ISRO brings down decommissioned weather satellite: What is a controlled re-entry? Why is it done?

Source: The post is based on the article “Isro brings down decommissioned weather satellite: What is a controlled re-entry? Why is it done?” published in The Hindu on 13th March 2023.

What is the News?

The Indian Space Research Organisation (ISRO) for the first time brought down a satellite – Megha Tropiques-1, in a controlled manner after its end of life.

What is Megha Tropiques-1?

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What is Controlled re-entry?

Usually, satellites are left in their orbit after their end of life and because of the gravitational pull of the earth, they come down to the atmosphere over years.

It takes debris from the low earth orbit 20 to 30 years to fall to the atmosphere and generations for those in geosynchronous or geostationary orbits to fall.

A controlled re-entry is possible only for satellites in low-earth orbit – at about 1,000 kms over the surface of the earth. It involves de-orbiting the satellite to very low altitudes to ensure the impact occurs within a targeted safe zone.

However, these manoeuvres are not usually attempted because fuel reserves have to be maintained in the satellite after mission life is over.

Moreover, this controlled re-entry is impossible for satellites placed in geo-stationary or geosynchronous orbit – where the time taken by the satellite to orbit the earth matches Earth’s rotation – because they are at altitudes of nearly 36,000 kms.

For attempting to bring down a satellite from such an orbit, a huge fuel reserve would be needed, which will only make the satellite heavier and costlier at launch.

Therefore, instead of bringing them down, they are shot upwards at the end of life. These orbits are like parking lots in space where all old satellites are put in.

Why did ISRO go for controlled re-entry of a satellite?

Other than extra fuel remaining in the satellite after the mission life ended, ISRO attempted the control re-entry to demonstrate and understand the process of doing so.

With several satellites/other objects/debris moving at extremely high speeds in low earth orbits, it has become imperative to keep the space clean as even the smallest debris can destroy active satellites.

Kessler syndrome – A scenario where the amount of space debris reaches a point where they just create more with one collision triggering others – is even scarier.

ISRO was also following the guidelines of the UN and the Inter-Agency Space Debris Coordination Committee (IADC) that say satellites should be deorbited after mission life.

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