Hot Rain from the Heavens

A Bit Close for Comfort

Throughout the Earth’s history, rocks from above have been a constant threat to what is on the Earth. 3.8 billion years ago, just 700 million years into Earth’s history, the Late Heavy Bombardment took place, where debris from planets being made, smashed into Earth, keeping it in a molten state.

Then came the fall of animals such as most of the dinosaurs, 65 million years ago, where a 10km wide asteroid fell into the Yucatán Peninsula, in the south east of Mexico, south of the Gulf of Mexico, created the 300km diameter Chicxulub crater, bringing about a global winter and global fires, among other things [1].

This turbulent past has shaped our planet, bringing humans to be Earth’s dominant species. But the threat of a future impact is always there, though 10km wide asteroids are, luckily, very uncommon. That said, even relatively small asteroids can sometimes cause a threat if they enter the atmosphere.

Chicxulub Crater Gravitational Anomalies due to Asteroid Impact. Credit: US Geological Survey (white line is coast)

Natural Defence

However, our Earth has ingenious ways at tackling some of these beasts of space: it’s called Earth’s atmosphere. It’s a relatively thick layer of gasses (compared to other rocky planets in our solar system, except Venus), mainly made of nitrogen (~78%) and oxygen (~21%) (shown by how the sky is blue, since blue light is scattered more than other colours due to this composition [2]) [3].

Because of the air resistance this layer of gas causes, objects like meteors burn up easily, before hitting the ground, leading to phenomena such as meteor showers, caused by trails of debris left by comets’ dust tails and asteroids [4], as well as fireballs (but only some of them get burnt up), leading to temperatures as high as 1,650ºC (hence why rockets need heat shields) [3].

To see the whole story, however, we must travel to the far side of the Moon; an old surface, untouched for millennia, without the thick atmosphere we have.

The Moon’s volcanism has covered many of the craters on the near side in lava flows and left large 'sea-like' maria, whilst the far side has a thicker crust, so lava can’t so easily escape and cover up its blemishes, showing just how frequently hit the Moon is, and how, due to lack of weathering, erosion and plate tectonics, these marks still remain, unlike on Earth, where craters are still being discovered to this day, or have fallen into the mantle because of subduction (one plate going underneath another plate) [5] [6].

This means that the Earth could be impacted more than we think by these monsters in the sky (over geological time, that is), so the question then arises: what would happen if an asteroid big enough to get though the Earth’s thick atmosphere reached the ground?

Far Side of Moon by Lunar Reconnaissance Orbiter (LRO). Credit: NASA/GSFC/Arizona State University

Fire in the Sky

In fact, this question is answered every year, in regular events called Fireballs. This name may sound quite cinematic, and indeed they are, from time to time, quite cinematic objects (when seen from a distance!).

In Chelyabinsk at 3:20:26 GMT on 15th February 2014, the biggest fireball since 1908 (in Tunguska) -with both events happening in Russia (2,420km from each other -1,500 miles)- fell from the sky, with an impact equivalent to 440 kilotons of TNT [7], and a diameter of ~15 meters, weighing ~7,000 tons and travelling 64,800 kph (40,260 mph). It wounded ~1,000 and damaged many buildings by shattering glass, for instance [8].

This event was something that may make it seem unlucky to live in Russia, since these event are really very rare, but they still caused only local impacts, unlike the global impacts of the meteorite that wiped out most of the dinosaurs.

In fact, fireballs are usually much less dramatic, and occur over the sea (since there is more sea than land on Earth), but in totally random locations (as seen by the map of fireballs since 15th April 1988 by NASA's JPL Centre for Near Earth Object Studies).

One such event happened just 3 days ago, at 10:21:24 GMT on 18th November 2020, over the South Tasmanian Sea, on nearly the opposite side of the world. As is with most fireballs, this one fell over the sea; specifically over the Southern Ocean, just south of Tasmania (an island south east of Australia). The live stream camera on a research vessel 'Investigator' (operated by the CSIRO or ‘Commonwealth Scientific and Industrial Research Organisation’) picked up the fireball, black and white in the image but bright green to observers. Events like these usually go unnoticed, with “over 100 tonnes of natural space debris” entering the atmosphere daily (NAGLE Glen, 2020), with a few of them becoming fireballs like this one. You’ve just got to be at the right place at the right time to observe them (or sometimes the wrong place at the wrong time, as was the case for many in Chelyabinsk!) [9] [10] [11].

