Skip to content
Starts With A Bang

Why Is It So Hard To Rule Out Future Asteroid And Comet Strikes On Earth?

Only 17 years after its discovery have we learned we’re safe from asteroid Apophis.

Ever since its 2004 discovery, asteroid 99942 Apophis has threatened planet Earth.

Asteroid Apophis has been measured many times over the timespan of multiple years quite precisely, leading scientists to determine its orbit to a remarkable precision. Of course, additional effects, such as offgassing or gravitational encounters, have the potential to alter that orbit beyond mere Keplerian effects. (UNIVERSITY OF HAWAII)

At 1100 feet (340 meters) across, an impact would release the energy equivalent of 1.2 Gigatons of TNT.

The 1961 Tsar Bomba explosion was the largest nuclear detonation ever to take place on Earth, and is perhaps the most famous example of a fusion weapon ever created, with a 50 megaton yield that far surpasses any other ever developed. An impact from asteroid Apophis would release approximately 60 times as much energy as this blast. (ANDY ZEIGERT / FLICKR)

That’s ~100 times as energetic the Meteor Crater-creating impact.

Meteor (Barringer) crater, in the Arizona desert, is over 1.1 km (0.7 mi) in diameter, and represents only a 3–10 MegaTon release of energy. A 300–400 meter asteroid strike would release 10–100 times the energy; asteroid Apophis is about 450 meters along its long axis, expected to release ~1200 MT of energy if it strikes Earth. (USGS/D. RODDY)

Initially, observations indicated a 2% chance of a 2029 collision with Earth.

Generically, asteroids under ~1 km in size are irregularly shaped, will rotate with respect to the Sun, and will move in elliptical orbits as dictated by the law of gravity, but can be perturbed by effects such as uneven solar heating or the gravitational influence of other solar system bodies. Potentially hazardous asteroid Apophis will be subject to all of these factors. (NASA/JPL-CALTECH)

That alarmingly high probability arose from insufficient data.

In April of 2029, asteroid Apophis will pass close to Earth, well within the orbit of the Moon and just a few Earth radii away from our world. Although the odds of a collision are negligible, the close encounter will drastically change the orbit of Apophis for its future close encounters, predicted to occur in 2036 and 2068. (NASA/JPL)

In orbital mechanics, small positional uncertainties compound over time.

Orbit of asteroid Apophis (pink) in contrast to the orbit of Earth (blue). The yellow dot represents the sun. Apophis takes 323.6 days to orbit the sun. Earth takes 365.3 days, but the April, 2029 encounter will dramatically change Apophis’s orbit thereafter, where it will now take more than one Earth-year to complete a revolution about the Sun. (PHOENIX7777/ WIKIMEDIA COMMONS)

Gravitational encounters — including with major planets — further alter trajectories.

The idea of a gravitational slingshot, or gravity assist, is to have a spacecraft approach a planet orbiting the Sun that it is not bound to. Depending on the orientation of the spacecraft’s relative trajectory, it will either receive a speed boost or a de-boost with respect to the Sun, compensated for by the energy lost or gained (respectively) by the planet orbiting the Sun. This happens naturally for all small bodies, like comets and asteroids, that encounter a large mass. (WIKIMEDIA COMMONS USER ZEIMUSU)

So do outgassing and interactions with unresolved objects.

Even asteroids contain substantial amounts of volatile compounds, and can often develop tails when they approach near the Sun. A small amount of outgassing can substantially change an asteroid’s or comet’s trajectory over long timescales. (ESA–SCIENCEOFFICE.ORG)

Many high-resolution observations over long timescales can enable accurate predictions.

These images of asteroid Apophis were recorded by radio antennas at the Deep Space Network’s Goldstone complex in California and the Green Bank Telescope in West Virginia. The asteroid was 10.6 million miles (17 million kilometers) away, and each pixel has a resolution of 127 feet (38.75 meters). (NASA/JPL-CALTECH AND NSF/AUI/GBO)

Now, in 2021, Apophis’s future trajectory is known through 2029: within ±2 km.

All potential impacts this century are confidently ruled out.

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. In order to accurately know the orbital characteristics of an asteroid (or any near-Earth object), its position and velocity must be measured at many different points over time. (NASA/JPL-CALTECH)

Still, many potentially hazardous objects, plus unidentified threats, remain.

The comet that gives rise to the Perseid meteor shower, Comet Swift-Tuttle, was photographed during its last pass into the inner Solar System in 1992. This comet, which gives rise to the Perseid meteor shower, also displayed a spectacular green coma. (NASA, OF COMET SWIFT-TUTTLE)

Comet Swift-Tuttle, the Perseids’ parent body, remains Earth’s most dangerous object.

The orbital path of Comet Swift-Tuttle, which passes perilously close to crossing Earth’s actual path around the Sun. While there is no danger to Earth for at least ~2400 years, the meteors from the cometary debris will grace our skies every year for the foreseeable future. In 4479, there’s the real threat of an impact. (HOWARD OF TEACHINGSTARS)

A possible 4479 collision could be 28 times worse than the historic Chicxulub impactor.

This graph shows the energy released by potential impactors of various sizes along with estimates of the frequency of such impactors. There are many uncertainties with this graph, which do not reflect the outcomes we can anticipate for collisions over the next few decades to millennia. (Encyclopaedia Britannica/UIG Via Getty Images)

Observational identification with well-characterized trajectories are required to enact mitigation plans.

The LSST at the Vera C. Rubin observatory, shown here in a 2018 photo, is currently being constructed and approaching readiness for its first observations. Even if satellite darkening were to take place according to SpaceX’s stated plans, this world-class, first-of-its-kind observatory will be forced to alter its operations to account for Starlink. (LSST PROJECT/NSF/AURA)

The Vera Rubin Observatory could help, but must overcome satellite megaconstellation pollution.

Otherwise, our fate will be to endure unforeseen damages, and then rebuild.

Workers repair a power line near the wall of a local zinc plant which was damaged by a shockwave from a meteor in the Urals city of Chelyabinsk, on February 15, 2013. This small meteor injured over 1000 people and caused more than a million dollars in property damage. (OLEG KARGOPOLOV/AFP via Getty Images)

Detection and prevention offers the only catastrophe-free solution.

In 1860, a meteor grazed Earth, and produced a spectacularly luminous light display. It is eminently possible that some of the meteors that strike Earth have their origins outside of our Solar System, and a complete characterization of small objects in the near-solar neighborhood will be necessary to understand the potential risks to our planet from asteroid, comet, and interstellar body collisions. (FREDERIC EDWIN CHURCH / JUDITH FILENBAUM HERNSTADT)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.


Up Next