Meteor burns: How Earth deals with millions of shooting stars every day

Do you know that as many as 25 million meteors may arrive in Earth’s atmosphere on a daily basis? According to the American Meteor Society, the speed of an approaching meteor can range between 25,000 mph and 160,000 mph (40,200 kmh and 257,500 kmh). Most of these meteors or shooting stars are made from sand-grain-sized rocks; they never reach the surface and are burned up in the atmosphere. However, recent research suggests that around 5,200 metric tons of micrometeorites fall to Earth every year.

Meteoroids are lumps of rock that orbit the sun. Many are formed by the collision of larger asteroids, which orbit between Mars and Jupiter in the asteroid belt. As asteroids smash into each other, pieces break off to produce meteoroids. The force of the collision can also throw the meteoroids out of their regular orbit and on a collision course with Earth.

Meteoroids can also be made up of the debris shed by comets as they travel through space. Meteoroids shed by a comet often orbit together in a formation called a meteoroid stream. A very small amount of meteoroids are made up of pieces broken off the Moon and Mars after they have been hit by asteroids or other meteoroids.

Most meteoroids are made of silicates - minerals that contain silicon and oxygen, along with metals like nickel and iron. When a meteoroid enters the Earth’s atmosphere, the atmospheric gases surrounding its path are compressed, and this air resistance causes the temperature to increase.

This heat is enough to vaporize and ionize the surface of both the meteoroid and the surrounding atmospheric gas, causing a bright glow. When seen from the Earth’s surface, this appears like a moving streak of light, which is called a shooting star or a meteor. Most meteoroids that enter Earth’s atmosphere to become meteors disintegrate before they reach the ground. 

The heat of reentry also causes the surface of the meteoroid to lose mass through vaporization, a process called ablation. Many meteoroids also break up under aerodynamic pressure. So, only a relative few such space rocks or meteors can survive the atmospheric heat and reach Earth’s surface; these rocks are called meteorites. 

Types of meteors

Meteors are not just rocks but they also contain minerals such as iron, magnesium, nickel, etc. The elements inside a meteor and its velocity affect the color of light a shooting star emits. For example, a meteor rich in nickel is likely to appear green, whereas a shooting star with abundant iron content would emit yellow light. Based on their size, brightness, and distance from Earth, the International Astronomical Union (IAU) uses the following classifications.

Fireballs

Large, bright meteors that appear brighter than any of the planets, with an apparent magnitude greater than -4, are called fireballs. They either have a smoke trail (which looks like a chemtrail) made up of non-luminous particulate matter stripped off during ablation, or an illuminated trail consisting of ionized air molecules called a train. Fireball trains are most visible during the night; however, smoke trails can be seen during the day. 

Trains usually occur at heights greater than 80 km) altitude and are most often associated with fast meteors. Smoke trails usually occur below this altitude.

Most fireballs go unnoticed because they occur over ocean or uninhabited areas, or are masked by daylight. Between around 15 to 20 km (9-12 miles) altitude, the meteoroid will decelerate to the point that ablation stops, and visible light is no longer generated, making the meteorite essentially invisible during the final portion of its fall, called “dark flight”.

According to the American Metro Society, several thousand magnitudes of fireball meteors occur in the Earth's atmosphere each day. "Experienced observers can expect to see only about 1 fireball of magnitude -6 or better for every 200 hours of meteor observing, while a fireball of magnitude -4 can be expected about once every 20 hours or so."

Sometimes bright fireballs that have a magnitude of -8 or greater explode in the stratosphere and produce a sonic boom (thunderous sound that is heard from aircraft like F-104 Starfighter and SR-71 Blackbird). Some astronomers classify these as bolides.

Earthgazers

Earthgazers are meteors that skim the top of Earth's atmosphere like a stone skipping across the surface of a pond and then bouncing back into outer space. An earthgazer has a colorful trail, and due to ablation, it also experiences a change in its mass and velocity from its brush with the atmosphere. The small pieces of matter from earthgazer sometimes reach the surface and are found as meteorites. 

The Great Daylight Fireball is a famous earthgazer that was seen on August 10, 1972, in some parts of the U.S. and Canada. According to eyewitnesses, the meteor had a smoke trail that lasted for several minutes and it produced a double sonic boom. Great Daylight entered the skyline above Utah and left our planet’s atmosphere when it reached over Alberta, Canada. 1994’s American television film Without Warning features an actual video recording of the Great Daylight meteor.

