Many people will remember the day in 2013 when one asteroid stole the limelight from another. On 15 February 2013, all eyes of astronomers were on the close approach of a 30 metre-sized Apollo asteroid 367943 Duende (then known as 2012 DA14). The asteroid was not expected to be visible to the naked eye. Nor was it expected to impact the Earth, this time.
Eager to determine the physical properties of this future potentially hazardous near Earth object, a concerted observing event was underway by astronomers using multiple telescopes at different observatories around the globe.
Everyone was therefore caught by surprise when an exceptionally bright meteor and airburst occurred over Chelyabinsk in Russia about 16 hours before the expected close passage of Duende which was orbiting 28,000 km further away.
With very different calculated orbits, these two coinciding events were unrelated to each other.
The Chelyabinsk meteoroid went undetected before its atmospheric entry partly because its radiant (that is, its originating path on the sky as viewed from Earth) was close to the Sun. You can watch a video of the entry here.
DUENDE and CHELYABINSK ORBITS
Positions at 03:20 UTC on 13 Feb 2013
(credit: vissiniti using NASA data)
The Chelyabinsk meteoroid is calculated to have had a total pre-atmospheric impact kinetic energy equivalent to 0.4 mega tonnes of TNT (see the red blob on the fireball map below). There were no fatalities, but there were 1,500 casualties caused by building damage, mostly windows shattered by the shockwave.
Information gathered from the Chelyabinsk event has without doubt informed the data being used for asteroid impact simulations run by the International Academy of Astronautics every other year at its Planetary Defense Conference (PDC), although the Chelyabinsk meteoroid was much smaller than the one in the most recent PDC simulation. The fictitious asteroid 2019 PDC in the simulation is between 100 to 300 metres in size, whilst modelling has shown that the Chelyabinsk meteoroid, which broke up 30 km above the surface, was probably only about 20 metres in size.
Scientists recovered significant meteorite falls from the Chelyabinsk event, including a 0.6 metre rock from the bottom of a lake, weighing in at 645 kg. Unravelling the history of the Chelyabinsk meteoroid is fascinating, as the following discussions reveals.
Based on the extent of exposure of the recovered meteorites to cosmic rays in space, the incoming 20 metre meteoroid is likely to have been a piece of the interior of a rubble pile asteroid that broke up 1.2 million years ago in an earlier near-Earth encounter.
Based on analysis of the shock veins running through the meteorites, the rubble pile asteroid may have formed 4.45 billion years ago when its own parent body was involved in a impact event that caused melting and shock veins to permeate the body. You can read about that in this 2013 article in Science.
The meteorites have been identified as LL5 ordinary chondrites in composition. Based on the fact that most LL chondrite meteorites and asteroids are spectrally similar to the Flora family that reside in the inner asteroid belt, and based on the Chelyabinsk meteoroid’s trajectory, scientists think that the Chelyabinsk meteoroid is also from the Flora family. Furthermore, based on the combined size of the inferred Flora family members, their parent body (also the Chelyabinsk meteoroid’s parent body) may have been 200 km in diameter. You can read about that in this paper which appears in Asteroids IV, the latest edition of the bible of asteroid studies.
Another event, also over Russia, which has also undoubtedly fed into the PDC simulations, is the Tunguska event of 1908.
This event was an airburst, considered to be the result of an object exploding and mostly disintegrating in the atmosphere, resulting in no discernible surface impact (unlike Chelyabinsk).
The airburst did, however, release enough energy to flatten an area of forest 2,150 square kilometres around the hypocentre, with trees flattened radially outwards. Modern estimates suggest the energy released was equivalent to around 4 mega tonnes of TNT (ten times higher than Chelyabinsk).
1908 TUNGUSKA EVENT
Top: survey 1929. Bottom: survey 1938
(credit: Leonid Kulik)
In terms of casualties, the Tunguska region was sparsely inhabited, and just three unconfirmed fatalities were reported.
The nature of the incoming Tunguska object remains a mystery and there is much ongoing scientific debate, mostly due to the shortage and uncertainty of information and data. Suggestions for the perpetrator include asteroid (rubble pile or otherwise), comet (icy body, extinct core or otherwise), space snow, antimatter, mini black hole and, of course, alien spacecraft. The perpetrator’s size is suggested to have been anywhere between 60 to 190 metres, depending on composition and structure.
Whatever caused it, there are reports that the volume of dust or water vapour turning to ice that was ejected into the atmosphere, reflected so much sunlight that people in London could read by the noctilucent clouds at night.
Seventy years after the event, scientists recovered minerals from peat bogs at the hypocentre. In 2013, these minerals were identified as being iron-bearing micrometeorites. You can read about that in these papers here and here).
One enduring conclusion since 1977 is that the object was a piece of comet 2P/Encke. This 2008 article provided a retrospective on the different theories as the debate continued on 100 years after the event.
And it still continues. One 2019 paper favours the incoming object as a rubble pile asteroid that split in two, with a larger piece causing the airburst, and a smaller piece impacting the ground to form a crater, which is now a lake a few kilometres from the hypocentre.
A geophysical survey conducted in 1999 and 2009 indicates that there is a seismic and magnetic anomaly at the bottom of the lake consistent with a stony object, however nothing has been recovered. An impact origin for the lake is as much disputed as it is proposed.
Another 2019 paper maintains that the energy required to create the vast area of flattened trees and the lack of a confirmed impact crater can plausibly be produced by either an icy comet or a rocky asteroid, under a range of model scenarios of different size, speed and entry angle.
The current consensus on what caused the Tunguska event is that there is no consensus.
