A Look at Earth's Planetary Defense Systems in Preparation for Doomsday

Within our solar system, there are thousands of objects that cross Earth's path, some of which could even collide with us someday. How are we ensuring this doesn't happen?
Matthew S. Williams
The Manicouagan impact crater (center-right)International Space Station

It has become something of a trope thanks to Hollywood, science fiction writers, and fans of doomsday scenarios alike. A sizeable comet or asteroid is on a collision course with Earth, and news of its impending impact causes widespread panic and hysteria.

While the people of Earth dig in and prepare for the worst, the nations of the world come together in a last-ditch effort to destroy it and save the planet. As the plot of a major motion picture or novel, the stuff practically writes itself!

However, as with any good story, there's a strong element of truth to this scenario. For billions of years, planet Earth has come into contact with asteroids, comets, and other pieces of debris.


Granted, the vast majority of these were so small that they burned up in our atmosphere, or caused little to no damage on the surface. And more often than not, asteroids that exist in near-Earth space (known as Near-Earth Objects or NEOs) will pass us by at a safe distance.

But on occasion, there have been some impacts that were so powerful that they did more harm than a thermonuclear bomb.

Every few millions of years, there have even been impacts that have triggered mass extinctions.

It is little wonder then why space agencies around the world make it their business to track and monitor any and all NEOs that we know of. It is also understandable that for decades, these same agencies and government planners have been working on strategies for deflecting or destroying any asteroids that come too close to Earth, also known as planetary defense.

Which begs the question: how prepared are we for a doomsday-type asteroid-impact scenario?

What are NEOs?

The term Near Earth Object (NEO) refers to any small body in the Solar System whose orbit periodically brings it into close proximity with Earth. They typically consist of comets and asteroids that have been nudged by the gravitational attraction or nearby planets into orbits that cross Earth's orbit around the Sun.

Whereas comets are composed mostly of water ice and embedded dust particles and formed in the cold outer reaches of the Solar System (the Kuiper Belt), most rocky asteroids are believed to have formed in the warmer inner Solar System between the orbits of Mars and Jupiter (the Main Asteroid Belt).

Over time, planets like Jupiter and Neptune would have caused objects in these belts to be kicked out, which then made their way towards the Sun and the inner Solar System. Because they are bodies that are composed of relatively unchanged material leftover from the formation of the Solar System (4.6 billion years ago), they are the subject of scientific interest.

However, scientists are also interested in NEOs because of the collision risk associated with them. While collisions are quite rare, the fact that NEOs will occasionally cross Earth's orbit means that sooner or later, one of them could slam into Earth.

According to the European Space Agency's (ESA) NEO Coordination Center, there are currently 20,304 known NEOs. Of these, an estimated 868 could pose a collision risk to Earth. For this reason, there are several agencies that are responsible for tracking these objects and alerting the public in the event of a threat.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Artist's impression of 1999 KW4, a binary asteroid that recently passed by Earth. Source: ESO

Potential Risk

Put simply, the odds of any known NEO colliding with Earth are about as good as winning the lottery or getting hit by a falling piece of plane debris. But when you consider the possible outcomes of a collision, it makes sense to be prepared.

As an example, consider (101955) Bennu, an asteroid that was discovered in 1999 and has been the subject of study by NASA's OSIRIS-REx spacecraft since 2018. This 246-meter (807 ft) asteroid is currently orbiting the Sun at a distance of 87 million km (54 million mi) and a speed of about 101,400 km/h (63,000 mph). 

While this body has only a 1 in 2,700 chance of colliding with Earth, the resulting impact could generate a blast as powerful as 1.15 gigatons. That is over 23 times more powerful as the largest thermonuclear test ever conducted, which was 50 megaton RDS-220 (Tsar Bomba) detonated over Novaya Zemlya Island by the Soviet Union in 1961.

