Avoiding Big Rocks From Space – Asteroids Edition feat. Apophis

The stage is set. Our planet is in peril because there is a big piece of rock coming straight at it. Cue worldwide panic. It is survival of the fittest. Humanity is at its lowest. But there is hope as one individual rises above all others.

Add a dash of unpredictability, an indestructible pickup, an uncanny ability to foresee and track debris fields, and of course, out of the box survival skills and we have a genuine hero to cheer on.

All I need to do is turn left and right, every few seconds, and nothing will hit me.

This hero goes on to somehow beat all the odds to save his family and a band of survivors, who will rebuild the world after its destruction. Humanity survives. The end.

Sound familiar? Yeah, asteroids have their share of paparazzi when it comes to mainstream cinema but that is not what we are interested in here at The Procrastinating Scientist.

We talk about the science. And what does it say? Well, if an asteroid (and one big enough to match those seen in these films) were to impact our planet at our present state, it is mostly game over. Survival after parties are mostly an after thought.

Ok, ok, it doesn’t have to be all gloom and doom. Let us take a step back and see what we can do about this, starting off by asking,

What beef do asteroids have with us?

None, actually.

Much like a non-player character (NPC) in an RPG or RTS game, asteroids do their own thing, as stoic space rocks, orbiting the Sun and occasionally passing by our planet.

A visitor? Just smile and wave everyone! (For those Age of Empires fans)

Asteroids are like minor planets and there are millions of them. They are not to be confused with all the other rocks in space like comets, meteors, and meteoroids.

Asteroids are remnants of planetesimals, bodies that never grew large enough to become planets, during the early formative stage of our solar system. Asteroids whose orbits bring them close to Earth are classified as near-Earth objects (NEOs). The drama escalates when these occasional passes become an actual visit.

It is at this point that we get all anxious because we are, as of now, not really prepared for an actual impact event. That was the consensus over this summer in May 2021, when NASA astronomers spent a week playing a simulation where they pretended to crash an asteroid into Earth. Their report concluded that we would need at least 5 to 1o years of preparation to avoid an asteroid impact.

What do we mean by an impact?

Like, how bad is bad?

Awareness of the threat presented by NEOs has grown over the last few decades thanks to the observation of select astronomical events like the Shoemaker-Levy 9 impacts on Jupiter in 1994, the formation of fresh craters on the Moon and Mars, and most recently the 2013 Chelyabinsk meteor impact that proved its predecessor in the Tunguska event was no freak phenomenon.

What made the Chelyabinsk event so concerning was that it demonstrated the possibility of an Earth-bound object large enough to be cause for considerable destruction of property (thousands of buildings and some 1500 people injured) and yet small enough to escape detection (as its approach coincided with a daylit sky).

The potential threat of a NEO and the damage it may cause depends not only on how big it is but also what it is made of, its density, and its ability to survive its trip through our atmosphere (which serves as a nice bunsen burner). In the case of the Tunguska and Chelyabinsk events, the asteroids were 20 m and 65 m in diameter, respectively.

Silhouettes of two tall buildings and two smaller spheres all marked with size in meters.
Size comparisons of the asteroids at the Tunguska and Chelyabinsk impact events versus the Empire State Building and the Eiffel Tower.

Now, the larger the object, the lower the probability of an impact event. In fact, on the larger extreme of asteroids more than 300 m in diameter, the impact probability per year is one in 70,000. Why? Because there are many more smaller asteroids than bigger ones.

Most of the shooting stars we see at night are small debris (like grains of sand) that strike our planet’s atmosphere in beautiful displays. The Tunguska and Chelyabinsk events refer to the slightly larger rocks that mostly survive their descent to the surface. The real dangerous objects are ones large enough to cause global catastrophe when they hit.

Enter, Mr. Apophis

The Lord Buckethead of the asteroid family, Apophis is a NEO with a diameter of 370 meters. It was cause for concern when first observed in December 2004 indicating a 2.7% impact probability on April 13, 2029.

Apophis swung by silently over the spring of 2021 when space news was largely focused on the successes of the Mars Perseverance Rover. Thankfully, this latest swing fully supported confirmations over the last decade that our planet is safe from the asteroid for another 100 years.

Animation of Asteroid Apophis’ 2029 Close Approach with Earth

NASA’s Center for Near-Earth Object Studies (CNEOS) states that the uncertainty in our observations of Apophis’ orbit has been minimized from hundreds of kilometers to just a few kilometers. With this improved knowledge and precision of the asteroid’s position, the team at CNEOS confirmed that Apophis can be removed from the Sentry Impact Risk Table (a database of NEOs whose impact with Earth cannot be ruled out).