The figure below from the website of the NASA/JPL Center for Near-Earth Object Studies in California (CNEOS) shows all fireball events (unusually bright meteors) observed and reported over the last three decades for which location data is available. The large red blob is the Chelyabinsk event in Russia in 2013, discussed above. Not all fireballs are reported though.
The CNEOS website also holds information about potential Earth impact risks of known near-Earth asteroids calculated for at least the next 100 years, and some calculated further into the future. It is fed by the NASA/JPL automated Sentry system which is continually scanning for new near-Earth asteroid observations and orbit data from the IAU Minor Planet Center. As well as NASA’s Sentry database, there is ESA’s NEODys database. As new data become available, some objects may be removed from the lists and new ones appear and the risk of impact may be increased or downgraded.
The visualisation below by data enthusiast Caroline Pierce uses the NASA Sentry data for the 850 known near-Earth asteroids with a risk of impacting Earth in the next 100 years (data as at 18 August 2018). The size of the circle represents the relative size of the asteroid; the colour of the circle represents the cumulative impact probability (see the legend) which means the total probability of the object impacting Earth given all its potential impact risks over the next 100 years. The highest cumulative probabilities (> 1 in 1,000) are shown in bright orange in the centre. What does that probability actually mean? Well, the probability of getting heads every time on 10 consecutive flips of a coin is 1 in 1,024, but it might take less flips or it might take a lot more flips (I’ve never been able to achieve more than four heads or tails in a row). Here’s the link to the web page for this visualisation where you can hover over each blob to see the name and risk data for each asteroid.
Because the above visualisation is limited to 100 years, the very highest potential impact risks on the NASA Sentry database from asteroid 101955 Bennu (1999 RQ36) (the near-Earth asteroid currently being visited by NASA’s OSIRIS-REx mission) and asteroids 410777 (2009 FD) and 29075 (1950 DA), are not shown because the impact risks for those asteroids are more than 100 years into the future.
Simulating an asteroid impact disaster
Recall that this post is about real and simulated asteroid and comet impacts and, as referred to earlier, every other year since 2013 the International Academy of Astronautics (IAA) has played out its own hypothetical asteroid impact scenario. Expert participants at its biennial Planetary Defense Conference (PDC) are challenged to save Earth from destruction in a non-screen version of Asteroid vs Earth which is as real as we can get in the absence of having experienced an actual catastrophic impact on Earth.
The PDC simulations are fed by data from the NASA/JPL Center for Near-Earth Object Studies in California (CNEOS) who compute the high-precision orbits of near-Earth asteroids and comets, their close approaches to Earth, and their probability of impacting Earth.
Orbit of fictitious asteroid PDC 2019 ( NASA/JPL)
The 2019 Planetary Defense Conference was held in Maryland, USA, from 29 April to 3 May. The scenario presented to participants in that year involved a hypothetical 60 metre asteroid fragment which was on course to impact Earth with absolute certainty on 29 April 2027, heading for New York’s Central Park. The incoming rock was a fragment of a five times larger asteroid named 2019 PDC, which was originally on course to impact Denver, Colorado.
A global effort using multiple kinetic impactors launched from US, Japanese, and Russian spacecraft had failed to deflect the whole of the larger asteroid; part was deflected away from Earth, but the 60 metre fragment broke off into a different trajectory, heading for New York.
Simulated PDC 2019 airburst damage zone (2019 IAA PDC)
That will all sound familiar to movie buffs out there, who may now be hearing one of Aerosmith’s biggest mainstream hits playing in their head. But unlike the task of the space-bound oil rig workers in that 1998 movie, launching a nuclear explosive device to break-up the remaining 60 metre fragment of PDC 2019 could not be implemented by this year’s conference participants, due to the insufficient lead time after the original discovery of the asteroid.
Despite managing to save Boulder from the boulder, the conference participants didn’t manage to save New York from the 60 metre fragment. The meteoroid exploded in an airburst over New York city, leaving an unsurvivable red zone of about 83 square kilometres surrounding Central Park, killing in the region of two million people. For those not vaporised in the red zone, or buried in the general devastation, people’s clothes ignited, with burns victims reported over 700 square kilometres. Then they were the wider, environmental effects to consider.
Unlike all the movies with similar plots, the PDC exercise should not be considered a drama. It’s a real-life possibility, played out as close to the real thing as the experts know how today. And for each year the exercise is run, the better they get at understanding how to mitigate the threat posed by these Potentially Hazardous Objects with Earth’s name on them.
Simulating a comet impact disaster
There is a similar hypothetical comet impact scenario on the PDC website for the hypothetical long-period comet C/2019 PDC, discovered on 4 April 2019 with a risk of impact on 28 February 2021.
Orbit of fictitious comet C/2019 PDC (NASA/JPL)
Obtaining a good idea of the size of the comet (nucleus) as it moves in closer to the Sun is hindered for months by its coma, although some idea of size can be obtained from the amount of outgassing.
But the downside to the outgassing is that it contributes to non-gravitational acceleration of the comet’s orbit, meaning the impact risk can remain uncertain up until the final few months prior to its encounter with Earth.
Unlike the hypothetical asteroid 2019 PDC impact scenario described earlier, where the impact region could be predicted with some certainty, the large uncertainty of the comet’s impact region means there are hundreds of potential impact sites over an entire hemisphere of the Earth.
Potential impact hemisphere for C/2019 PDC (2019 IAA PDC)
Lights, camera and asteroid!
That’s the end of my post on Real and Simulated Asteroid and Comet Impacts. If you want clarification on any of the key terms used in this article, read my post Meteoroids, Meteors and Meteorites. If you want to read behind all the apocalyptic tabloid news headlines, read my post Asteroid Warning! To see how all this plays out in the movies read my post Making an Impact: Lights, Camera and Asteroid!
Don’t forget Asteroid Day is on 30 June.