The fireball created by the detonation was 8 km (5 mi) wide and was visible from up to 1000 km (620 mi) away. The resulting mushroom cloud reached 67 km (42 mi) high - that's seven times the height of Mount Everest! - and was 95 km (59 mi) wide at its peak and 40 km (25 mi) wide at its base.

All buildings within 55 km (34 mi) of ground zero were obliterated, structures hundreds of kilometers away were also destroyed, and it is estimated that anyone standing 100 km (62 mi) of ground zero would have suffered third-degree burns.

That's a surface area larger than the city of New York, which means that a destructive force of 50 megatons would be enough to wipe over 9 million people off the map in mere seconds. When you multiply that by 23 times, you begin to see just how deadly and devastating a serious impact could be!

Risk Assessment

In order to measure the risk of collisions from individual NEOs, scientists rely on the Torino scale and the more complex Palermo scale.

The former, known officially as the Torino Impact Hazard Scale was adopted by the International Astronomical Union (IAU) in 1999 and consists of an integer scale ranging from 0 to 10 with five associated colors.

  • White Zone (0, "No Hazard"): The category establishes that there is "No Hazard" - i.e., likelihood of a collision is zero or is so low as to be negligible. It also applies to meteors and small bodies that enter the atmosphere and either burn up or rarely cause damage. 
  • Green Zone (1, "Normal"): This category applies to "Normal" discoveries that will pass near the Earth and where the chance of collision is extremely unlikely with no cause for public attention or public concern.
  • Yellow Zone (2-4): This category involved bodies that are judged to be "Meriting Attention by Astronomers," where a close flyby will take place but a collision is judged to be very unlikely.
  • Orange Zone (5-7): This category applies to bodies that are considered "Threatening." These are those that will conduct a close flyby with Earth, but for which the chance of a catastrophic collision is still unknown.
  • Red Zone (8-10): This final category is reserved for "Certain Collisions," where an object will not only cross Earth's orbit but will definitely collide with Earth, causing anywhere from localized damage to global destruction.

This more simplified scale captures the likelihood and consequences of a potential impact but does not consider the time remaining until the impact could occur. It is intended primarily to facilitate public communication by the asteroid-monitoring community.

For more complicated assessments, scientists rely on the Palermo Technical Impact Hazard Scale. This scale was developed to enable NEO specialists to categorize and prioritize potential impact risks by combining two types of data - the probability of impact and estimated kinetic yield—into a single "hazard" value. 

Like the Torino scale, the Palermo scale employs integer values from 0 to 10, but which are based on the predicted impact energy as well as the likelihood of the event. The scale also compares the likelihood of a specific potential impact with the "background risk" - the average risk posed by objects of the same size or larger until the date of the potential impact. 

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Image of Itokawa taken by Hayabusa in 2005. Source: NASA/JAXA

In contrast to the Torino scale, the Palermo scale is logarithmic, meaning that a Palermo Scale value of zero is just as threatening as the background risk. Those events that have a value of -2 indicates that the potential impact event is only 1% as likely as a random background event while a value of +2 indicates an event that is 100 times more likely than any other impact.

The Palermo Scale is used by NEO specialists to quantify in more detail the level of concern warranted for future potential impacts. Much of this scale's utility is due to its ability to carefully assess the risk posed by less threatening Torino Scale 0 events, which comprise nearly all of the potential impacts detected to date.

Major Impacts in the Past

Suffice it to say; Earth has a very long history of asteroid and meteor impacts. In fact, astronomers estimate that shortly after the formation of the Solar System, asteroids and comets pelted Earth and the other planets of the inner Solar System with extreme frequency.

Fortunately for us, impacts have become a much more rare phenomenon in recent eons. And the particularly large craters caused by bigger impacts in the past have all but been covered up thanks to geological activity and surface renewal.

Nevertheless, there are still many impacts that have made a mark on Earth's terrestrial and biological evolution, the evidence of which is still contained in Earth's geological record. And there have been several that have occurred since the emergence of humanity, which also had a drastic effect on our history and evolution. Here are just a few examples.

