Somewhere, a large asteroid is on course to eventually hit Earth. Will humanity be ready?

Michael Byers holds the Canada Research Chair in Global Politics and International Law. Aaron Boley holds the Canada Research Chair in Planetary Astronomy. They teach at the University of British Columbia and co-direct the Outer Space Institute. Their new book is Who Owns Outer Space? International Law, Astrophysics, and the Sustainable Development of Space.

An asteroid has been discovered on an Earth-impact trajectory. Astronomers estimate its diameter at 300 to 800 metres and precisely calculate an impact date of Oct. 22, 2036.

This hypothetical scenario was put before the 2023 Planetary Defense Conference, held at the United Nations offices in Vienna. Astrophysicists, planetary scientists, engineers, diplomats and international lawyers used it to explore responses to the threat posed by asteroids and comets.

“This is an exercise” was repeated at every stage, and for good reason. A 300-metre asteroid would strike with an energy equivalent of a billion tonnes of TNT.

Earth has been bombarded by asteroids and comets throughout its history. These leftovers from planet building are formed from metals, rocks and ice that coalesced in the solar nebula – the disc of dust and gas that surrounded the nascent sun.

An “astronomical unit” (AU) is the distance from the sun to Earth’s orbit. Asteroids and comets that pass within 1.3 AU of the sun are called near-Earth objects (NEOs). An AU is a vast distance – 149.6 million kilometres, about 21,640 times the distance between Vienna and Toronto. Not all NEOs will actually come “close” to Earth. But if you wait long enough, some will come, and can do so at relative speeds of 20 kilometres a second or more.

Approximately 31,000 NEOs have been identified so far, including most of those with diameters greater than one kilometre. However, only about 40 per cent of NEOs with diameters above 140 metres have been identified. The percentage is even lower for smaller NEOs.

Each year, dozens of asteroids with diameters around a metre explode harmlessly in the upper atmosphere. Larger asteroids have consequences. A decade ago, in February, 2013, an asteroid about 19 metres in diameter exploded 30 kilometres above the Russian city of Chelyabinsk, with the energy of about 500 kilotons of TNT. The airburst injured more than 1,000 people, most of them hurt by shattered glass after they rushed to windows to observe the bright flash in the sky.

The larger the Earth impactor, the longer the typical timescale between events. The average time between impacts causing widespread damage is measured in tens of thousands of years, but nothing precludes a catastrophic impact this century.

As low-probability, high-consequence events, impacts by asteroids and comets fall outside the lived experience of most people and therefore seem fantastical – the Bombardment of Earth in The Expanse, or a plot line from a Bruce Willis or Jennifer Lawrence movie.

Others’ imaginations take them elsewhere. On the first day of the conference, a UN guard spotted “planetary defense” on a security pass and asked, “Should I be watching out for aliens?”

Nature also has a sense of humour: On Friday the 13th, April, 2029, the need for planetary defence will become visible, when a large, very real asteroid will – with 100-per-cent certainty – narrowly miss Earth.

This is an exercise. As the hypothetical scenario unfolds, real-life diplomats play the role of decision makers. They approve a “reconnaissance” spacecraft to gather information about the asteroid’s size, composition and trajectory. The robotic spacecraft is launched the next year and reaches the asteroid in 2025.

The asteroid is indeed large and headed for Lagos, Nigeria, population 28 million. The combination of size and short timeline leaves just one possible deflection method: a nuclear explosive device – a NED. The idea, however, is not to blow the asteroid up, as in Armageddon. That could only increase the number of objects hurtling toward Earth.

The NED will be detonated alongside but not too close to the asteroid, so the surface of the asteroid is partially vaporized. As the surface expands rapidly outward, this creates a “rocket reaction” that subtly changes the asteroid’s orbit, causing it to steadily fall behind or get ahead of its original position. The hope is that as the orbits progress, the difference in motion ensures that the asteroid and the Earth are not at the same place at the same time.

As the asteroid hurtles toward the Earth, it’s possible to deflect it, but when it’s deflected it will either go east or west, along an arc. You hope you can deflect it enough that it will miss the whole planet, but by doing so you set the asteroid on a course over other parts of the world. In the case of the hypothetical scenario, substantial populations, including Maputo, the capital of Mozambique, would be imperilled if the deflection is only partially successful.

A deflection westward passes over the Atlantic Ocean, the south-central United States and northwestern Mexico. The U.S. and Mexican populations at risk from an unsuccessful deflection are smaller than those in southern Africa.

This is an exercise. The scenario’s designers have drawn on philosophy to create a version of the “trolley problem.” Because the U.S. supplies the NED, it is the person who sees a runaway train careening toward a three-way, multitrack junction. Doing nothing sees the train plow straight ahead, killing three strangers.

Pulling a lever left (westward) places the train on one of two different tracks. One is clear, but one has their child on it. The person cannot guarantee that the train will turn onto the clear track.

