# Large Asteroid Impacts Earth…

## … And Hardly Anyone Notices!

Five years after the Chelyabinsk asteroid impact, a three-in-a-century event happens again over the Bering Sea.  And almost no-one notices.  I say “no-one”… but the Earth is a planet under constant scrutiny.

18 December 2018.  Around midday local time, near the Russian Kamchatka Peninsula, an asteroid barrels at 32 km/s through the atmosphere, on a steep trajectory of 7 degrees, before exploding over the Bering Sea.

A “huge fireball”, according to NASA scientists, that went largely unnoticed…

### Space Dust

Every day, Earth is bombarded with over 100 tons of dust and sand-sized particles.  About once a year, an automobile-sized asteroid hits Earth’s atmosphere, creates an impressive fireball, and burns up before reaching the surface.

Asteroid: A relatively small, inactive, innocuous rocky body orbiting the Sun…  Innocuous enough, until it crosses path with Earth and impacts a populated region.

Ground-based observers and CCTV cameras sometimes register these events at night, and more rarely during daylight hours, as impressive atmospheric light displays, and sometimes accompanying shock wave.

“It was over the Bering Sea so it didn’t have the same type of effect or show up in the news.”

Kelly Fast, near-Earth objects observations programme manager at NASA.

A few people will remember the 20-metre bolide that ripped through Russian skies over Chelyabinsk in 2015.

But many such meteors remain unseen, and that is mostly because they occur far out at sea.

However, Earth is a well scrutinised planet with constellations of satellites always gazing down at our planet.

## Looking Down From Up Above

Military satellites picked up the blast last year.

NASA scientists were notified of the event by the US Air Force.

Opposite, Terra EOS AM-1) captured images of the Bering Sea bolide.  The Terra satellite  is a multi-national NASA scientific research satellite in a Sun-synchronous orbit around the Earth – the flagship of the Earth Observing System (EOS).

Dr Lindley Johnson, NASA Planetary Defence officer, said the fireball was detected over an area close to commercial flight routes between North America and Asia.

Researchers have been checking with airlines to see if there were any reported sightings of the event.

## Vital Statistics

The Bering Sea bolide is the second largest asteroid of its kind in only 30 years.

Himawari 8 – a Japanese weather satellite – captured the large asteroid crashing through the atmosphere.  The orange meteor trails in the centre of the animated photographs.

The biggest since the Chelyabinsk impact six years ago in Russia.

Several metres in size, the space rock exploded 25.6 kilometres above the Earth’s surface, with an impact energy of 173 kilotons.

#### The Bering Sea event released an energy equivalent to 40% the energy of the Chelyabinsk fireball.

How do we know that?

## Earth Under Scrutiny

A visualisation of the sizeable asteroids that hit the Earth in recent years was released in 2014 by the B612 Foundation – a United States-based group of researchers and former NASA astronauts campaigning on the issue of space protection.

Their presentation relied on data from the Comprehensive Nuclear-Test-Ban Treaty (CTBT) – a multilateral treaty that bans all nuclear explosions, for both civilian and military purposes, in all environments.

An array of sensors monitors any clandestine atomic explosions.

The infrasound network registered 26 large explosions on Earth, between 2000 and 2013, with magnitudes ranging from 1 kiloton to 600 kilotons!

All 26 events were the result of various asteroid impacts.

A single one had been anticipated, and the course of the object had been detected just hours before the event.

### TNT Equivalent

TNT equivalent is a convention for expressing energy, used to describe the energy released in an explosion.  It compares the destructiveness of an event with that of traditional explosive materials, such as TNT (Trinitrotoluene).

The “ton of TNT” is a unit of energy defined by that convention to be 4.184 gigajoules, which is the approximate energy released in the detonation of one metric ton (1,000 kilograms) of TNT.

For example, the atomic bomb that destroyed the Japanese city of Hiroshima was a 15-kiloton device.

The explosion over the Bering Sea released over 10 times the energy released by the Hiroshima atomic bomb.

Dr Johnson confirmed that this type of impact event is normally expected about 2 or 3 times per century only.

## Problems without Passports

Such large-scale space rocks are so-called “problems without passports” because they are expected to affect entire regions of the World, should they collide with Earth.

To be fair, most of these space rocks disintegrate upon entry into terrestrial atmosphere and cause little problem at ground-level.  Fortunately.

Nevertheless, in 2005, the United States Congress tasked NASA with finding 90% of near-Earth asteroids of 140 metres (460 ft) in size or larger by 2020.

Scientists now estimate it will take them another 30 years to fulfil this congressional directive.

## Fireballs and Bolides

Fireballs and bolides are astronomical terms describing exceptionally bright meteors that are spectacular enough to to be seen over a very wide area.

The Center for Near Earth Objects Study (CNEOS) published an online dynamic application showing a map of asteroid impacts throughout the World.

A dynamic world map shows a visual representation of the data table that provides a chronological data summary of fireball and bolide events provided by U.S. Government sensors, over the past four decades: from 15 April 1988 to 15 March 2019.

The map shows all the reported fireball events for which geographic location data are provided.

Each event’s calculated total impact energy is indicated by its relative size and colour.

