A rocket body discarded in space 48 years ago re-entered the atmosphere last Friday night (23), probably over the Pacific Ocean. The rocket was expected to re-enter the atmosphere over the weekend of July 24th.

This is the second stage of the rocket soviet Kosmos-3M, which was released on December 26, 1973 from the Plesetsk Cosmodrome.

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The rocket placed in orbit the satellite DS-U2-GKA (or Aureole-2), which aimed to investigate the upper atmosphere of the Land at high latitudes and study the nature of polar auroras.

Kosmos-3M rocket (left) and DS-U2-GKA satellite (right)
Kosmos-3M rocket (left) and DS-U2-GKA satellite (right). Source: kosmonautix.cz

re-entry

Re-entry was scheduled to take place on Friday (23) or Saturday (24). According to engineer Joseph Remis, it should take place at 21:47 pm (Eastern time) on Friday. The forecast has a margin of error of +-13 hours, which means it can occur between Friday morning and Saturday morning.

Within that margin of error, the rocket's body, also called the SL-8 R/B, must complete 17 orbits around the Earth. In 4 of them it will pass over Brazil. This means that there is a small possibility of re-entry over Brazilian territory. The most accurate calculations should only be released on Thursday or Friday, and may increase or remove the possibility of re-entry into our country.

SL-8 R/B passes (yellow lines) within the margin of error of the re-entry prediction
SL-8 R/B passes (yellow lines) within the margin of error of the re-entry prediction. Reproduction: satflare.com

Why the rocket must re-enter the atmosphere

To orbit the Earth, any object must travel at a speed of about 27 kilometers per hour, which generates a centrifugal force that compensates for the gravitational pull. After accomplishing their mission, many rockets are left in low orbit, where there are enough atmospheric gas particles to produce drag. This reduces the rocket's speed, gradually causing gravity to prevail.

Centrifugal force (v) and gravity acceleration (a) generating orbital motion
Centrifugal force (v) and gravity acceleration (a) generating orbital motion.
Source: wikimedia.org

With each orbit around the Earth, the drag further reduces the rocket's speed. As a result, its altitude decreases, which causes the rocket to reach even lower and denser layers of the atmosphere, which offer even more resistance. It is a path of no return. Its orbit deteriorates until it reaches the breaking point, when intense heat and mechanical deceleration will cause the rocket to shatter and burn, leaving generally few tracks.

In the case of the SL-8 R/B, it was left in a not-so-low orbit that still suffered some drag. As a result, its orbit took nearly 50 years to deteriorate to the point where it was about to reenter the atmosphere.

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Re-entry should not be risky

When released, the second stage of the Kosmos-3M was over 20 tons. But after burning and draining its fuel, it was left with “only” 1,4 tons of mass. It is a cylindrical piece 6 meters long and 2,4 meters in diameter. Nothing so small and not so light as to make us feel comfortable knowing that it will fall to Earth at any moment.

However, there is nothing to worry about: thanks to the enormous speed at which this occurs, around 28 thousand km/h, the Earth's atmosphere acts as a shield, almost completely disintegrating the object. During the re-entry process, atmospheric gases are heated and ionized, generating a huge ball of fire that can be seen hundreds of kilometers away. The heat is so high that it completely vaporizes up to 80% of the object. What little is left must be fragmented by the resistance of the air, and must reach the surface practically harmlessly.

Fireball generated on ATV-5 re-entry in 2015
Fireball generated on ATV-5 re-entry in 2015. Credits: ESA/Nasa

Only the most massive components must withstand atmospheric passage, such as the engine frame and booster tanks. Although they are not so light parts, the risk that they can cause some ground damage is still extremely small. Two thirds of the planet's surface is covered by oceans and continental areas still have a huge amount of sparsely populated or completely uninhabited regions.

Therefore, this re-entry should not generate great concerns. The only risk we run is to be able to observe a beautiful spectacle in the sky, especially if we are lucky if we are lucky that it occurs at night and close to us.

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