We briefly mentioned asteroids in our previous blog about the astronaut John Young and how in his later years he dedicated his time to publicising the danger of asteroid impact. In 1992 NASA-sponsored the Near-Earth-Object Interception Workshop which evaluated issues involved in intercepting celestial objects that could hit Earth. One of the outcomes was a survey being commissioned to discover and identify Earth-crossing asteroids. The result of this and later surveys lead to NASA in 1998 formally adopting the goal of finding and cataloguing, by 2008, 90% of all near-Earth objects (NEOs) with diameters of 1 km or larger that could represent a collision risk to Earth. NASA has now identified and tracks 2300 Potentially Hazardous Objects or PHO’s which are comets or asteroids bigger than 140 meters in diameter that could cause regional devastation if they impacted the Earth. The discovery of 2024 YR4 on the 27th of December was a reminder of just how big a risk this is. As of the 6th February 2025, 2024 YR4 is rated 3 on the Torino scale with a 1 in 43 (2.3%) chance of impacting Earth on 22 December 2032. This rock is smaller than 100 meters but has the potential to strike the Earth releasing energy equivalent to 7.7 megatones of TNT. Its not going to threaten civilisation unless by some pure fluke it strikes a nuclear power station. Scientists are still researching its make up but at the moment the consensus is that it is more likely to explode in an airburst similar to the Chelyabinsk meteor in Russia in 2013, but on a way bigger scale, lets hope that is correct if the worst comes to the worst.
Those of you who are fans of Adam McKay’s film Don’t Look Up will have heard of the Planetary Defence Coordination Office. It was created in January 2016 and was given the job of cataloguing and tracking potentially hazardous near-Earth objects (NEO), such as asteroids and comets, larger than 30–50 meters in diameter. Not so long ago the average McDonalds employed more people than the total global number of scientists looking for potentially civilisation ending asteroids and comets, thankfully that has now changed. The statistics are pretty sobering. Asteroids with a 1 km diameter strike Earth every 500,000 years on average. Large collisions – with 5 km objects happen approximately once every twenty million years. The last known impact of an object of 10 km or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago. Large impacts correlate very highly with large scale extension events. The Chilexhiub crater that resulted is crater is roughly 200 kilometres in width and a kilometre deep. You are reading this on a PC or a phone today because of that impact event. Dinosaurs largely became extinct – except for the birds – and the mammals came out of their burrows and diversified to fill all the ecological niches left empty by the demise of the dinosaurs. The periodicity of large impacts is estimated, but its not an exact science, we can see in the geological record of the earth significant impacts have happened on numerous occasions creating iridium layers in the rock strata, the only question is whether past periodicity will continue to hold true into the future.
Asteroids are rocky, metallic, or icy bodies with no atmosphere, they are classified as Carbonaceous, Metallic or Silicaceous depend on what dominates their composition. The size and shape of asteroids vary significantly, ranging from loosely bound small rubble piles to objects as large as the dwarf planet Ceres which is almost 1000 km in diameter. One of the definitions of a planet is a body that has cleared its orbit around a star. The Earth has undergone several periods of heavy asteroid and cometary bombardment in its history, delivering a large percentage of the water on the Earth in the process, contributing significantly and possibly even delivering the rudimental building blocks for life on Earth. Recent missions like OSIRIS-Rex and Hayabusa2 have brought asteroid samples back to Earth. The Hayabusa2 samples from the asteroid 162173 Ryugu found ten types of amino acids in the samples, including glycine and L-alanine which are fundamental to life. It might surprise people to learn that the Earth gets hit by roughly 100 tons of dust and sand sized particles daily. Yes daily. Its estimated that the impact of an object much larger than 1 km diameter would result in global devastation and would have significant consequences for civilisation as it currently exists. The impact of a rock 5km or greater in diameter would in all likelihood result in the extinction of the human race.
