute orbit of AstroSat around the Earth. The orbit is roughly equatorial (top right), inclined at around 6 degrees to it (top left). This results in each orbit being slightly displaced from the previous one (bottom). Pic Credits: Leo Jackson John, Operation Director, AstroSat, ISTRAC, ISRO

When seeing images from AstroSat, have you ever wondered where exactly is the satellite, how does it move, and how do astronomers get their hands on the data? This month's APOM is here to answer those questions for you.

AstroSat was launched https://www.youtube.com/watch?v=hZCRiNpfB-8 by ISRO on 28 September, 2015 from Sriharikota https://www.isro.gov.in/about-isro/satish-dhawan-space-centre-sdsc-shar, on board the PSLV-C30 into its current orbit. This is a low-earth equatorial orbit, at a height of 650 km above the Earth. This orbit is not exactly over the equator, but is inclined at angle of about 6 degree to it.  In the top left image, the green line marks the equator and the yellow line marks the orbit of AstroSat and the top right image is a view from over the north pole. But why was this orbit chosen?

Our Earth has a magnetic field (https://en.wikipedia.org/wiki/Earth%27s_magnetic_field), which behaves overall like a bar magnet (http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/MagEarth.html), with its poles a few degrees from the poles defined by our rotation. These magnetic fields trap charged particles within them, which form the Van Allen belts (https://www.space.com/33948-van-allen-radiation-belts.html). These belts are much closer to the Earth over the southern Atlantic Ocean. An equatorial orbit reduces the effect of this South Atlantic Anomaly  https://en.wikipedia.org/wiki/South_Atlantic_Anomaly, on AstroSat which carries very sensitive instruments. Making the inclination exactly zero requires more resources and hence a 6 degree inclination was chosen.

AstroSat takes about 97 minutes to orbit the Earth once. Hence, it will not pass directly overhead the same point in successive orbits. Each orbit, therefore, will be slightly shifted with respect to the previous one. The individual orbits shown in red in the bottom image mark orbits. One orbit per week is plotted for clarity. Data is beamed down from an antenna on the satellite once every orbit, when it passes over India. The data is received by ISRO's dedicated Indian Deep Space Network http://isrohq.vssc.gov.in/VSSC_V4/index.php/ground-segment/82-chandrayaa... antenna in Byalalu https://en.wikipedia.org/wiki/Indian_Deep_Space_Network near Bengaluru (marked as BLR). All orbits of AstroSat fall within the visibility of this antenna, which is marked by a circle centred at Byalalu. ISRO can also use an antenna in Indonesia, marked BIK, to monitor the satellite when needed. All the command, control and tracking of AstroSat is done by ISTRAC https://www.isro.gov.in/about-isro/isro-telemetry-tracking-and-command-n... in Bengaluru.

As of 30 January 2019, AstroSat has completed more than 18,000 orbits around the Earth, acting as our high energy eye,  uncovering the nature of neutron stars, black holes, hot star, and many strange celestial objects. May it continue to do so for many more orbits!