On 30th of June, the world observes International Asteroid Day. Why should we care about asteroids? Good question. We should care about them because we can. The dinosaurs could not and they went extinct. Near Earth Objects are planetary threats.

What happened on the 30th of June?

On 30th of June 1908 an asteroid impacted Earth in Tunguska, in Siberia, now in the Russian Federation. It was Earth’s largest asteroid impact in recorded history. Larger events occurred in prehistory but nobody was there to record them.

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We see their indirect effects because Earth is now populated by humans. We are here because when another asteroid impacted Earth 65 million years ago and our ancestors survived while the dinosaurs went extinct. Not only the dinosaurs died then but 99.999 % of life on Earth and about 75% of all species became extinct. The carbon cycle came to a halt (Preston D, 2019, The Day the Dinosaurs Died).

The asteroid that caused the extinction of the dinosaurs was between 11 and 81 km large. That’s anything from the size of a large village to the size of a city. It left a crater behind it, the Chicxulub crater. The meteoroid that fell in Tunguska was 50-190 metres, big enough not to destroy a planet but a city.

Trees knocked over by the Tunguska blast. Photograph from the Soviet Academy of Science 1927 expedition led by Leonid Kulik By CYD – From English Wikipedia.

Fast forward to February 2013, a date most of us can remember, another fireball, the Chelyabinsk meteor – a superbolide of about 20 m in diameter entered the atmosphere of Earth. The light from the meteor was brighter than the Sun and visible up to 100 km away. Some eyewitnesses also felt intense heat from the fireball. It exploded with the power of 400–500 kilotons of TNT that is 26 to 33 times as much energy as that released from the atomic bomb detonated at Hiroshima (Leonard, D, 2013. “Russian Fireball Explosion Shows Meteor Risk Greater Than Thought”)

The Day the Dinosaurs Died

It was only in the 1980’s when geologist Walter Alvarez and his father, Nobel Prize-winning scientist Luis Walter Alvarez, advanced their hypothesis that a large extraterrestrial body had struck Earth at the time of the Cretaceous–Paleogene (K-Pg) boundary (Alvarez, et al. 1980, Extraterrestrial Cause for the Cretaceous-Tertiary Extinction). New Zealand is by the way, one of the few countries where the boundary can still be seen (Thompson, J, GNS, geotrips).

Putting a finger on the K-Pg boundary. J.Thomson / GNS Science

Luis Alvarez, who also contributed to a number of World War II radar projects, and his son Walter Alvarez proposed that the extinction event that wiped out the dinosaurs was the result of an asteroid impact, and that Near Earth Objects (NEO) are a threat to our civilisation. Over time, other scientists argued that the dinosaurs were on their way to extinction when that happened anyway (Greshko, M, 2019, What actually killed the dinosaurs? Volcanic clues heat up debate.). A study published on 29 June 2020 demonstrated, using computer modelling to combine climate and ecological modelling tools, “a substantial detrimental effect on dinosaur habitats caused by an impact winter scenario triggered by the Chicxulub asteroid” (Chiarenza et al, 2020, Asteroid impact, not volcanism, caused the end-Cretaceous dinosaur extinction). Asteroid it is!

Since the Alvarez published their paper in 1980’s possible asteroid collisions with Earth are now considered examples of planetary threats. This kind of threat is dealt with by Planetary Defense. But it took until 2015 for NASA to be the first space agency to create a Near Earth Objects programme — prompted by the paper published by Alvarez and building on previous military capabilities and knowledge. NASA conducted a workshop in 1992, followed by another report three years later, and in 1998 it formally embraced the goal of finding and cataloguing NEOs with a diameter bigger than 1 km, which have the potential to destroy the human species. In 2005 a NASA Authorization Act was passed to, among other things, direct NASA to “detecting, tracking, cataloguing, and characterizing near-Earth asteroids and comets in order to provide warning and mitigation of the potential hazard of such near-Earth objects to the Earth.” (Authorization Act 2005, SEC. 321. GEORGE E. BROWN, JR. NEAR- EARTH OBJECT SURVEY) (NASA 2007) (US Government 2005) (Mogosanu, H, 2015 Stars without wars, the institutionalisation of civilian space research).

A year later, the United Nations were adopting a resolution in December 2016 to observe “at the international level the anniversary of the Tunguska impact over Siberia, Russian Federation, on 30 June 1908… to raise public awareness about the asteroid impact hazard”. Currently the global coordination for detection is led by the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG), established by the UN in 2014 to keep watch. As for protection, we still have some time to come up with a plan.

Near Earth Objects – we know them by many names

Asteroid: a small rocky body orbiting the Sun. Large numbers of these, ranging enormously in size, are found between the orbits of Mars and Jupiter, though some have more eccentric orbits.

Meteoroid:meteoroid is a piece of stone-like or metal-like debris which travels in outer space. Most meteoroids are no bigger than a pebble. Large meteoroids are believed to come from the asteroid belt. Some of the smaller meteoroids may have come from the Moon or Mars. (NASA, 2020, Meteoroids).

Meteorite: A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. (Wikipedia)

Meteor: When meteoroids enter Earth’s atmosphere (or that of another planet, like Mars) at high speed and burn up, the fireballs or “shooting stars” are called meteors. (NASA, What’s the difference between a meteor, meteoroid and meteorite?)

Bolide: A bolide is an extremely bright meteor, especially one that explodes in the atmosphere. In astronomy, it refers to a fireball about as bright as the full moon, and it is generally considered a synonym for a fireball. In geology, a bolide is a very large impactor. Wikipedia

Barringer meteor crater https://meteorcrater.com, Photo by Jimmy Conover on Unsplash