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Starts With A Bang

What Are The Real Odds Of ‘Doomsday’ Asteroid Apophis Striking Earth In 2068?

And if it does strike us, how much damage will it cause?

Will near-Earth asteroid Apophis strike us in 2068?

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)

Despite widespread affirmative reports, the collision threat is likely negligible.

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)

Many observations, over long timescales, are required to precisely reveal an asteroid’s trajectory.

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)

Close gravitational encounters will also induce significant orbital changes.

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)

Finally, solar heating will lead to volatile offgassing, accelerating asteroids with shape irregularities and rotational motions.

These two radar images (above and below) of asteroid Apophis show this large asteroid, about as large as the Empire State Building or the Eiffel Tower (but far more massive), rotating, tumbling, and with an irregular shape as it orbits the Sun. (NASA/JPL)

Discovered in 2004, Apophis presently orbits the Sun every 323 days.

This five-exposure image shows asteroid Apophis (circled), with star trails seen in the background, as the asteroid (tracked) is moving relative to the background stars. Apophis was discovered in 2004, and will come very close to Earth three times this century: in 2029, 2036, and 2068. (D. THOLEN, M. MICHELI, G. ELLIOTT, UH INSTITUTE FOR ASTRONOMY)

Initial measurements indicated Apophis had a 1-in-37 chance of impacting Earth in 2029.

When a potentially hazardous object is first measured, it will have a large cone of uncertainty to its impact parameters, leading to a moderate probability of collision with Earth if our planet’s position is included in that cone. As the error ellipse narrows with improved precision, the calculated probability of striking Earth rises, only to fall again with Earth becomes excluded. (LOU SCHEFFER/PUBLIC DOMAIN)

That’s untrue; Apophis will miss Earth by 47,000 km (29,000 miles), with gravity drastically altering its orbit thereafter.

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)

Newly acquired Subaru telescope observations of Apophis revealed a surprise: the Yarkovsky effect.

When an asteroid or comet rotates with respect to the Sun, its Sun-facing side heats up while the opposite side cools down. The radiation coming from the different sides of the asteroid will have different magnitudes as a result of having different temperatures, causing an additional acceleration known as the Yarkovsky effect. (GRAEVEMOORE AT ENGLISH WIKIPEDIA)

Sunlight heated the rotating, irregular asteroid, causing unexpected accelerations.

Similar to the effects of sunlight on the nucleus of Comet 67P/Churyumov-Gerasimenko, the combination of solar heating, irregular shape, and rotation will cause an additional force to be exerted on asteroid Apophis, leading to alterations in its future orbit from the current predictions. (ESA/ROSETTA/NAVCAM)

Prior odds of a 2068 collision were estimated at 1-in-150,000; new odds will require a reanalysis.

The close encounter between Apophis and Earth in 2029 will change the asteroid’s trajectory, but the addition of the Yarkovsky effect means that the previous calculations of just a 1-in-150,000 chance of a collision in 2068 has been thrown into doubt. Additional, more accurate calculations will be needed to compute the new odds. (NASA/JPL-CALTECH)

New satellite megaconstellations complicate ground-based observations near dusk and dawn: the most critical asteroid-tracking window.

Hypothetically, a collision would release ~1200 Megatons of TNT of energy: enough to create a ~5 km (3 mile) crater.

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)

However, with just 0.001% the mass of the legendary dinosaur-killing asteroid, it poses no extinction threat.

The asteroid redirect vehicle demonstrates the “gravity tractor” planetary defense technique on a hazardous-size asteroid. The gravity tractor method leverages the mass of the spacecraft to impart a gravitational force on the asteroid, slowly altering the asteroid’s trajectory. The science of planetary defense could literally prevent the catastrophe of the millennium from occurring on Earth. (NASA)

Possible mitigations include a future deflection mission: potentially in 2029 or 2036.

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. A similar impact could potentially redirect a potentially hazardous asteroid, such as Apophis. (PAUL STEPHEN CARLIN / NASA / JPL)

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.


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