Asteroids, meteors, and us
March 2013
Earthlings have had exciting times these past few weeks, as we've been targeted by a stream of space rocks. The asteroid/meteor that exploded over Chelyabinsk, Russia on February 15 has been estimated at a mass of 10,000 tons, with a diameter of about 15 meters, or nearly 50 feet, about the size of an average five storey building. Because of its relatively modest size, it had not been observed by telescopes, and so came without warning. On the very same day, the full attention of NASA (U.S. National Aeronautics and Space Administration) was focused on an even more spectacular event: the extremely close fly-by of the asteroid 2012 DA14. This much larger object – 40-50 meters long, covering half a football field – flew as close as 17,000 miles (27000 km) to the Earth. That's about two Earth-diameters away, or about one fourteenth the distance to the moon:Abstract:
The Earth's recent shower of meteors and close misses by asteroids should remind us that if we want to mitigate a collision disaster that is sure to come – it's only a question of when – we need to enlist world-wide resources in the effort. It is no longer sufficient that the U.S. bear the burden. There are plenty of resources available, currently wasted on fighting one another. Our most serious enemy is not our neighbor, but the mass of inert and deadly rock hurtling around the sun.
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| Earth | asteroid | moon |
That's close. It's so close that in the two decades that NASA has been tracking near-Earth space objects, nothing of that size has come that close. NASA estimates that the size of the fly-by asteroid was of the same order as the one that caused the devastation at Tunguska, Russia in 1908, where it leveled 830 square miles (2150 sq.km) of spruce forest, or the meteor that resulted in the famous "Meteor Crater" in Arizona, some 50,000 years ago. Though these two were of roughly the same magnitude, the Arizona meteor had a high iron content which allowed it to survive the heating by the atmosphere and so reach the ground, while the Tunguska meteor was evidently of a less dense mineral type, so that it exploded above the ground. Clearly both types caused massive destruction on the ground, with the explosive energy of multiple atom bombs.
Predictably – and unfortunately – the world's news media covered these events in their usual style: a couple of days of front page drama, and then you move on to something else. They're called "news" media for a reason. When it's no longer news, the subject moves off the radar. I say "unfortunate" because these events gave us a golden opportunity – perhaps now already wasted – to carry on a serious debate about the threat of disaster from space. It's doubly unfortunate that politicians are generally so occupied by our own human-caused and largely unnecessary conflicts that they are unable to take such threats seriously.
There was nothing surprising in the space bombardment of the past weeks. We know that there are statistically determinable odds of the Earth being struck by asteroids and comets of various sizes. We can safely say that there are trillions (i.e., millions of millions) of space rocks zooming about in the solar system, in various orbits around the Sun. Of those, only a small percentage is of interest as possible collision hazards with the Earth, since most lie in orbits far outside ours. The vast majority of those that are on a potential collision course with Earth are very small fragments, and these hit the Earth's atmosphere at the rate of tens of thousands each day (about 100 tons of material daily, according to NASA). Almost all these objects are burned up in the atmosphere without being seen; the larger of them provide us with our meteor ("shooting star") spectacles. Those the size of a car may spatter the ocean or the ground with rock or iron fragments – meteorites – generally doing no harm. But asteroids the size of a house – like the Chelyabinsk meteor – may cause significant local damage, and the near-miss asteroid of last month could have leveled a large city if it had hit the Earth.
We know that solid – rocky or metallic – asteroids are fragments of planets or protoplanets (smaller bodies in the process of accreting into a larger body), because dense rock can only form on a body large enough to compress the mineral constituents under its force of gravity. Such a fragment would have been ejected from its parent body by various mechanisms, the most probable being a collision with another body. In the early solar system, two to four billion years ago, such collisions were much more common than today. Collisions between planet-sized objects would likely have occurred and resulted in large masses of rock fragments spread in the general orbit of the larger body, and such collisions may be the origin of the current asteroid belt between the orbits of Mars and Jupiter.
An interesting point in this connection is that the currently favored explanation of the formation of the Earth's Moon is that it coalesced from fragments ejected from the Earth as a result of Earth's collision with another planet perhaps the size of Mars (one tenth the Earth's mass), nearly four and a half billion years ago, shortly after the Earth solidified. This phantom planet has been called Theia, appropriately a Greek Titan goddess, mother of the Moon goddess. (This major hit may also have been responsible for the Earth's axial tilt (obliquity) of 23.4 degrees from the ecliptic – the plane of our orbit around the sun, since its original axis of rotation would likely have been in the orbital plane.) It's the nature of orbits that they tend to repeat themselves, though in the complex gravitational field of the solar system, orbits – especially of smaller bodies – are routinely altered. Nevertheless, there's a good chance that a certain amount of the material constantly raining down on the Earth from space is original Earth material, come home after a four billion year sojourn. Most analyzed meteorites and asteroids, though, suggest a non-terrestrial origin.
Many concerned scientists, both in the U.S. and around the world, have spoken out on the need to beef up our effort to detect and deflect any near-Earth object that might be a serious hazard. NASA's program to locate such objects has been funded for about two decades, and has done a very good job: they estimate that they have located and calculated orbits for about 95 percent of near-Earth objects of a diameter (or length) of 1 kilometer (0.6 mile) or more. These are the objects that could cause a world-wide ecological disaster and threaten our species' survival if they hit the Earth. But the funding has not been sufficient to get very far with locating the much more numerous smaller objects, such as February's near-miss asteroid, that could still cause a regional disaster if a collision should occur. NASA estimates that they have only located about ten percent of objects between one kilometer and 140 meters in size. Thousands of these undetected objects are potentially hazardous, and may not get picked up by telescopes until a few days before impact, a time that is much too short to do anything to prevent it. (The minimum warning time needed to actually alter the course of an incoming object is at least on the order of several months.)
In recent years additional countries have joined the hunt for near-Earth objects. Japan, Canada, the European Union, and a few others have funded modest programs. These help, but they are by no means enough. It's important to note that the efforts that have been made to date have concentrated on detecting the threats. This is a reasonable priority, but the effort to deflect the threat has received much less attention, though NASA and the European Space Agency (ESA) have theorized a good deal on the subject. No one has yet performed a practical test of sending a probe to nudge, tow, push, hit, explode or otherwise attempt to change the orbit of any near-Earth object. Certainly, a number of such missions will need to be flown in order to be confident of having found an effective means of predictably altering the body's orbit. It further complicates matters that different methods will probably be needed for the different compositions, consistencies, sizes, tumbling rates, and orbits of asteroids and comets.
All of which is to say that defending ourselves from a tragedy that we know is coming from space – though we don't know when – and for which we are unprepared ought to have a higher priority than military posturing against our earthly neighbors. The world's bloated military budgets are where we should find the funds to launch a world-wide, concentrated effort to develop the defense we need against our common enemy, that fateful rock somewhere in the dark of space, like a messenger from the gods carrying our address. Our military forces are called "departments of defense." It's time to defend us all.
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