Halley’s comet is one of the most well-known comets, appearing in Earth’s sky about every 75-76 years. The last time it was visible from Earth was in 1986, and it’s expected to return again in 2061.
Halley’s comet is named after the astronomer Edmond Halley, who analyzed reports of a comet approaching Earth in 1531, 1607 and 1682. He concluded that these appearances were actually the same comet returning over and over again in an orbit around the Sun. Halley predicted the comet’s return in 1758, and it reappeared right on schedule.
The comet is composed of dust, ice and small rocky particles. When it approaches the Sun, some of the ice turns to gas which forms the comet’s tail. The nucleus of Halley’s comet is relatively small, estimated to be only about 10 miles (16 km) long and a few miles wide. However, its tail can stretch for millions of miles.
What would happen if Halley’s comet collided with Earth during one of its orbits? Let’s take a look at some of the potential consequences.
Impact Effects
If Halley’s comet struck Earth, the effects would depend on a number of factors, including:
– The comet’s mass, composition and velocity at impact
– The angle at which it strikes
– The location of impact on Earth
Halley’s comet has a very small nucleus, so the total mass hitting Earth would likely be relatively small compared to other cosmic impacts like asteroid strikes. However, the high velocity at which comets travel means there could still be devastating effects.
Some researchers estimate that if Halley’s comet hit a landmass, it would explode with a force of about 1,000 to 10,000 megatons of TNT. This is about 10 to 100 times more powerful than the asteroid impact that likely caused the extinction of the dinosaurs 65 million years ago.
The immediate devastation from the impact explosion would level trees and structures for hundreds of miles in every direction. A comet strike in the ocean could generate massive tsunamis spreading outward from the impact site.
If the comet struck at an angle rather than head-on, it might skip off the atmosphere and back into space, like a rock skipping off water. This would lessen the impact but could still cause significant damage.
Impact Craters
An impact from Halley’s comet would carve out a sizable crater in the Earth’s crust. Craters give us clues about the size and nature of cosmic impacts.
Smaller comets and asteroids produce craters up to a few kilometers wide. For example, Meteor Crater in Arizona which is about 1.2 km (0.75 miles) across was created by a relatively small metallic asteroid estimated to be only about 50 meters (164 feet) in size.
Larger impact craters like Chicxulub in the Yucatan Peninsula are over 180 km (112 miles) wide and were caused by objects estimated to be 10-15 kilometers (6-9 miles) across.
Based on its relatively small size, if Halley’s comet struck Earth it would probably create a crater somewhere between these extremes, perhaps 5-10 km wide. This would cause regional destruction beyond the initial blast zone.
Climate Effects
Beyond the immediate destructive effects from the impact explosion, an Earth collision with Halley’s comet could also cause long-term climate changes.
When the comet explodes, huge amounts of dust and debris get thrown into the atmosphere. This could block sunlight and rapidly cool the planet’s surface.
In 1815, Indonesia’s Mount Tambora erupted and spewed so much ash into the atmosphere that 1816 became known as the “Year Without a Summer” across parts of Europe and North America. Crops failed and people starved in what was the worst famine of the 19th century.
A Halley’s comet impact would likely release much more debris and dust than even the largest volcano eruption. The cooling effects could last for years, severely affecting global agriculture. Photosynthesis in plants would decrease, disrupting food chains and ecosystems.
In the longer term, the release of chemical compounds like nitrogen and sulfur oxides would cause acid rain damaging plant and animal habitats.
It might take decades for the climate system to recover from a comet collision. Extinctions would likely occur, especially among organisms unable to quickly adapt to rapidly changing conditions.
Firestorms
A Halley’s comet strike could also potentially ignite regional and even global firestorms.
The heat from the comet’s impact explosion could directly ignite wildfires over hundreds of miles. But fires could also be sparked by electricity from the plasma generated as the comet crashes through the atmosphere.
These fires would loft more smoke and soot into the air, compounding the climate cooling effect. In the case of a comet impact in a populated area, massive fires could also ignite in urban areas and industrial facilities. Firestorms incinerated entire cities like Dresden, Hamburg and Tokyo during World War II.
The damaging effects of global firestorms might persist for years after the initial impact. The complex chemistry of smoke particles and aerosols could slowly destroy the ozone layer, exposing Earth to more harmful UV radiation.
Tsunamis and Earthquakes
If Halley’s comet struck an ocean, the impact would generate enormous tsunamis fanning out from the collision site.
Impact craters form rapidly – within seconds or minutes of a cosmic collision. This rapid depression of the sea floor would displace massive amounts of water, creating waves potentially hundreds of meters high.
The total energy released in a comet strike is estimated to be equivalent to over a billion of the strongest nuclear bombs. For comparison, the 2004 Indian Ocean tsunami which killed over 200,000 people was generated by an earthquake calculated to have “only” the force of 23,000 Hiroshima bombs.
Tsunamis from the comet impact would inundate coastlines around the entire planet. Wave heights would decrease with distance from the impact point, but even 30 cm (1 foot) of water is enough to knock people off their feet and cause casualties and widespread flooding.