Say for the sake of an argument a collision is imminent, what can be done?

Asteroid Impact Avoidance

The size of a NEO threat that requires an actual deflection remains a matter of debate. A lower size threshold for such an intervention accounts for asteroids around 30-50 m in diameter. While our current network of NEO observation and search programs are progressing well, it is deemed highly unlikely that objects in this lower threshold would be identified far enough in advance for deflection action to be feasible. On the other extreme, it is unlikely that present governments would be willing to invest taxpayer money to investigate how to deal with objects the size of Apophis when the probability of impact is quite low.

So for now, we get to play with Apophis’ smaller cousins.

We begin by observing. We need to know the danger well in advance to prepare for it. Multiple ongoing projects such as the NEOshield initiative, SpaceGuard, Asteroid Terrestrial-impact Last Alert System, and many other global collaborations are all hands on deck with this common initiative. In terms of pushing these buggers away from the Earth’s orbit, three realistic methods have been identified among various others: the Kinetic Impactor, the Gravity Tractor, and Blast Deflection.

Starting off with the most obvious, the blast deflection technique refers to the use of an explosive either close to, on the surface of, or buried beneath the surface of the NEO. The impending blast should provide deflection of the asteroid or break it up into smaller pieces that can burn over in atmospheric entry.

This basic and largely extrapolated idea was executed by these dudes in the 1998 film Armageddon.

The best scenario would be to bury the explosive on the asteroid before setting it off as it would remove the most surface material from the asteroid. Unfortunately, this would require prior knowledge about the asteroid’s composition and sub-surface structure. Altogether, blast deflection is not considered top of the list to start with in our arsenal of deflection techniques though it may sound wildly exciting.

The kinetic impactor technique involves ramming an object into a collision course with the asteroid and knocking it off course. One way to do this is to ram it with an actual spacecraft, when it is still far from the Earth, and altering its velocity and direction enough to avoid impact. In space, even the slightest change can provide wide margins of deflection. The 2005 Deep Impact collision did exactly this when NASA’s Deep Impact spacecraft utilized a 370 kg space probe and smashed it into Comet Tempel 1 at 37,000 km/h. This resulted in a crater about 150 m wide. The probe delivered the equivalent impact of 5 tons of TNT causing the comet’s velocity to change by 0.0001 mm/s and its orbit by several centimeters.

View of the collision as taken by the Deep Impact spacecraft. The ejecta from the surface of the asteroid is the white haze.

Lastly, the gravity tractor technique (my personal favorite) is the equivalent of towing one’s car except in space. Here, a spacecraft would hover near an asteroid and using the gravitational attraction between it and the asteroid, gradually tow the asteroid off course.

The procedure could also be executed by multiple spacecraft set in orbit around the asteroid. The caveat is that this technique works mainly for NEOs in the smaller range around ~ 50 m in diameter. Seeing that smaller NEOs outnumber the larger ones, gravity tractors might be the most ideal and likely scenario for NEO deflection.

All Is Well?

Phew, sounds like we are quite prepared. Well, we have ideas BUT there is still a lot of work to be done. The ideas and foundations are there but consistent progress is a necessity. Our NEO tracking systems are getting better by the day. With the current progression in our technological endeavors, there is “space” for realizing and optimizing the deflection techniques discussed above.

It all comes down to the fundamental observations we can make of these space rocks and their physical properties. The other side of the coin is more bureaucratic and financial – having the relevant funding to pursue these investigative routes.

Thankfully, with the recent surge of interest in the space industry over the last decade, grounded and backed by public and private industries, the future looks bright in our ability to build and work on our knowledge and skills on space dodgeball with these rocks.


[1] https://cneos.jpl.nasa.gov/pd/cs/pdc21/

[2] https://earthsky.org/space/what-is-the-tunguska-explosion/

[3] https://elib.dlr.de/100120/1/3004_Harris_reprint.pdf

[4] https://stardate.org/astro-guide/faqs/what-chance-earth-being-hit-comet-or-asteroid

[5] https://www.space.com/planetary-defense-asteroid-impact-scenario-exercise-2021

[6] https://www.nasa.gov/feature/jpl/nasa-analysis-earth-is-safe-from-asteroid-apophis-for-100-plus-years

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