Is Apophis bigger than asteroid that killed dinosaurs?

Asteroid 99942 Apophis was initially designated as 2004 MN4 when it was discovered on 19 June 2004, by astronomers at the Kitt Peak National Observatory in Arizona, USA. The 370-metre wide asteroid is both a Near Earth Object whose orbit crosses that of the Earth, and a Potentially Hazardous Asteroid (PHA), that can impact the Earth in the distant future. Apophis pales in comparison to the Chicxulub impactor, providing a death knell 66 million years ago to the dinosaurs, after populations were already stressed by the excessive volcanism that formed the Deccan Traps. 

Apophis measures about 350 metres across, and is primarily made up of silicates, iron and nickel. (Image Credit: NASA). 

The Chicxulub impactor measured between 10 and 15 kilometres across, and slammed into what is now the Yucatan Peninsula in Mexico. Even if Apophis does strike the Earth, the impact would release the energy equivalent of 1,000 megatons of TNT, which is significant but far less than the millions of megatons released by the Chicxulub impactor, which caused global climate upheaval including wildfires, tsunamis and a nuclear winter, where the dust from the impact blocked out the Sun. Apophis is not in the same league as the dinosaur killer. Apophis at best can wipe out a city or cause widespread regional devastation, not a global catastrophe. Scientists have ruled out any large asteroid impacting the Earth for the next millennium, including Apophis. 

The likelihood of an asteroid ending humanity in the foreseeable future is low. Within a century, humans are expected to develop more advanced strategies for deflecting threatening asteroids, including gravity tractors, ion beam shepherds and kinetic impactors, the last of which has already been demonstrated by a NASA mission. The capabilities for detecting asteroids and tracking their orbits is also expected to improve, with dedicated programmes aimed at discovering over 90 per cent of Near Earth Asteroids measuring larger than 140 metres across. 

The asteroid that killed the dinos measured between 10 and 15 km across. (Image Credit: NASA). 

Historically, large impacts have reshaped life on Earth, with the Chicxulub impactor being the most notable. An asteroid of similar size can theoretically threaten human civilisation by causing global climate disruption, famine and societal collapse. However, no known asteroid currently poses such a risk. A number of space agencies around the world track thousands of objects, including Apophis, and none of these are on a collision course. Smaller asteroids, measuring under one kilometre across can cause regional devastation, such as the 1908 Tungaska event, but not extinction. A massive, untracked ‘planet killer’ remains a remote possibility, but our vigilance and ingenuity makes it unlikely that humans will meet their demise through an asteroid impact. 

Based on extensive observations, Apophis will not hit the Earth. Radar observations by the Goldstone and Arecibo facilities in 2013 and 2021 helped refine its orbit, definitely ruling out collisions during future close approaches in 2029, 2036 and 2068. Astronomers have also meticulously modelled the drift to the asteroid caused by the Yarkovsky effect and its encounter with the Earth, which is expected to widen its orbit. Apophis has the best-tracked orbit among all the Near Earth Objects. Any uncertainties in the orbit are shrinking as more data accumulates on the object. The close encounter in 2029 provides humans with a rare chance to study a large asteroid from up close. 

Apophis, measuring 370 metres across is not large enough to end the world. It is still formidable enough to dwarf the largest nuclear bombs, but falls well short of the scale needed for a global extinction. A strike on land could destroy a city, or cause regional devastations, while an impact on water could cause devastating tsunamis. The effect would be catastrophic, but only locally, and an impact will not trigger a planet-wide climate collapse required to end humans in particular and life on Earth in general. Scientists estimate that the ‘Planet Killer’ asteroids have to be between five and ten kilometres across to threaten global civilisation. Apophis is a serious hazard, but not an apocalyptic one. 

A ‘planet killer’ asteroid is defined as one large enough to cause widespread global catastrophe, typically measuring between five and ten kilometres across. The Chicxulub impactor measuring between 10 and 15 kilometres across is considered a benchmark. After the impact 66 million years ago, it vapourised rock, ignited global wildfires, spawned tsunamis, collapsed food chains, and lofted dust that blocked out the sunlight for years, wiping out 75 per cent of all species on the planet, including non-avian dinosaurs. Their ability to alter global climate makes these asteroids existential threats. At 370 metres across, Apophis is less than a tenth of the size of the Chicxulub impactor, lacks the heft necessary to disrupt the entire planet. A one kilometre wide asteroid can still trigger a climate cooling effect.