The two most notable asteroid impact events on the CNEOS map happened just recently, and within a comparatively short time span:

• Bering Sea (172.4°, 56.9°), 18 December 2018, 173 kiloton
• Chelyabinsk (61.1°, 54.8°), 15 February 2013, 440 kiloton!

For each reported fireball event, the accompanying data table provides information on the date and time with its approximate total optical radiated energy and its calculated total impact energy.

When reported, the event’s geographic location, altitude and velocity at peak brightness are also provided.

Note. Data are not provided in real-time and not all fireballs are reported.  A blank (empty) field in the table indicates the associated value was not reported.

Although Chelyabinsk was a terrifying experience for those caught up in it, the impact event was small by comparison with some of the incomers recorded throughout Earth history.

## Statistics for Historical Impacts

There are plenty of examples of asteroid impacts at the surface of the Earth, or at the bottom of the sea.

### The Chicxulub Example

The 66-million-year old Chicxulub event is believed to be the cause of the Cretaceous-Paleogene extinction.

The Chicxulub impactor had an estimated diameter between 11-81 kilometres (6.8-50.3 miles).

The asteroid delivered an estimated energy of 21-921 billion Hiroshima A-bombs (between $1.3 \times 10^{24}$ and $5.8 \times 10^{25}$ joules.

The object dug a hole 100 kilometres (62 mi) wide and 30 kilometres (19 mi) deep, leaving a crater mainly under the sea and covered by 600 metres (2,000 ft) of sediment by the 21st century.

## Anticipating the Next Asteroid Impact

Once an incoming object can be identified, NASA has had notable success at calculating where the impact will occur on Earth, based on the precise determination of its orbital path.

An asteroid’s orbit is calculated by finding the elliptical path about the Sun that best fits the available observations of the object.  That is, the object’s computed path about the sun is adjusted until the predictions of where the asteroid should have appeared in the sky at several observed times match the positions where the object was actually observed to be at those same times.

As more and more observations are used to further improve an object’s orbit, we become more and more confident in our knowledge of where the object will be in the future.

## The Torino Scale

The Torino Scale is a method for the categorisation of the impact hazard associated with near-Earth objects (NEOs), such as asteroids and comets.

This communication tool for astronomers and the public assesses the seriousness of collision predictions, by combining probability statistics, and known kinetic damage potentials, into a single value.

A score of 0 indicates an object with a negligibly small chance of collision with the Earth, compared with the usual “background noise” of collision events, or one that is too small to remain intact upon entry into the atmosphere.

A 10 indicates that a collision is certain, and the impacting object is large enough to precipitate a global disaster.

Time is a critical consideration.  The sooner an Earth-bound asteroid is detected, the greater the chance of mitigating its risk successfully.

The 3 metre (10ft) asteroid 2018 LA was discovered by a ground-based observatory in Arizona, just eight hours before impact.

The Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory (JPL) made a precision determination of its orbit, which was used to calculate a probable impact location.  The result showed the rock was likely to hit southern Africa.

As the calculation suggested, a fireball was recorded over Botswana by CCTV camera on a farm.

Later, asteroid fragments were found in the area.

## Three-in-a-Century Event

The CTBT data suggest that Earth is hit by a multi-megaton asteroid, large enough to destroy an entire city if it hit a populated area, about once every 100 years.

The latest event over the Bering Sea shows that larger objects can collide with us without warning.

A more robust network is needed to ensure enhanced monitoring, dependent not only on ground telescopes, but space-based observatories also.

### Space-Based Solutions

The Sentinel Space Telescope would have tracked 90% of Earth-orbit-crossing asteroids larger than 100 metres, and 50% of 30-metre space rocks.

The craft would have been placed in an orbit similar to that of Venus, allowing it to clearly survey the night half of the sky every 20 days.  It would have picked out inner Solar System rocky bodies that are currently often difficult, if not impossible, to see in advance from Earth because the glare of the Sun hides them.

Sentinel would have had an operational mission life of from six and a half to ten years.  After NASA terminated their funding agreement with the B612 Foundation in October 2015, the Foundation opted for an alternative using a constellation of smaller spacecrafts.

NASA/JPL’s NEOCam (Near-Earth Objects Camera) is currently the proposed option.

NEOCam would survey from the Sun–Earth L1 Lagrange point, a specific gravitational balance point, allowing it to look close to the Sun and see objects inside Earth’s orbit.

NEOCam is the successor to the WISE mission.

The principal investigator is also WISE’s principal investigator, Amy Mainzer of NASA‘s JPL.  Recently, she indicated that if the mission did not launch:

[The search for near-Earth asteroids] would take us many decades to get there with the existing ground-based surveys.

With an IR-based telescope, it goes a lot faster.

The mission concept would see NeoCam launched to a gravitational balance point in space, where it would discover and characterise potentially hazardous asteroids, larger than 140 metres.

The idea for the scientists is to get as close as possible to that 90% goal of finding the 140-metre and larger near-Earth asteroids given to NASA by Congress.

While previous findings suggested that a little over 90% of the really large problematic asteroids out there have already been found, NASA’s WISE telescope indicates that space rocks in the size range 100-1,000 metre number 20,000 with the majority of them yet to be identified and tracked.

The hope of researchers is that the visualisation tools they created will increase understanding of the problem scale, and drive home the realisation that large asteroid impacts are indeed much more common than previously thought.