Footage from the ship 'Investigator' of fireball. Credit: CSIRO


Luckily for us, the world is ready for the next big fireball, with many telescopes scanning the skies for potentially hazardous objects, such as the Arecibo radio telescope, talked about in the last article for the site's use in sending the the Arecibo Message into space. It has recently been decommissioned due to damage done to the cables that support the platform suspended over the dish below, bringing an end to this iconic telescope [19].

However, the current projects of Pan-STARRS 1 and 2, (which stands for the “Panoramic Survey Telescope and Rapid Response System”) are carrying on its work. They're located on top of Haleakala on the island of Maui in Hawaii and are funded by NASA’s Near Earth Observation Program, designed to take 4 exposures of 1,000 square degrees of the sky every hour during the night. Then, data on objects with unusual motions is sent to the Minor Planet Centre, where information like the orbit and size of the object is determined, to see if it is a threat [12]. Then, around 3 weeks prior to an impact, information can then be sent to people in the ‘at risk’ area, preventing injury, so events like that seen in Chelyabinsk, with injury from shattered glass, won’t be seen [13].

Then, in the extremely unlikely event that a very large asteroid is bound for Earth, there are various different options available to either deflect it or break it up. One such method, submitted to US Congress in 2007 and called the most effective method by NASA, would be to: launch a nuclear bomb into space, detonate it, and so deflect the asteroid away from the Earth. However nice that sounds, global nuclear fallout would be a slight issue, so a “kinetic impactor” has also been proposed, being a projectile (like a rocket) which would nudge the asteroid in the right direction (although key facts like the momentum, trajectory and composition of the asteroid must be known) [14].

One mission that will help our understanding of asteroids to better our chances of surviving an impact, and to get some cool science, is the recent OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) mission, which, on 28th October this year, collected a sample of the asteroid Bennu (named after the Ancient Egyptian god of the Sun. Discovered in 1999, this asteroid is now classed as potentially hazardous (with an orbit getting close to the Earth -within 7.5 million km or 0.03x the distance from the Earth to the Sun- and a size of ~1500 sq. metres, or, as NASA put it, the size of a car park with 100 spaces), but also is rich in Platinum and Gold, along with possibly holding some secrets to the origins of life on Earth, since it has gone billions of years undisturbed. The samples (which were nearly lost to space [15]) are going to get back on 24th September 2023, shedding new light on these dangerous, yet useful and exciting objects [16] [17].

Sample Return Capsule before & after collection. Credit: NASA/Goddard/Arizona University/Lockheed Martin

Glittering Comet Dust

While we’re waiting for the results of that mission, there are some meteors that you can observe yourself over the coming months. The peak of the Leonids has just passed us by, happening on 16-17th November (although you might still be able to see some), but the Geminids are on there way in mid December (13-14th December), caused by the dust left by the asteroid 3200 Phaethon, currently in the middle of Venus’ and Mercury’s orbits, and visible in the early hours heading through Virgo at an apparent magnitude of 16.11 (needs a good telescope). This is a pretty good shower too, with, at its peak, 120 meteors per hour [18].

By George Abraham, ADAS member

#NEO #FireBall #MeteorShower #Asteroid #Meteor

Click here for the previous news article

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Click here to find the current location of asteroid Bennu, and here for 3200 Phaethon (click here for a list of websites like JPL Small-Body Database Browser, to help with amateur astronomy)

Click here for information on what the current sky is like, to see if you’ll have a chance to observe some NEOs (Near Earth Objects).

Click here to see members’ pictures of NEOs (please send in your own pictures using the link at the bottom of the gallery page)

if you want to appear in our member gallery, or send in your observers reports, which will appear on our news page).


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  17. "Ten Things to Know About Bennu". NASA. Archived form the original on 21st November 2020.

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