Theia Impact:
In accordance with the Giant Impact Hypothesis (the most widely-accepted theory of how the Earth-Moon system formed), Earth was struck by a Mars-sized astronomical body roughly 4.5 billion years ago.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Artist's impression a massive impact. Source: NASA

This was just 100 million years after the formation of the Earth, and it resulted in the surface of both bodies becoming hot magma. Some of this magma was thrown into space, where it cooled and coalesced to form the Moon.

This theory emerged as a result of the Apollo lunar missions, which brought back samples of lunar rock that were surprisingly similar in composition to those on Earth, indicating that they had a common origin.

Warburton Impact:
With the exception of Theia, the impact event that formed the Warburton Basin in southern Australia is thought to be the largest impact in the history of planet Earth. Based on geological evidence, the impact is believed to have been caused by two asteroids measuring 10 km (6 mi) across.

While the crater from the impact has long since disappeared, the Warburton Basin - which measures 400 km (250 mi) in diameter and was discovered about 3 km (1.86 mi) beneath the Earth’s crust - is evidence of this ancient event.

Chicxulub Impact:
Perhaps the most well-known impact event, the Chicxulub impactor struck Earth roughly 66 million years ago. This body measured between 11 and 81 km (7 to 50 mi) across and is thought to be what caused the Cretaceous–Paleogene extinction event (the K-T extinction event).

This is none other than the extinction level event (ELE) that caused the death of most species of land-dwelling dinosaurs and allowed for the rise of mammalian species.

The Chicxulub impact crater is located in the Yucatán Peninsula in Mexico, at depths ranging from 10 to 30 km (6.2 to 18.6 mi) beneath the Earth's crust. The crater is estimated to measure about 150 km (93 mi) in diameter and 20 km (12 mi) in depth.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Artist's impression of the Chicxulub impact. Source: NASA/JPL-Caltech

Tunguska Impact:
This event, which took place on June 30th, 1908 in Eastern Siberia, was the largest impact event on Earth in recorded history. And while the meteoroid that was responsible did not technically strike Earth, but exploded in our atmosphere (an air burst), it is still classified as an impact event.

The resulting explosion caused widespread damage to the Eastern Siberian Taiga, flattening 2,000 km² (770 mi²) of the forest. Luckily, since the explosion happened over a sparsely populated region, it is not believed to have caused any human casualties.

Different studies have produced different estimates for the size of the meteoroid, ranging from 60 to 190 m (200 to 620 ft), depending on whether it was a comet or an asteroid. The object is thought to have disintegrated at an altitude of 5 to 10 km (3 to 6 mi) above the surface.

Chelyabinsk Meteor:
This impact event is the most recent on record, which involved an extremely bright meteor (superbolide) entering Earth's atmosphere and exploding over the small southern Ural town of Chelyabinsk, Russia, on February 15th, 2013.

This event was caused by a NEO measuring approximately 20 m (66 ft) in diameter which was traveling at speeds of about 20 km/s (12.5 mi/s). The resulting airburst caused a shockwave that inflicted damage to 7,200 buildings in the region, as well as causing 1,500 injuries (but no reported deaths).

The light from the meteor was temporarily brighter than the Sun and could be seen by observers up to 100 km (62 mi) away. Some eyewitnesses also reported feeling the intense heat from the fireball, despite the otherwise freezing conditions at the time.

Current Strategies

At present, all mitigation strategies for possible collisions involve careful monitoring and public alerts. There are two independent systems that calculate orbital intersections in order to determine if there is a risk of collision. These include NASA's Sentry system and the ESA's Near Earth Objects Dynamic Site (NEODyS).

Observations and orbit solutions of NEOs are regularly received from the Minor Planet Center (MPC) in Cambridge, Massachusetts. When new NEOs are discovered that are deemed to be a potential risk, they are posted on the Sentry Impact Risk Page.

In the vast majority of cases, newly-discovered objects will be removed as new observations become available, our understanding of the object's orbit is improved, and its future motion is more tightly constrained.