Bolide 

The biggest and the brightest meteors that have a magnitude greater than -14 are often referred to as bolides, although there is no single definition for a bolide. According to some estimates, about 5,000 bolides enter Earth’s atmosphere annually. Many of these meteors produce sonic booms, and some "superbolides" can even give rise to large shockwaves that become hazards on the ground. 

On February 15, 2013, a superbolide exploded over Chelyabinsk, Russia at an altitude of around 12 miles (20 km), releasing 500 kilotons of energy, around thirty times the yield of the Hiroshima nuclear bomb. The shockwave caused damage to a distance of 75 miles (120 km), breaking the windows in hundreds of buildings and injuring around 1,500 people, mostly due to cuts from the flying glass. Around five tons of material reached the ground, including a 1,400 lbs (650 kg) meteorite that fell into Lake Chebarkul.

The Chelyabinsk meteor sparked a debate among scientists and government agencies on the need for setting up a system to monitor threats from outer space. In 2016, NASA established a Planetary Defense Coordination Office in order to track objects in space that could pose a risk to life on Earth. Fortunately, since the superbolide incident in Chelyabinsk, no dangerous object has shown up.

What is a meteor shower?

When a comet (an object in space with a nucleus of ice and dust) passes close to the Sun, the heat causes part of it to vaporize and form a "tail" of dust and gas. Most comets travel in orbits that form highly elongated ellipses. As a result, some comets have orbits that intersect the earth's path. When our planet crosses the comet’s tail, it runs into this debris, which produces a visible shower of meteors.

Meteor showers associated with particular comets occur at around the same time each year. The point from which meteors in a meteor shower appear to emanate is called the radiant. 

Since the name of a meteor shower include the name of the constellation in which the radiant is located, some people believe that star constellations produce meteor showers but this is not true. All meteor showers have comets as their source. 

For example, the Orionid meteor shower received its name because its radiant is located in the Orion constellation, but its source is the debris from Halley’s comet. Similarly, the Andromedids meteor shower, whose radiant is observed in the Andromeda constellation is believed to have originated from Beila’s comet.

Every time the Earth passes through a comet’s path, the meteor shower associated with the comet appears in our skyline. NASA, the Royal Observatory, and various other space agencies across the globe release their meteor shower calendars every year; by going through them, you can know when a meteor shower is scheduled to take place. 

Some meteor showers are so intense that they lead to the occurrence of thousands of shooting stars per hour in the sky; such cosmic events are called meteor storms. The Leonid meteor shower, which occurs every 33 years, is one of the most popular events because more than 100,000 shooting stars may appear every hour.

Some interesting facts about meteors and meteorites

Meteors and meteorites may be composed of minerals and elements that existed before our solar system originated. So they are important for studying our universe, here are some interesting facts and studies related to these exciting space objects:    

• The largest meteorite known is called Hoba and is located in Namibia. Hoba is mostly made of iron, weighs more than 60 tonnes, and is believed to have arrived on Earth 80,000 years ago. 
• If you have found a meteorite, then for the purpose of its nomenclature and authentication, you need to submit a sample weighing 20 g, or 20% of the meteorite, to the Nomenclature Committee under the Meteoritical Society. 
• One plan to deflect a very large meteor from hitting the Earth is to explode a nuclear device in its vicinity. The resulting radiation pulse from such an explosion would vaporize the meteor's surface. As the vapor streamed away, it would deliver thrust that could throw the body off course. This process is called an X-ray slap.
• According to NASA, until now, more than 60,000 meteorites are known to have arrived on Earth but only around 126 of them came from Mars. The martian meteorites are considered quite valuable as they are sometimes sold at a price of $1000 or more per gram (far more valuable than gold). Interestingly, you can sometimes buy meteorites on eBay as well.   
• The Allende meteorite that landed in Mexico in 1969 is believed to be 4.6 billion years old. Numerous studies have been conducted on Allende and so far hundreds of its broken pieces have been recovered. It is composed of water, silicates, sulfates, and many other elements.

Since water covers more than 70 percent of the Earth's surface, most of the meteors that enter the atmosphere pass over oceans and other places uninhabited by humans, ending up in the water, deserts, and the Arctic or Antarctica. That means that we are already missing out on seeing a lot of beautiful shooting stars so the next time you have the chance to see one, don’t miss it.