Pulling the lever right (eastward) could also place the train on a clear track, or on a track with three different strangers. Pulling the lever right is not the same as condemning those three strangers to death, but it would imperil them.

More than ethics are in play. Governments could object to having the asteroid moved over their populations. In the absence of consent, imposing a deadly risk on a country’s population is illegal – sovereign states are not required to make sacrifices to achieve global utilitarian outcomes. If those states are bribed or coerced into providing consent, difficult human-rights issues arise.

The scenario was never completed during the conference. Confronted with these daunting moral dilemmas, the “decision makers” packed up and went home. A real-life planetary defence emergency won’t offer that luxury.

Last September, NASA tested its ability to deflect an asteroid by slamming a 500-kilogram spacecraft into a 170-metre asteroid at a relative speed of 6.6 kilometres a second. The asteroid was chosen because it is the smaller component of a “binary,” two asteroids that orbit each other as they both orbit the sun. This particular binary produces a distinctive, periodic brightness variation as the asteroids pass in front of and behind each other, which astronomers can measure from Earth. Doing so confirmed that the spacecraft’s impact did change the asteroids’ orbital dance.

The same deflection method could be used against a small to medium-sized asteroid heading toward Earth. For a larger asteroid, multiple spacecraft might be needed, which could require participation from Russia and China.

After the 2013 Russian airburst, two organizations were created to help co-ordinate planetary defence efforts. The International Asteroid Warning Network connects astronomers and observatories that identify and study potentially hazardous NEOs.

The Space Mission Planning Advisory Group prepares for an international response to an impact threat, including by studying deflection options. Its collaborative nature is emphasized by the pronunciation of its acronym – SMPAG – as “same page.” It is currently composed of 18 space agencies, including those of China and Russia. The Canadian Space Agency is planning to join them.

SMPAG is not Starfleet. It does not marshal emergency planetary defence spacecraft, nor would it fulfill a decision-making role. Such decisions are the purview of national governments, whether acting unilaterally, in some “coalition of the willing,” or through an existing mechanism such as the United Nations Security Council.

Security Council resolutions can be vetoed by any of the five permanent members: China, France, Russia, Britain and the U.S. Consensus among great powers can be difficult to achieve, but there is reason to hope that all 15 permanent and non-permanent members of the council would come together to address a clear and present common danger. Just one day after the terrorist attacks of Sept. 11, 2001, they unanimously adopted a resolution that identified a “threat to international peace and security,” recognized “the inherent right of individual or collective self-defence,” and expressed their “readiness to take all necessary steps to respond.”

As the principal decision-making body of the United Nations, the Security Council has the authority to assert control over any situation it deems a threat to international peace and security, including an incoming asteroid or comet. It can also legalize what would otherwise be an illegal action, such as the use of a NED for planetary defence.

Still, the question of “Who decides?” will cloud the future of planetary defence until states adopt a decision-making protocol in advance of an actual emergency. Such a protocol could take the form of a UN General Assembly resolution, enabling all 193 member states to express their support. It could even take the form of a multilateral treaty, if Russia, China and the U.S. were persuaded to join.

More co-operation is also needed on astronomical observations – the first line of planetary defence – with more eyes on the ground and in space. In 2013, Canada launched the Near Earth Object Surveillance Satellite (NEOSSat), a 15-centimetre-diameter telescope intended to detect NEOs at small angles from the sun. Because of design and planning flaws, NEOSSat never met its NEO mission requirements, although it has been successful in tracking space debris. Costing just $25-million, the spacecraft has yet to be replaced.

Protecting detection capabilities from light pollution is also necessary. Companies and governments have begun to fill orbits with satellites and space debris, reflecting sunlight that could soon degrade nightly NEO sky surveys.

Mission-ready planetary defence assets should be another priority, including a set of low-cost, reliable reconnaissance spacecraft. A few spacecraft designed for deflections would also be good. After all, who waits for a fire to build a fire truck?

In April, 2029, the Planetary Defense Conference will be held in Nice, France, because of a spectacle in the Mediterranean sky. Apophis, a 370-metre-diameter asteroid, will be clearly visible to the unaided eye as a moving point of light. It will momentarily be just 32,000 kilometres above us, closer than satellites in geosynchronous orbit. The flyby will present amazing science opportunities. It will also necessitate that governments and media explain, clearly and repeatedly, that this particular asteroid – while coming very close indeed – does not pose a risk to anyone.

Potentially hazardous NEOs are still being discovered. Asteroid 2023 DZ2 was identified in February and passed harmlessly by the Earth just a few weeks later – at less than half the distance to the moon. The asteroid is between 40 and 90 metres in diameter, a size that could severely damage a large city.

Asteroids and comets are existential threats, but humanity should be ready. We already know what needs to be done. And unlike the non-avian dinosaurs, which perished when a 10-kilometre-wide asteroid struck Earth some 66 million years ago, we have a space program.