Lots of factors impact on how much damage an incoming body does. Its speed and the kinetic energy it holds is one of the most important. Its composition is another key factor, Metallic asteroids tend to survive re-entry better than Carbonaceous ones. The angle of entry is also key in terms of determining how long an object has to burn up in the atmosphere. People might remember the videos of when the Chelyabinsk meteor struck in Russia, it turned night into day as it burned up. That object was estimated to be about 18 metres in diameter and about 9,000-tonnes. It entered the atmosphere at quite a shallow 18‐degree angle with a speed relative to Earth of 19 kilometres per second largely burning up before it hit the ground. Based on its entry direction and speed of 19 kilometres per the Chelyabinsk meteor probably originated in the main asteroid belt between Mars and Jupiter, most likely originating as a result of a collision between two bodies. The irony is that the global Astronomical community was focusing on and actively preparing for the close approach of a slightly larger known asteroid 367943 Duende which happened 16 hours later. The Chelyabinsk meteor struck out of a clear blue sky and was not picked up before its entry into the atmosphere. It exploded with the equivalent energy of 440/450 Kilotonnes of TNT or roughly 30 times the power of the Hiroshima bomb. Even though the meteor exploded at a height of 30 km, nearly 1500 people were injured when the shock wave hit the ground smashing windows over a wider area. The 1908 Tunguska event is thought to have resulted from a similar air burst explosion of an object about 3 times the size of the Chelyabinsk one, which exploded with roughly the power of 1000 Hiroshima bombs flattening 830 square miles of forest in roughly the shape of a giant butterfly. Tunguska is an incredibly remote area, even today. The impact of flattering 800+ square miles of a densely populated urban area are catastrophic to put it mildly. This recently discovered asteroid 2024 YR4 is about 4 times the size of the Chelyabinsk one. An air burst explosion generating a shock wave from release the energy equivalent of 7.7 megatones of TNT over somewhere very densely populated like Bangkok, Delhi, Cairo, Dhaka, Sao Paulo, Jakarta, Beijing, London, Mexico City, Paris, Moscow, Lagos, Manilla, Shanghai, Seoul, Tokyo or Karachi would result in fatalities in the millions and damages in the trillions. It does not bear thinking about.
In terms of the recent history of the solar system even something as powerful as 1000 times the impact of the Hiroshima bomb is actually relatively small beer. Comet Shoemaker–Levy 9 was a comet that was identified in 1993 after it had been captured by Jupiter’s gravity, ultimately breaking apart under the intense pressure of Jupiter’s gravitational forces and lining up like the carriages of a train. Shoemaker–Levy 9 impacted on Jupiter over a 6 day period in July 1994. 21 distinct impacts were observed heating the atmosphere to temperatures as hot as 30,000 to 40,000 degrees Celsius. When Fragment G struck Jupiter it created a giant scar on Jupiter’s surface over 12,000 km wise, roughly the diameter of the Earth, and it was estimated to have released energy equivalent to 6,000,000 megatons of TNT or 600 times the world’s entire nuclear arsenal. The Hellas crater on Mars is 1,400 miles across and approximately 23,500 feet deep, because the impact created an impact ring the depth top to bottom is 30,000 feet, in other words deep enough to hide Mount Everest. These numbers, the forces involved and the scars left behind are almost too big to comprehend.
It is 100% certain the Earth will be impacted again in the future. Suffice to say Bruce Willis and Ben Affleck won’t be hopping into a space shuttle and drilling a hole and setting off a nuclear bomb. (Never mind the fact the shuttle is no longer operational) Current asteroid mitigation measures need years of advance notice. The DART Project (Double Asteroid Redirection Test) in 2022 was our first attempt to study the effect of a high speed impact on an asteroids orbit, that project started in 2015. Scientists are currently examining 2024 YR4’s orbit and the impact of gravitational events on it to see if the 1 in 43 change of impact in 2032 goes up or goes down. Chelyabinsk showed despite all the efforts being put into Near Earth Observations the danger remains that a big rock with our name on it and the potential to end civilisation in all likelihood doesn’t have any other name yet, especially if it approaches the Earth from the inner solar system.
It is noteworthy that someone as bright as Stephen Hawking, in his final book Brief Answers to the Big Questions, remained convinced that an asteroid collision continued to be the biggest threat to civilisation and the planet.
EDIT
As of 24 February 2025, with better observations over a longer period 2024 YR4 is now rated as having an estimated 1-in-59,000 chance of hitting the Earth on 22 December 2032. Relax!