Seismic shaking from the impact could also trigger earthquakes and volcanic activity around the globe. Earth’s crust and mantle would reverberate from the collision like jello in a bowl. This could go on for weeks and months, damaging structures and infrastructure.
Megatsunamis
Some scientists have proposed even more dire scenarios: a Halley’s comet strike in the Atlantic or Pacific ocean could displace enough water to create akilometer-high megatsunami. Waves this enormous have occurred in the past, like the 58m (190 ft) high wave that swept the North Sea during the Storegga submarine landslide some 8,000 years ago.
Megatsunamis hundreds of meters high would inundate coasts on both sides of the ocean basin, travelling inland for tens or even hundreds of kilometers. The tsunami that hit Japan in 2011 after the magnitude 9.0 Tohoku earthquake reached heights up to 40 meters (133 feet) and travelled up to 10 km (6 miles) inland.
A megatsunami from a Halley’s comet ocean impact would make even the 2011 Japan tsunami seem small by comparison. Nothing would survive in the paths of these cataclysmic waves.
Long-term Consequences
The long-term effects from a Halley’s comet collision would persist for years and decades after the actual impact. Some consequences would include:
Mass Extinctions
The immediate destruction, fires, tsunamis and earthquake would obliterate ecosystems in the impact zone. Dust and debris thrown into the atmosphere would block sunlight and disrupt photosynthesis. Global cooling would make survival difficult for many species.
Even a small change in global temperatures can have devastating effects on plant and animal populations. It’s estimated 70% of species went extinct after the dinosaur-killing asteroid impact.
Targeted conservation efforts might be needed to prevent total extinction of some critically endangered species. Ecosystems could take centuries to fully recover.
Infrastructure and Economic Collapse
Modern civilization depends on interconnected infrastructure systems that are highly vulnerable to disruption. Transportation, power generation, fuel production, manufacturing, agriculture and trade would all be negatively impacted by a comet collision.
Widespread infrastructure damage would lead to economic stagnation or collapse, disrupting supply chains. Financial markets would be thrown into chaos. It could take years to fully restore essential services and reconnect global economic systems, especially if workforce populations have been decimated.
Disease Outbreaks
Poor sanitation, lack of health services, respiratory issues caused by dust particles, and contamination of water sources could all contribute to disease outbreaks in the aftermath of a Halley’s comet strike.
Cholera, typhoid, dysentery and other infectious diseases could become epidemic in heavily impacted regions. With global trade and travel links disrupted, containing and treating outbreaks will be challenging.
Preparing emergency medical strategies and stockpiling vaccines and supplies would help mitigate disease impacts. But global cooperation would be essential to prevent rampant spread of illness.
Political Instability
Widespread devastation, infrastructure collapse and disease outbreaks would severely test political institutions. There could be a breakdown of law and order if essential public services can’t be maintained.
Some nations might exploit instability and disasters in other countries for political or military advantage. Diplomacy and cooperation will be crucial to prevent conflict.
But a global catastrophe could also unite humanity to work together. Providing aid, sharing resources, opening borders for refugees and planning reconstruction would help avoid complete societal collapse.
Preparing for a Comet Collision
While the chances of Halley’s comet or other comets impacting Earth may be small, cosmic collisions have happened in the past and will inevitably occur again in the future.
Advance preparation and planning could help humanity better handle an unexpected comet strike. Some steps that could help mitigate risks include:
Detection and Monitoring
Ongoing observation and detection efforts by astronomers are critical to tracking near Earth objects like comets and asteroids. With enough advance warning, a threatening object could potentially be deflected or destroyed before impacting our planet.
Impact Preparation
Identifying areas and infrastructure at high risk allows steps to be taken to reinforce and protect critical facilities like power plants. Vulnerable coastlines can be prepared for tsunamis. Early detection systems and evacuation plans are also vital.
Disaster Response Planning
Emergency efforts will need to provide aid, shelter, medical care and sanitation on a massive scale. Simulation exercises with international organizations and national governments can help identify gaps and improve response coordination.
Environmental Monitoring
Studying past extinction events gives insights into ecological impacts and recovery timelines. Scientists will need to closely monitor the environment in a comet’s aftermath to assess damage and guide restoration efforts.
Climate Intervention
Adding aerosols to the atmosphere or taking other measures to counteract global cooling might help mitigate some climate effects, but risks must be carefully weighed.
Food Stockpiles
Maintaining emergency food reserves could help avoid starvation in the event of widespread crop failures. But distribution challenges must also be addressed.
Conclusion
The possibility of Halley’s comet colliding with Earth conjures up frightening doomsday scenarios. But even a relatively small comet impact could have catastrophic effects, from the initial explosion and tsunamis to long-lasting climate changes and ecosystem damage.
While such an event may seem improbable, humanity must take steps to monitor risks, make advance preparations and have response plans to help soften the blow from an unexpected comet strike.
With scientific ingenuity and global cooperation, it’s possible our civilization could one day prevent or withstand even inevitable cosmic disasters. But much work remains to build resilience, protect our planet and become what Carl Sagan famously called “a species with a long-term future.”
