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Starts With A Bang

The biggest challenges of redirecting a killer asteroid

Sooner or later, Earth is going to be hit by a large enough space object to cause significant damage to humanity. Stopping them isn't easy.
This illustration shows the asteroids Didymos and Dimorphos, with NASA's DART mission on course to impact the latter, smaller asteroid. This collision should be recorded by the cubesat LICIACube, as well as ground-based telescopes and a follow-up mission. If the mission goes as expected, Dimorphos's orbit will change by the predicted amount: a great step forward in the science of asteroid redirection.
(Credit: NASA/Johns Hopkins APL)
Key Takeaways
  • There are over 25 million asteroids in the Solar System with the power to create a Tunguska-like event, at minimum, if they were to strike Earth.
  • The best hope for averting such a crisis lies in asteroid redirection, which NASA's DART mission will perform as our first attempt toward such efforts. 
  • But the combined problems of threat identification, reaching the hazardous object quickly, and delivering a safe and effective solution remains beyond our capabilities at present. To survive, we must remain lucky.

On September 26, 2022, NASA’s DART mission collides with asteroid Dimorphos (Watch it live here).

This infographic shows the original orbit of asteroid Dimorphos around the larger asteroid Didymos, along with the trajectory of NASA’s DART spacecraft and the presumed new orbit that will result. The new orbit changed by a much greater amount than simulations and calculations predicted, indicating that a better understanding of these redirection efforts are required before we begin relying on them to save our planet.
(Credit: NASA/Johns Hopkins APL)

This ~170 meter (560 foot) asteroid provides the perfect testing grounds for asteroid redirection.

A variety of objects from Earth shown for comparison with NASA’s DART mission, the asteroid Dimorphos that it will strike, and the asteroid Didymos which Dimorphos orbits. Although there are some ~25 million asteroids of 100 meters in diameter or larger, none of the asteroid strikes recorded in human history have been larger than ~80 meters.
(Credit: NASA / Johns Hopkins APL)

Only ~100,000 civilization-killers potentially exist, but over 25 million Dimorphos-sized bodies threaten Earth.

Although we’ve catalogued most of the large (greater than 1 km) asteroids in the Solar System, the population of inner near-Earth asteroids that’s greater than 0.1 km has not been well-determined at all. The number density of the smaller objects on this graph has only been estimated; a mission like NEO Surveyor will be vital toward learning what truly poses a predictable hazard to Earth.
(Credit: Marco Colombo, DensityDesign Research Lab)

Many are already near-Earth asteroids, with most others one Jupiter-assist away from creating terrestrial impacts.

While the near-Earth asteroids are already posing potential hazards to Earth, most of the asteroids that are out there are heavily influenced by Jupiter. The wrong gravitational interaction, which can always occur as time goes on, could turn any of these asteroids into potential Earth-orbit-crossing hazards.
(Credit: Pablo Carlos Budassi/

These bodies move fast: at ~45,000 mph (72,000 kph) relative to us.

A comparison of the scale of various objects, including the size of three famous meteor strikes on Earth: the Chelyabinsk event that struck Russia in 2013, the Tunguska Event of 1908, and the event that created Meteor/Barringer crater tens of thousands of years ago. None of these objects were big enough to even be counted among the ~25 million asteroids that are out there at 100+ meters in diameter.
(Credit: cmglee, Wagner51, domdomegg/Wikimedia Commons)

With such masses and speeds, impacts equate to ~10+ Megaton explosions on Earth.

Barringer Crater, also known as Meteor Crater, is an impressive crater located in the Arizona desert, more than a mile in diameter. Although this crater was made tens of thousands of years ago, it was caused by a relatively small impactor estimated at a mere 50 meters across: less than a third the size of the asteroid that NASA’s DART mission will collide with. Although these “city-killer” sized objects are dangerous, one about three times the diameter would be large enough to cause regional devastation for tens to up to a hundred miles in all directions, similar to the Tunguska event.
(Credit: Grahampurse/Wikimedia Commons)

Asteroid redirection efforts could avert such events but face many challenges.

This diagram maps the data gathered from 1994-2013 on small asteroids impacting Earth’s atmosphere to create very bright meteors, technically called “bolides” and commonly referred to as “fireballs”. Sizes of red dots (daytime impacts) and blue dots (nighttime impacts) are proportional to the optical radiated energy of impacts measured in billions of Joules (GJ) of energy. The largest impactor over this time period, the Chelyabinsk meteorite, was only ~20 meters in diameter.
(Credit: Planetary Science, NASA/JPL-Caltech)

1.) Early identification.

At present, nearly 30,000 potentially hazardous asteroids have been identified, with about a third of them above ~140 meters in diameter. The overwhelming majority of asteroids, including near-Earth asteroids, have yet to be found and characterized.
(Credit: Alan B. Chamberlin, NASA/JPL-Caltech)

Identifying and characterizing potentially hazardous objects early is key.

