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In June 2004, astronomers Roy Tucker, David Tholen and Fabrizio Bernardi discovered a 340-meter asteroid while working at the Kitt Peak National Observatory in Arizona. The object was nicknamed Apophis, named after the snake-like enemy of Ra from the Egyptian myth. At first glance, Apophis’ trajectory suggested a minimal possibility could have an encounter with Earth in 2029, but experts quickly ruled out this possibility.
Step 2029 will bring it close, about 20,000 miles from the Earth’s surface. It’s close enough that you can see it with the naked eye, as long as you live in the Southern Hemisphere.
Earth’s gravity will alter Apophis’s orbit, widening it, and it is expected to make another flyby in 2068. The probability that it will impact Earth at that time was originally calculated as 1 in 150,000, but based on recent observations, those models need to be reorganized.
New data informs us that Apophis is moving outside of our previous projections and the Sun is the likely culprit. Irregular solar heating causes objects like Apophis to radiate heat from one side more than the other, and that little bit of energy gives it an extra boost. Over time, this can greatly alter its orbit. This is known as the Yarkovsky effect.
At present, drift represents a variance of about 557 feet per year from previous models and is enough to question our impact projections. That said, there’s no reason to worry about an Apophis impact in 2068 or any other year. This is great news for us, because Apophis isn’t old enough to be a planet killer, but if he were to come in contact, it wouldn’t be our best day.
CALCULATION OF IMPACT PROBABILITIES
You may wonder why we don’t know for sure if Apophis or any other known Near-Earth (NEO) object will eventually land. After all, classic mechanics are pretty well understood. It is a rock, whizzing through space at the mercy of gravity and other natural forces. If we can predict a solar eclipse, if we can land a probe on an asteroid, why can’t we know for sure? What does it mean for an object to have a one in 150,000 chance of having an impact?
The problem is not in physics, it is in the available data. Large objects such as planets, with orbits that remain stable for long periods of time, allow for extensive data collection. Each observation is an opportunity to refine our calculations. Objects like Apophis aren’t that cooperative. Recently discovered objects offer fewer opportunities for observation, and relatively small objects pose the challenge of resolution. As for Apophis, we are not yet able to see it clearly enough to predict its orbit accurately.
Instead, possible orbits are modeled on the basis of available information. If the data allows 1,000 possible orbits and one of these orbits impacts the Earth, the probability of impact is calculated at 1 in 1,000. We know where Earth will be in 2029 and in 2068 when the flybys will happen, only we are not sure where Apophis will be.
Over time, with more observations or as we learn more about an object (as if driven by the Yarkovsky effect), our predictions become more accurate. Some of these possible orbits are excluded and, in general, the probability of impact decreases.
In the very, very unlikely possibility that it hit Earth, though, what could we do about it?
PLANETARY DEFENSE
The Center for Near-Earth Object Studies (CNEOS) at NASA’s Jet Propulsion Laboratory tracks all known NEOs and their likelihood of impact so you can act quickly if the need arises. Let’s imagine, then, we draw the cosmic short straw and our new calculations place Apophis firmly in the “He’s coming-for-a-visit” category and we had years to prepare and prevent impact, what could we do?
The obvious and safest answer for something like Apophis is to evacuate the area. An object of that size would not threaten humanity as a whole, but it would be devastating to a considerable portion of the planet. If we can’t stop it or change its trajectory, we should get out of the way. An effort of this magnitude would require global cooperation as millions of individuals would become the first space refugees. And depending on the size, density, speed and angle of approach of the asteroid, such an evacuation may not even be feasible.
For the kind of impact it has inspired films like Armageddon (1998) and Strong impact (also 1998), we would need to dig deeper into the science to modify the asteroid’s path so that it avoids Earth entirely.
One option involves nuclear bombs, because of course it does, but probably not in the way you’d expect. In Armageddon, a team of drillers travels to an asteroid that kills the planet in a desperate attempt to plant a bomb and split the object in two. In the film, the bomb splits the asteroid in two and both halves just miss the planet.
While a nuclear bomb is a plausible tactic for deflecting an oncoming asteroid, blowing it up is not the best strategy. Research suggests it would have little effect.
Planting a nuclear explosive inside an asteroid could cause it to rupture and rupture, but the gravitational pull of the remaining core could put it all back together. And it may not do enough damage to significantly alter the asteroid’s trajectory. Instead, we could detonate a nuclear device near the object. The explosion would heat that side of the asteroid and push it off course in the same way that the Yarkovsky effect is altering the path of Apophis.
The main challenges here are overcoming public discomfort with nuclear explosives and the danger of the device exploding on Earth if the launch goes wrong. Fortunately, we have other options and the laws of physics are on our side.
An impact with the Earth depends not only on the intersection of the two orbits, but on the fact that the two objects are in the same point of their orbit at the same time. So, to prevent an impact, all we need to do is change the position or speed of one of the objects along its orbit. Moving the Earth would be a huge undertaking (pun intended) and probably not the best idea. We should move the asteroid.
We could move it one way or the other so that its orbit changes, or we could speed it up or slow it down so that it passes the intersection point before or after the Earth is there. One way to do this is to park a large spacecraft nearby such as a gravity tug to take the asteroid off course.
This plan depends on an incredibly large vessel or a long period of time, but it would work. We could also send a series of vehicles on a collision course with the object, using kinetic energy to push it away. Finally, a series of high-powered lasers could heat parts of the asteroid or detonate parts of the asteroid, using the energy of those interactions to propel it.
All of these plans are based on the same general concept: acting on the object in a meaningful way and over a period of time long enough to make it fly rather than collide.
There are countless objects gushing into the great cosmic darkness. Some of them, like Apophis, we are aware of and we study them. Others, we don’t even know exist. Fortunately, the larger objects are the easiest to see and plan. Given sufficient time and sufficient cooperation, the problem is not beyond our ability to handle if we keep our eyes in the sky.
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