Is it a UFO? No, it's a man-made wonder! Let's take a look at the beautiful rocket cloud

We often see some netizens post some strange cloud photos online. These clouds are different from the clouds we usually see. They are obviously directional, often dragging long tails at first, and suddenly start to expand when they reach a certain length. There is often a bright spot on the top moving forward. This kind of astronomical phenomenon is often called an unidentified flying object, or UFO, by witnesses, and then it triggers a heated discussion.

On June 29, 2018, the Falcon 9 carried the Dragon spacecraft to the International Space Station

In fact, this kind of thing happened many times as early as the Cold War. This strange astronomical phenomenon once caused many people to think that the other camp's nuclear missiles were coming and caused panic. However, people familiar with astronomy and meteorology should not be surprised by this, because this kind of astronomical phenomenon is often caused by the launch of rockets or missiles. It is a man-made astronomical phenomenon-rocket cloud.

What is a rocket cloud?

On Christmas Eve, December 23, 2017, SpaceX's Falcon 9 rocket carrying 10 Iridium satellites was launched from the Cape Canaveral Space Force Base in the United States. The rocket cloud caused by the rocket scared many people, and the local media also reported it to reassure everyone.

On December 23, 2017, the Falcon 9 rocket was launched from the Cape Canaveral Space Force Base in the United States.

The rocket cloud formed during launch

Sometimes we can see the contrail clouds caused by airplanes when we look up. They are essentially the same as the rocket clouds caused by rocket launches, but rocket clouds are often much higher than airplane contrail clouds. In English, the unique astronomical phenomenon caused by rockets can be called space jellyfish, which is translated into Chinese as space jellyfish because its expansion section is very similar to the appearance of jellyfish.

Falcon 9 launches from Cape Canaveral Space Force Station before dawn on June 29, 2018

If described from the meteorological perspective, this type of cloud can be called a noctilucent cloud. Under natural conditions, noctilucent clouds are often composed of tiny water ice crystals with a diameter of up to 100 nanometers. Generally speaking, noctilucent clouds exist at an altitude of about 76 to 85 kilometers, which is very close to the edge of the atmosphere. It is the highest cloud that can exist in the atmosphere and is a type of high-altitude cirrus cloud. Under natural conditions, it can often only be observed in high-latitude areas, and the time of observation is often in the early morning or dusk. Because the height of this type of cloud is very close to space, if we regard sunlight as a tangent to the earth, when the ground has entered the night, the noctilucent clouds in the sky are outside the tangent and can still be illuminated by the sun. At this time, people on the ground can see a shining cloud in the dark sky. If you can see it with your own eyes, it should be a rare experience in your life.

Rocket clouds can be said to be an artificial astronomical phenomenon that we create unintentionally during rocket launches. They are created during the flight of the rocket and can be observed by people within the visible range, but the best observation direction is parallel to the launch direction of the rocket. In this way, you can get a glimpse of the beautiful rocket clouds caused by rocket launches.

When talking about rocket clouds, we need to talk about the fuels currently used in rockets. Comprehensively considering the current rockets at home and abroad, the main fuels are: nitrogen tetroxide/hydrazine compounds; liquid oxygen/kerosene; liquid oxygen/liquid hydrogen; liquid oxygen/methane and solid fuels.

Among these fuels, solid fuels can produce a large amount of particulate matter when ignited; other fuels can produce a large amount of water vapor during the combustion process. Water vapor at high altitudes will turn into ice crystals due to the temperature below zero degrees. Particles, water vapor, and ice crystals are often important factors in the formation of clouds. During the rocket launch process, the rocket continuously discharges exhaust gas backwards, and the rocket cloud is created. In recent years, the number of rocket cloud sightings in China has gradually increased, because my country's aerospace industry is launching rapidly and the number of rocket launches is increasing.

The "doorway" to see rocket clouds

The shape of rocket clouds is often closely related to the model and type of rocket. For example, my country's Long March 5B rocket is a very unique one-and-a-half-stage rocket, consisting of a core stage and a booster. The core stage can be ignited once to directly send the payload into orbit. There are no other stages, so the clouds produced by this rocket launch will not be intermittent. However, most rockets in the world have two or more stages, so we will still discuss the rocket clouds that most rockets can create.

The discontinuity of rocket clouds, that is, the state of the clouds when the first and second stages separate, is closely related to the ignition time of the rocket core stage. It can be said that the clouds created by each type of rocket are unique. The clouds created by the same rocket are also very diverse due to the launch trajectory, weather conditions, wind intensity, etc., so it is impossible to generalize. In short, when you are lucky enough to see rocket clouds, you have to analyze the specific situation.

SpaceX's Falcon 9 is a very special reusable rocket. After the first and second stages separate and the second stage ignites, the first stage will return to the launch site or to a specific recovery ship. At this time, there will even be a strange sight of two opposite "space jellyfish" appearing at the same time.

When my country launched the Long March 7A rocket on December 23, 2021, a pilot (Weibo account: @天边的贝洛斯) was lucky enough to see the rocket launch process, and saw a very perfect rocket cloud and took a video. For convenience, we will use the middle interruption as an interval and divide it into the first half and the second half for analysis.

The Long March 7A rocket cloud photographed by a pilot on December 23, 2021

The first half of the rocket cloud is usually straight and dense, which is mainly caused by the rocket just taking off. In this stage, the rocket needs to leave the atmosphere as quickly as possible, almost parallel to the ground and continuously increase its vertical and lateral speed. If we look at it in an abstract way, this process is very similar to the first quadrant image of lnX in the logarithmic function in mathematics. However, since there is often wind on the ground and in the sky, the clouds in the first half will soon be blown like a curved thin snake and then gradually disappear in the sky.

In the second half of the rocket cloud, the booster and core stage of the rocket are separated from the second stage, and the second stage is ignited. The atmosphere at the height of the rocket is already very thin and the air pressure is very low. At this time, the exhaust gas ejected from the second stage nozzle can expand much more than the exhaust gas at sea level. The water vapor produced by the second stage of the rocket condenses at high altitude and diffuses due to the thin air pressure. The rocket cloud produced by it also gradually becomes larger and thinner, like an arrow piercing the sky. This cloud will continue until the rocket completely leaves the atmosphere. After that, the rocket will no longer produce a visible trajectory and fly towards the predetermined orbit.

What Filipino netizens saw when the Long March 7A was launched

Afterwards, the clouds will gradually deform under the influence of the wind, becoming a curved snake shape, and then gradually disappear in the sky.

This is the process of a rocket cloud from its creation to its end.

This process changes very quickly. Just like naturally occurring noctilucent clouds, rocket clouds only take a few hours from their creation to their disappearance, so their lifespan is not long.

Some readers may notice that some photos and videos of rocket clouds only show the second half. This is because the rocket is already flying very fast after the first and second stages are separated. It flies very far in a very short time. The distance the rocket travels from takeoff to payload separation can even cross a continent. When it passes over the observer's head, it has already reached the second half.

On December 25, 2021, the European Space Agency's Ariane 5 rocket launched the James Webb Space Telescope. It took off from Kourou, French Guiana, South America. When the telescope separated from the second stage, the payload had already crossed the Atlantic Ocean and the entire African continent. If we were standing at the westernmost corner of the African continent to observe the rocket cloud, we would definitely not be able to see the first half of the rocket cloud caused by the first stage and booster that had already separated, and at most we could only see the second half.

In addition to rockets, ballistic missiles, which are closely related to rockets, can also create similar effects. After all, the flight process of ballistic missiles is quite similar to that of rockets, except that rockets send payloads into space, while missiles send payloads to the top of the target. Their formation principles are the same.