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Sunday, October 5, 2014

How was MOM/Mangalyaan placed in Martian orbit?

An update on MOM operation ( updated on 7th March 2016 )

ISRO and its Ahmedabad based unit SAC have released latest images of MOM:


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In the previous post we had shown how the Martian Orbit Insertion ( MOI ) will be carried out and also reviewed it in light of  MAVEN MOI that was carried out 2 days prior to MOM( Mars Orbiter Mission ..  also called Mangalyaan )  MOI .

A quick recall is here:

MOM entered Mars' gravitational sphere of influence on 22Sept2014. During its descent its speed increased very rapidly ( i.e. acceleration also increased ) as it approached near the surface of Mars. At this point when the MOM was about 2500 kms from Mars' surface the retro rockets started reducing the speed and slowly it entered in the regular orbit around Mars.

Although it appears as simple a thing as pressing the brake pedal in the car , it is an involved and complex  process and we will see the actual course of incidents that have taken place and how and why behind it in this post.

Antenna switch and Orientation reversal:

Much before the MOI time ( at 2248 Hrs GMT on 23rd Sept ) the High Gain Antenna ( HGA ) of MOM was switched off and in its place the Medium Gain Antenna ( MGA ) was brought into operation. This was done because MGA has a wider beamwidth than HGA and it would allow communication with Earth even if MOM  antenna is not exactly pointing to Earth due to some unexpected catastrophe.

Adjacent figure shows the MOM moving from left to right. Under normal configuration the Liquid Apogee Motor ( LAM , the green portion on the leftside face in figure ) which had helped in raising the orbits while orbiting Earth is pointed to the back.  ( MOM is moving from left to right In the adjacent figure and the LAM and  8 other small thrusters are pointed to the rear  ).

The LAM would be used to create a braking torque during MOI and so it has to be pointed to the front. This change in attitude was initiated at 0127 GMT on 24th Sep and lasted 20 minutes. Technically this is termed as ' Forward Orientation'
A period of 1 minute is given after completion of Forward Rotation  for the system to stabilize in the new state and actual MOI is initiated at 0148 GMT.

Immediately after the MOI is over, the MOM rotation is started to normalize the orientation. This 'Reverse Maneuver  lasted 22 minutes ending at 02:34 GMT.

Eclipse and Occultation:

A small deviation from the current topic:

Even as the attitude change was going on, MOM went in the shadow of Mars @0143 GMT technically termed  as 'MOM enters Eclipse'.

Also 4 minutes after MOI had started ( 0152 GMT ), the MOM disappeared from Earth view as it went behind Mars at that time and therefore it couldn't communicate with Earth ,This phase is technically called as occultation. Since MOM is already in eclipse there is no Solar Energy to charge batteries and so to conserve energy the telemetry transmitter is switched off because anyway no communication was possible during this occultation.

The eclipse continued upto 0207 GMT  and occultation ended @0215GMT and telemetry was resumed  @0218GMT.

MOI Requirement defined :

The MOM has a planned regular orbit of 80000
kms near apoapsis ( maximum distance from the surface of Mars )  and  430 kms at periapsis ( Minimum distance from the surface ). Applying simple Kepler's laws near the periapsis, we obtain Height, Speed and Acceleration  that the MOM would have in this orbit near periapsis and it is plotted in dashed lines in this figure.

The Speed and Acceleration of MOM before MOI as it approached 2500 kms near the surface of Mars at 01:48 GMT is plotted as Solid lines in the left top corner. The speed at this instant has reached 5.18 kms/sec while acceleration is 0.89 m/sec/sec.  The aim of the whole MOI operation is to change these quantities and bring them to the required values shown in dotted lines. How is it done is shown in the next diagram.

MOI Operation:

Red line in adjacent figure shows acceleration during this critical operation ( numbers are in kms/sec/sec ). Height of MOM from the surface is a blue curve. Green line shows speed in kms/sec relative to Mars surface.
X axis is time axis from 0145GMT to 0225GMT of 24th Sept.

As soon as retro rocket start @ 01:48:40 GMT,  ( at about 2500 kms from the surface of Mars ) we can clearly see the sudden drop ( vertical portion ) in forward acceleration from 0.89 to 0.137 m/sec/sec.

This retarded the previously increasing speed from 5.18 kms/sec to 4.38 kms/sec,  a drop of 0.8 Kms/s over the burn duration.
Acceleration also drops from 0.137 to -1.55 m/s/s in this period.

1st Orbit start:

About midway through MOI, acceleration becomes -ve and MOM starts rising ..  this is the periapsis at the start of 1st orbit.

As soon as the retro rocket stops @ 02:11:50 GMT the MOM regains its natural speed around Mars ( as its satellite under the influence of Martian gravity ) and the acceleration which had fallen to -1.55 jumps to -0.73 m/s/s. and MOM starts its 1st orbit around Mars. This point becomes the periapsis.
A small observation worth to take note of:

 Notice that the LAM retro action  tried to reduce the speed but the fall is more powerful under the influence of Martian gravity, so MOM speed continues to increase despite retro action for sometime.( Peak speed of 5.18kms/sec @ 0156GMT) This is the gathered momentum  by MOM during freefall.  Finally however the thrusters succeed in reducing the speed and as soon as this speed matches with the required speed  ( 4.38 kms/sec ) in actual orbit    ( dashed and solid green lines match )  the thrusters are turned off .
Now the actual operation of MOI is over.

Autonomy of MOI:

At the time of MOI, MOM was at a distance 224 million kms from Earth ( Equivalent to 12.5 Light minutes ) and so there is a 12.5 minutes delay in one way signal transmission. It follows therefore that one can't control MOM online and therefore the whole operation of MOI had to be carried out by onboard computers without waiting for decisions from Earth.

Whole operation was as per the preloaded commands and so proves the maturity of ISRO scientists in correctly modelling the anticipated situations at Mars end and assimilation of Martian parameters.

Added to that is the fact that the MOM was in radio shadow from Earth during a major portion of MOI operation.This did not allow even to know what happened during MOI untill MOM emerged out of radio shadow.

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