The first stage of China’s Long March 5B rocket, launched on April 29, is scheduled to re-enter Earth’s atmosphere and crash into Earth on May 10. It is one of the largest objects to fall out of Earth’s orbit without control in recent times.
Normally when a rocket launches a satellite into Earth orbit, the large first stage of the rocket does not gain orbital velocity. The second stage is somewhat small in size which closes to the orbit. After launching into orbit & the satellite is launched, as a second step orbital removal engines perform a Deorbit Burn and safely enter the atmosphere above the ocean. Some rocket companies do not remove accept the second step from orbit, and if they are in lower orbit, they gradually slow down and enter the atmosphere after days or weeks due to atmospheric resistance (Drag). These are uncontrolled reentry and when left in the atmosphere, the remaining parts can fall to the earth. However as a result of the second stage which is smaller, the risk of something of such magnitude falling to Earth is lower.
The Long March 5B rocket which has 31.7 m long with a diameter of 5 m, has the final first stage fuel and a mass of 20,000 kg. Why did China launch such a large rocket into orbit? These Rockets are usually used to reach the Earth Orbit in several stages. These are called Multistage Launch Vehicles. So why aren’t there single-stage rockets? Does it means that the part that leaves the earth cannot be carried to orbit?
This is called “Single Stage to Orbit (SSTO)”. Though there have been ideas about this since the beginning of the Rocket era, none of these have been put into practise in practically. The main reason for this is that if a single-stage rocket were to be launched into orbit, the entire rocket would have to be taken into orbit. Which is need to send a 5-ton satellite into orbit, or send a 50-ton rocket into orbit with the satellite? The trick is to skip the big first stage after a part of the journey and move on to the rest of the smaller second stage. Then a small mass has to be accelerated to the second stage. So the first stage is smaller and eventually, the whole rocket becomes cheaper. Therefore, the most commonly used two Stage to Orbit (or three-stage) rockets.
However, between the SSTO system and the TSTO system there are One and a Half Stage to Orbit, that means, 1.5 Stage rockets are also available. In the middle of all there is a big first stage. But since these do not have a second stage, it is better to call it the Core stage than the First Stage. This goes from Earth to orbit. But this does not have the power to rise to the top alone. If built with that much power, the power of those engines will increase as they go up. Therefore, in the first part of the journey, several boosters are used parallel to the first stage (Core Stage). Shortly after launch, the boosters escape. The core stage travels up to the orbit.
But the other problem is that the very large core stage travels to Earth orbit. It would be a problem if there was no way to remove it from Earth orbit properly. The largest 1.5 Stage rockets in history are the American space shuttle and the Soviet Energia rocket. The Soviet Buran shuttle was launched from this Energia rocket. The first stage of the shuttle is a little different. The high-cost RS-25 engines needed to be reused, so the three engines were mounted in the Orbiter and the core stage simply turned into a tank. Then at the end of the trip, only the tank is removed. The engine lands back with the orbiter. The Energia rocket is not like that. It’s a normal rocket. There are boosters and a Core Stage. The Buran shuttle is one payload that carries it.
Somehow, the engineers who designed it had done a little bit to prevent both of these huge Core Stage / External Tanks from going into orbit. That is, these two rockets do not fully achieve orbital velocity. For example, when an American shuttle shuts down the main engine, in its trajectory when rises above the Pacific Ocean and returns to the atmosphere. This removes the orbiter from the outer tank and accelerates the engine to a full circular orbit (Circularization Burn). The 30-ton tank travels in the same direction as before, entering the atmosphere above the Pacific Ocean and falling into the ocean. The Energia rocket worked the same way. As a result, the massive first phase of any shuttle or Energia mission remained in Earth orbit.
But China’s Long March 5B stage one and a half rocket, which launches into Earth orbit in first stage. China has not yet implemented a way to remove it from orbit. Thus, during the first few days of orbiting the earth in an unstable lower orbit, the earth’s orbital forces gradually lower its orbit and eventually enter the atmosphere without control. The first phase of the Long March 5Brocket will consist of two large fuel tanks and two rocket engines, most of which are expected to burn up as they enter the atmosphere. But the remaining parts of the two main engines of the rocket will fall to the ground.
The principle of orbital mechanics is that if a satellite accelerates at one point, the orbit rises 180 above the position of the orbiting satellite. Also, if the speed is reduced at one point, the height of the other side of the orbit decreases. So the Long March 5B rocket launched the Tianhe module into an orbit of 170 × 375 km. The Tianhe module accelerated its engines at the top of the orbit (Apogee) and elevated the Perigee at the bottom of the orbit. Initially, it orbited at 284 × 384 km, and later by combustion engines, it orbited at 350 × 380 km. But the core stage of the rocket is still in old orbit. When it travels in the lowest region of the orbit (below 200 km), the speed decreases slightly due to atmospheric influences. The result is a decrease in the height at the top of the orbit. So this always lowers the top of the orbit as it travels in the lower region of the orbit.
April 29 – 170 375 km
April 30 – 169 × 363 km
May 1 – 168 × 352 km
May 2 – Kilometers 167 × 338
May 3 – 165 × 326 km
May 4 – 163 × 310 km
Thus, when the highest point of the orbit descends, not only the lowest point of the orbit but also the entire wind orbit increases. When the total orbit is less than 200 km, the speed of the core stage decreases throughout the entire orbit, and in a spiral path it approaches the thicker layers of the atmosphere.
Due to it is still in low orbit, it is not possible to say 100% exactly when and where it will enter the atmosphere. But according to the calculations that have been done so far, this will happen on May 10 at 2 p.m. It is estimated that 6.31 is likely to enter the Earth’s atmosphere. The uncertainty of this calculation is 41 hours. That is, it will enter the atmosphere in the 82 hours from 41 hours before to 41 hours after. During that time this will complete several orbits around the Earth. The rocket’s orbit has an inclination of 41.5 degrees, so it will crash between 41.5 degrees north latitude and 41.5 degrees south latitude. Sri Lanka is also within this range.
The first phase of the first Long March 5B rocket, previously launched on May 5, 2020, crashed into Earth in a similar manner. It was in low orbit for a few days and entered the Earth’s atmosphere west of the African continent. The wreckage crashed 2,100 miles [2,100 km] into the atmosphere. These parts fall into the village of Mohounou near the town of Bocanda in the state of Cote d’lvoire. No casualties or injuries were reported, but property damage was reported.
As this core stage gradually descends, the time and place it enters the atmosphere can be calculated more accurately. So by May 10, this will be able to calculate the time it will enter the atmosphere with a few hours of uncertainty.