NEO Surveyor
The NEO Surveyor mission, whose goal is to discover and categorize most of the potentially hazardous near-Earth objects, is a planetary defense mission that should find practically all of the Earth-crossing asteroids greater than 140 meters across, as well as many smaller ones. It’s a high priority mission, but one that needs to be fully funded to do its job.
(Credit: NASA/JPL-Caltech)

New low-Earth orbiting satellites severely hinder this already herculean task.

The Vera Rubin observatory, home to the Large Synoptic Survey Telescope, will soon become active, and will be humanity’s best tool for identifying and tracking the orbits of potentially hazardous objects. Although one of its main science goals is to track and identify potentially hazardous asteroids, this endeavor is severely curtailed by the recent deluge of new low-Earth orbit satellites. More than 50% of all low-Earth orbiting satellites have been launched since 2019.
(Credit: Todd Mason, Mason Productions Inc./LSST Corporation)

2.) Asteroid interception.

This image shows the parabolic trail left by a rocket after launch. If we can identify a potentially hazardous object that’s bound for a collision with Earth, the ability to intercept that object as quickly as possible is key to mitigating any damage that it could do to us, as changing its trajectory is much easier earlier than later.
(Credit: SpaceX/rawpixel)

Intervening quickly is key.

Comet 67P/Churyumov-Gerasimenko was imaged many times by the ESA’s Rosetta mission, where its irregular shape, volatile and outgassing surface, and cometary activity were all observed. However, the attempting landing of Philae was a failure; only two missions have ever had a successful, soft landing on either a comet or asteroid, a necessary step for many strategies in development for changing the trajectory of a potentially hazardous object.

Small changes in trajectory, early on, are equally effective to large alterations later.

Flyby spacecraft Deep Impact shows the flash that occurred when comet Tempel 1 ran over the spacecraft’s impactor probe. It was taken by the flyby craft’s High Resolution Instrument, Visual CCD camera (HRIV) over a period of about 40 seconds. Black borders are the result of image stabilization. The small change in momentum resulting from this impact did not appreciably alter the motion of Tempel 1.
(Credit: Paul Stephen Carlin, NASA/JPL)

3.) Momentum transfer.

The debris stream of asteroid 3200 Phaethon creates the Geminids. While Phaethon itself doesn’t appear to be particularly comet-like, its very close passage to the Sun helps fragment it, enabling the spectacular meteor shower we’ve seen every December for over 150 years now. Its relative youth indicates a gravitational encounter that changed the parent body’s orbit shortly before the arrival of the Geminids; another such encounter could make it an existential hazard to human civilization on Earth.
(Credit: Peter Jenniskens and Ian Webster)

This is the hardest problem of all, as each solution possesses drawbacks.

Schematic of the DART mission shows the impact on the moonlet of asteroid (65803) Didymos: Dimorphos. Post-impact observations from Earth-based optical telescopes and planetary radar would, in turn, measure the change in the moonlet’s orbit about the parent body, determining the effectiveness of a small impactor in changing the motion of the asteroid as desired.
(Credit: NASA/Johns Hopkins Applied Physics Lab)

DART-like impacts can create ejecta, failing to redirect the main body.

The asteroid Bennu, shown here, has a surface typical of most asteroids under ~1 km in diameter: it appears to be a volatile-rich pile of rubble. A detonation/explosion, either on the surface or from deep within, might simply kick up debris and create multiple fragments that will then collide with Earth, leading to a comparable amount of destruction to no intervention at all.
(Credit: NASA’s Goddard Space Flight Center / Conceptual Image Lab / Scientific Visualization Studio)

Detonations could create multiple impactors, compounding the problem.

Detonating a nuclear device close to or right against an incoming asteroid might not simply impart momentum to it, changing its trajectory, but could blast it apart into pieces and could irradiate it, creating a problem of multiple fragments with large amounts of nuclear waste landing on Earth, bringing destruction and pollution back simultaneously.
(Credit: NASA/JPL-Caltech)

Nuclear strikes could do both, while creating Earth-bound radioactive fallout.

The NEXIS Ion Thruster, at Jet Propulsion Laboratories, is a prototype for a long-term thruster that could move large-mass objects over very long timescales. If we had sufficient lead time, a thruster (or series of thrusters) like this could save the Earth from a potentially hazardous impact.
(Credit: NASA/JPL)

Long-term engine thrust is the safest strategy, but requires the most lead time.

The animation depicts a mapping of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years, and finishes with a map of all known asteroids as of January 2018. It’s vital that we recognize that the most hazardous asteroids of all, i.e., the ones that cross Earth’s orbit the most frequently, have largely not been characterized at all, particularly among the populations that lie largely interior to Earth’s orbit.
(Credit: NASA/JPL-Caltech)

Without a demonstrated technological solution, we can only hope our luck continues.

Comet Bernardinelli-Bernstein, the largest comet ever discovered, has a nucleus that’s approximately 119 kilometers across. If such an object were to strike Earth, the energy imparted to our planet would be thousands to ten thousand times as energetic as the K-Pg impactor that occurred 65 million years ago.
(Credit: NASA/Don Davis)

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