As a result, several new NEAs each month may be listed on the Sentry Impact Risk page, only to be removed shortly afterward.

However, there are objects that have been lost by trackers, which resulted in them being made permanent residents of the Risk Page (their future removal will depend entirely on rediscovery).

NEODyS, meanwhile, is an Italian and Spanish service that has provided a continuous and almost automatically maintained database of NEO orbits. Since 2011, the European Space Agency has been an active sponsor of NEODyS, which now pays for part of their operating costs.

The majority of work regarding NEO orbits and risk computations are performed by the University of Pisa's Department of Mathematics and by the National Institute for Astrophysics' Milan Institute of Space Astrophysics and Cosmic Physics (IASF-INAF) in Rome.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Near-Earth Object Bennu, photographed by the OSIRIS-REx mission on approach in. Source: NASA/Goddard/University of Arizona

Beyond NASA and the ESA, there are also many organizations around the world dedicated to tracking NEOs and developing the necessary technology to deflect or destroy those that pose a threat to Earth.

In 2013, the UN established the International Asteroid Warning Network (IAWN) to bring these organizations together. The UN also mandated the creation of the Space Missions Planning Advisory Group (SMPAG), which is tasked with coordinating joint studies to develop asteroid-deflection missions, and as well provide oversight for these missions.

In 2016, the Committee on Homeland and National Security within the National Science and Technology Council (NSTC) created the Detecting and Mitigating the Impact of Earth-bound Near-Earth Objects (DAMIEN) interagency working group. This body was charged with developing strategies and technologies for meeting the threat posed by future impacts by NEOs.

Potential Strategies

Beyond monitoring NEOs and keeping the public informed about possible collisions, a number of strategies for planetary defense are also being researched and developed by space agencies and private organizations.

These include everything from high-speed spacecraft that would collide with asteroids, to directed-energy (lasers) that would push an asteroid off course. There are even some options for using nuclear warheads to deflect or destroy them. Some examples include the following. 

A popular method is the concept of a Hypervelocity Asteroid Intercept Vehicle (HAIV) that would intercept an asteroid, collide with it at very high speeds, and redirect so that it would not collide with Earth.

A good example of this is the Double Asteroid Redirection Test (DART), a kinetic impactor demonstrator currently being developed by NASA. As the first mission of its kind, this mission will be launched in the coming years to test the effectiveness of using a spacecraft to change an asteroid's motion in space.

The target for this mission is the NEO that is known as (65803) Didymos, a binary asteroid consisting of a 780-meter (2,550 ft) primary body and a 160-meter (525 ft) secondary body (or "moonlet"). It is this secondary body that will be used to test DART, once it is operational.

The DART spacecraft will rely on a NASA Evolutionary Xenon Thruster – Commercial (NEXT-C) solar electric propulsion to achieve a speed of about 6.6 km/s (4 mi/s) - 23,760 km/h; 14,760 mph. It will use autonomous navigation software to deliberately crash itself into the moonlet while an onboard camera (DRACO) will record the process.

The collision will change the speed of the moonlet's orbit around the main body by a fraction of one percent, which will alter its orbital period of the moonlet by several minutes - which will be observed and measured by telescopes on Earth.

The DART spacecraft is scheduled to go in late July 2021 and intercept Didymos’ moonlet in late September 2022. At this time, the Didymos system will be within 11 million km (6.8 million mi) of Earth and observable using ground-based telescopes.

The DART mission is currently in Phase C of development, a process that is being led by NASA's Applied Physics Lab (APL) and managed under the NASA Planetary Defense Coordination Office (PDCO) and the Science Mission Directorate’s Planetary Science Division at NASA HQ in Washington, DC.

Directed Energy System for Targeting of Asteroids and exploRation (DE-STAR) is a proposed system to deflect asteroids, comets, and other NEOs using lasers. This project is the result of work conducted by the UCSB Experimental Cosmology Group (ECG), led by Professor Philip Lubin.

