Understanding the MARS launch window and different stages in an Inter-planetary voyage

Image credit: www.deccanchronicle.com

It has been a month since the Mars Orbiter Mission started its research around the red planet. India has joined the elite group of countries which succeeded in reaching MARS. India astonished the world with its frugal yet successful Mars mission that was never ever thought of.  ISRO has also broken the record of executing this herculean project in mere 15 months duration. So, why did India hurry in making its maiden voyage to our nearest neighbor?? Is it the unreasonable desire to prove its capabilities to the world? The answer is a big NO. The factor that made ISRO to execute this mission at a faster rate is the space dynamics. Earth takes 365 days to revolve around the sun while mars take about 687 days to complete a revolution around the sun. So we can say that a Martian Year (A time period of one year on Mars) is approximately equal to 1.88 Earth years (A time period of one year on earth). So, by the time Mars finishes its one revolution, Earth is already at the verge of completing its second revolution. Also these two planets revolve in the orbits with different orbital characteristics. Mars’s orbit has more eccentricity than that of Earth. To sum up all these points from orbital geometry to the relative dynamics between these two planets we can arrive at a conclusion that these two neighbouring planets come closer to each other every two years and thereby providing a chance for the launch of spacecraft. This time frame in rocket science’s jargon is termed as the LAUNCH WINDOW. It is impractical to launch a spacecraft out of the launch window and if you miss it once you should wait for another 2 years for the arrival of next launch window.
As the Government of India approved the project for ISRO on August 2012 and with the expected MARS launch window on November 2013, ISRO was left with two options, either to speed up the execution or wait for the next launch window (after 2 years), ISRO preferred the former and turned a Nation’s dream into reality in a matter of 15 months.

Deciding a launch window is a hectic job because of the multitude of factors to be considered to arrive at an optimal time slot. As both Earth and Mars are moving bodies, the physics calculations must be carried out considering a dynamic launch pad and a moving target in mind. Apart from this when once the MOM exits the earth atmosphere, it enters a heliocentric area where the dominating force is the gravitational pull by the sun. Due to the gravitational pull of the sun the orbiter takes a different trajectory and starts moving in space in an elliptical orbit. So, while calculating the launch window, we need to consider the sun’s gravitational pull and the expected trajectory in the heliocentric stage of the mission. Keeping aside all these technical aspects, it is important to consider the obstructions created by nature in the form of bad weather etc. So, deciding a launch date and time for an inter-planetary mission is a tedious job and requires complicated physics and the tools that simulate that.

 The Mars launch window usually lasts for a month and thus it is also crucial to pick a day and time in the existing launch window. In deciding the optimal launch date and time, the factors like earth’s west to east movement is also considered to utilize the earth’s additional pull to the fullest extent and also some reserved days are also to be considered for bad weather conditions at the launch site. To further understand the essence of launch window concept, let us consider the dates of the Mars launch window.  Earth and Mars came nearer to each other (Usually termed as Mars close approach) on April 14 2014 and this nearness is maintained for a period of almost 4 weeks before it actually moves away from earth in an elliptical orbit. This period of 1 month is the time in which the orbiter must elliptically intercept the Mars in space to accomplish the task with less energy and thereby less fuel and cost. So, let us do some reverse engineering!! Keeping the dates and duration of Mars close approach in mind (Usually happens every 2 years) and considering the time it takes to reach Mars from an earth’s orbit((Usually 300 days)), when launched at a predefined velocity and trajectory, the date of launch of the space craft from the surface of the earth is determined. So, the launch window is defined as the time span in which a space craft can be launched from the earth’ s surface so that it can travel in an elliptical orbit around the sun and successfully intersect the elliptical orbit of Mars after a year at a point nearest possible to Earth. It is not as simple as firing the orbiter towards the MARS based on its current position because both MARS and Earth are moving bodies in an elliptical orbit and so we need think of intersecting ellipse rather than intersecting straight lines because nothing in space can travel in a straight line due to the gravitational pull.   

          

           

    Stages of MOM image credit: ISRO

Once the launch date is finalized, the oribter is then placed into an earth orbit by a space craft. The orbiter revolves around the earth in the injected orbit before it is given enough velocity to travel towards Mars. In case of MOM, ISRO relied on its work horse PSLV for injecting MOM into an earth orbit (Parking orbit). For the interplanetary mission it is always advisable to inject the satellites into higher earth orbits which make it easy for the orbiter in the cruise phase. With the PSLV space craft, higher earth orbits cannot be reached. On the other hand GSLV, the space craft intended for higher earth orbit applications is still incompetent. So, ISRO relied on a modified PSLV design for the launch of MOM. PSLV injected orbiter into a Medium Earth orbit and from there it was brought to a higher earth orbit by orbit raise manoeuvres which was performed by firing the thrusters. After reaching a desirable earth orbit, a long firing session for more than 23 minutes was carried out to disconnect the orbiter from Earth’s influence and to sling shot the orbiter into a planned heliocentric trajectory. The timing of this firing is so vital because delay in this activity for a day may lead to a new trajectory for the orbiter all together and thus ISRO engineers had worked on different trajectories that the orbiter needs to take corresponding to different time of firing to account for the unexpected delays in firing activity in the earth orbit. Once the orbiter leaves the earth orbit, it comes in to the gravitational influence of the sun and starts revolving in a larger elliptical orbit around the sun and thereby moving close to the point that it is intended to meet its target-The MARS. This phase is termed as Cruise Phase and is usually the largest phase in the entire mission. In case of MOM it lasted for about 300 days. The velocity that is imparted to the orbiter will not vary much due to the negligible drag available in the free space. But due to the influence of different celestial bodies that the orbiter encounters in its voyage, the orientation, trajectory and sometimes even the velocity of the orbiter changes in the cruise phase. These parameters must be meticulously observed and corrective measures must be taken in case of deviation from normal. It is observed that the reduction in the velocity of the orbiter by a few miles/sec may miss the target by a long distance and thus the parameters like velocity and trajectory must be corrected on a regular basis by firing the thrusters.

          

       " For your Information: A thruster is a propulsive device or liquid fuel engine which ejects the exhaust gases resulted due to combustion of fuel inside it and thereby moving the orbiter in the direction opposite to the direction of exhaust gases (Newton’s Third law). It is the cardinal organ in a spacecraft to control the orientation, velocity or orbit rising. MOM is equipped with one big 440 Newton thruster for the purpose of orbit raising, orbit injection (which require more firing duration and fuel) and 8 small 22 Newton thrusters on different direction for the purpose of orientation control (Which require firing of smaller duration)."

 Cruising Phase of The Surveyor Image credit: NASA

If the planning and execution is perfect, by the time the orbiter completes its cruise phase, it should be near the target it was aimed at. In case of MOM, the planned point of injection is at an elevation (shortest) of 372 km from the Martian surface. As the orbiter elliptically approaches MARS (Just like two vehicles crossing a U- turn from opposite directions at the same time), it velocity must be brought down considerably such that it gets caught in the intended orbit. We all know that every orbit around a celestial body will have an orbital velocity and the orbiter which has that velocity will be captured in that orbit by that celestial mass and the orbiter continues to move in that orbit around the celestial body like a satellite etc. The velocity is reduced by firing the thrusters in the direction opposite to the motion of the orbiter by a method called “Aero braking”.  This phase is called “Mars Orbiter Injection Burn”. It is a very challenging fete as we will be having a very limited control over the happenings near Mars. The main obstacle is the limitation of the speed of radio waves. It takes about 13 minutes on an average for an instruction to reach from Earth to Mars and thus controlling this stage from Earth is impractical and whatever that has to be done is to be performed by MOM on its own by executing the pre-programmed instructions. In the cruise phase, the orbiter is subjected to different temperatures and conditions. Due to this reason, automatically activating the thrusters (to reduce orbiter’s velocity) on its own which were idle for almost a year is a tedious activity. If due to any reason, the orbiter fails to fire its thrusters at the right moment, the orbiter will either crash land on the Martian surface or may even burn out due to the heat generated by excessive atmospheric drag. In case of MOM everything went on well and it reached the target as intended. MOM’s orbit is currently a large elliptical orbit and it must be corrected (again by firing the thrusters) to a near circular orbit to carry out thorough investigation on Martian atmosphere.     

        I remember a chapter named Projectiles from my college, Newton’s laws and Equation for the Force of Gravity from my secondary school. All that I have shared with you till now are the mere applications of these concepts. Let it be a simple day to day activity or the most challenging task that the human race has ever tried- PHYSICS stays valid and that is the beauty of it.          

                

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