The plan calls for a modular phased array of kilowatt lasers powered by solar arrays that would be placed on orbital platforms. These would be capable of heating the surface of a potentially-hazardous object to the point of deflection or vaporization.

The ECG envisioned two possible versions of the technology, the larger "stand-off" DE-STAR arrays that would remain in Earth orbit and deflect targets from afar, and the much smaller “stand-on” DE-STARLITE system, which travel to the targets and deflect as they flew alongside.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Could directed-energy (lasers) be used to deflect or destroy asteroids? Source: UCSB ECG

In both cases, a highly-focused beam of laser energy would raise the temperature of a spot on the target’s surface to ~3000 K (2725 °C; 4940 °F). This would cause surface material to sublimate and become ejected (which would alter the object's orbit), or lead to the entire body vaporizing.

Ideally, Prof. Lubin and his colleagues have envisioned a system that could engage multiple targets simultaneously.

Going Nuclear!: 

In 1967, MIT Professor Paul Sandorff and a team of his graduate students conducted a study called Project Icarus - a hypothetical planetary-defense scenario. This is not to be confused with Icarus Interstellar's plan for an interstellar spacecraft.

For the sake of the study, Prof. Sandorff asked his graduate students to come up with a plan to deflect 1566 Icarus, a 1 km-wide (0.6 mi) asteroid that would make a close approach with Earth within a year.

Based on a hypothetical scenario where the asteroid would collide with Earth, the team proposed sending a Saturn V rocket (which was in development at the time) to deploy six or seven 100-megaton nuclear warheads that would detonate in close proximity to the asteroid's surface.

Based on their analysis, Prof. Sandorff and Project Icarus team concluded that their concept had a 71% chance of completely protecting the Earth and an 86% chance of reducing the damage a full impact would cause. Though Project Icarus was never tested, it laid the groundwork for future research on Nuclear Explosive Device (NED) deflection techniques.

This research continues in the form of the Hypervelocity Asteroid Mitigation Mission for Emergency Response (HAMMER), another concept currently being researched by NASA. It calls for spacecraft weighing about 8 metric tons (8.8 US tons) capable of detonating a nuclear bomb to deflect an asteroid if it was on a collision course to Earth.

The study is a collaboration between NASA, the National Nuclear Security Administration (NNSA), and two Energy Department weapons labs. At present, they are conducting the study using the asteroid Bennu as the modeling target.

A Look at Earth's Planetary Defense Systems in Preparation for Doomsday
Artist's rendering of an asteroid entering our atmosphere. Source: hub.jhu.edu


In 2018, Stephen Hawking released his final book to the world, titled Brief Answers to the Big Questions. In it, he stated how an asteroid collision was likely to be the biggest existential threat facing humanity.

In fact, one of the main reasons to colonize Mars, according to multiple statements made by Hawking, was to ensure that human civilization had a "backup location" in case such a cataclysmic event happened.

Also in 2018, the US National Science and Technology Council (NSTC) released a report titled "National Near-Earth Object Preparedness Strategy Action Plan," which was a follow-up to the 2016 report released by DAMIEN.

In addition to indicating that the US and its allies were not prepared for the threat of a large impact, it also stated that there was time to address this problem:

"Unlike other natural disasters (e.g., hurricanes), once a NEO is detected and tracked we can typically predict many years in advance whether it will cause a devastating impact, and, most importantly, we can potentially prevent impacts when detected with sufficient warning time. A NEO may be deflected via spacecraft systems designed to alter the NEO’s orbit such that it misses the Earth."

This is fortunate since space agencies like NASA would require at least five years of preparation before a mission could be launched (according to expert testimony heard by the US Congress in 2013).

In the meantime, the greatest weapon we have in the planetary-defense arsenal is still information.

The ability to track NEOs that are years away from crossing Earth's orbit is indispensable, and the primary means through which we can ensure that human civilization will survive a cataclysmic impact.

Further Reading:

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