The universe is mostly empty, but not quite. Asteroids, also called minor planets or planetoids, float around and can potentially be dangerous for the Earth. Just looking at a map of the asteroids in the solar system should be enough to convince most people that we are not as alone in our corner of space as we sometimes might think.
A study on asteroids from 2002 said:
Asteroids in our Solar System may be more numerous than previously thought, according to the first systematic search for these objects performed in the infrared, with ESA’s Infrared Space Observatory, ISO. The ISO Deep Asteroid Search indicates that there are between 1.1 million and 1.9 million ‘space rocks’ larger than 1 kilometre in diameter in the so-called ‘main asteroid belt’, about twice as many as previously believed.
Don’t panic, there’s this reassuring sentence:
However, astronomers think it is premature to revise current assessments of the risk of the Earth being hit by an asteroid.
But what comes right after it reveals how problematic things can be:
Despite being in our own Solar System, asteroids can be more difficult to study than very distant galaxies. With sizes of up to one thousand kilometres in diameter, the brightness of these rocky objects may vary considerably in just a few minutes. They move very quickly with respect to the stars – they have been dubbed ‘vermin of the sky‘ because they often appear as trails on long exposure images. This elusiveness explains why their actual number and size distribution remains uncertain. Most of the almost 40,000 asteroids catalogued so far (1) orbit the Sun forming the ‘main asteroid belt’, between Mars and Jupiter, too far to pose any threat to Earth. However, space-watchers do keep a closer eye on another category of asteroids, the ‘Near Earth Asteroids’ or ‘NEAs’, which are those whose orbits cross, or are likely to cross, that of our planet.
Though the asteroid belt itself, being relatively far from Earth, is not directly a problem (the inner edge of it is farther than Mars’ orbit, which is itself at half the distance between the Earth and the Sun, or 0.5 Astronomical Unit), it is important to better understand it because that’s where most of the “Near Earth Asteroids” (NEAs) or “Near Earth Objects” (NEOs) come from, and those share the same characteristics as belt asteroids (elusive “vermin of the sky”).
most NEA are believed to be former main belt asteroids. In the main belt there are four ‘special’ regions where Jupiter’s gravitational influence is especially disruptive; originally, most asteroids currently known as NEA suffered collisions which resulted in them ending up in one of those four key regions, and because of Jupiter’s gravitational influence their orbits quickly evolved into Earth-crossing orbits. Therefore, by studying the asteroids near these so-called ‘source regions’ in the main belt astronomers can learn about NEA. About 500 NEAs have been found so far*, and none of them pose any threat to Earth in this century.
* The study above was published in 2002. According to this NASA page, the number in 2002 was 523, but in 2007 (so far, up to April) it is up to 707 known “large” (one kilometer of diameter or larger) Near Earth Asteroids (NEAs).
The total number of known NEAs was 2165 at the end of 2002 and 4647 in April 2007. Here is a graph of the data from 1980 to November 2006:
Another interesting page on the NASA site is the NEO Earth Close Approaches page. It shows the recent and upcoming “close approaches” with the date, the miss distance (calculated in Astronomical Units, one being 149,597,870.691 kilometers, and “lunar distances”, one being 384,000 kilometers), the estimated diameter of the object, and the velocity of the NEO relative to the Earth’s (in kilometer per second).
The closest recent “close approach” detected was an object with an estimated diameter of 29 to 65 meters. It flew by the Earth at 9.32 kilometers per second on April 2, 2007. It missed by 5.3 lunar distances, or about 2.03 million kilometers.
The closest “upcoming approach” is for a relatively small object about 30 to 67 meters in diameter (about the size of a big house, think McMansion) that should pass by the Earth at about 2.1 lunar distances (806,400 kilometers) on April 16, 2007, moving at about 16.93 kilometers per second. Mark your calendar.
The Wikipedia article on meteorites says:
Most meteoroids disintegrate when entering the Earth’s atmosphere. However an estimated 500 meteorites ranging in size from marbles to basketballs or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists.
This NASA site says:
Millions of meteors occur in the earth’s atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers). [...]
The size of meteorites varies greatly. Most of them are relatively small. The largest meteorite ever found weighs about 66 short tons (60 metric tons). It fell at Hoba West, a farm near Grootfontein, Namibia. However, much larger bodies, such as asteroids and comets, can also strike the earth and become meteorites.
So an object the size of a large house would definitely not fall into the category of normal meteorites, and it would reach the Earth without completely disintegrating in the upper atmosphere (unless it is made of ice or something similar, I suppose).
What are the probabilities (as far as we can currently tell) that a NEO of “large” size could impact the Earth in the near future?
This page on the NASA site shows “impact risks”:
[JPL] Sentry is a highly automated collision monitoring system that continually scans the most current asteroid catalog for possibilities of future impact with Earth over the next 100 years. Whenever a potential impact is detected it will be analyzed and the results immediately published here, except in unusual cases where an IAU Technical Review is underway.
Right now, the whole page is a very relaxing sight. It is color-coded, with blue for objects that have an “estimated diameter 50 meters or less” and are “not likely to cause significant damage in the event of an impact, although impact damage does depend heavily upon the specific (and usually unknown) physical properties of the object in question” (so it depends), and white means:
The likelihood of a collision is zero, or is so low as to be effectively zero. Also applies to small objects such as meteors and bodies that burn up in the atmosphere as well as infrequent meteorite falls that rarely cause damage.
You can see the other colors on the Torino scale here.
Here is what they say about objects that would be categorized in the Red Zone:
Thankfully, the probabilities of a serious impact are very low, but we also have to remember that reality doesn’t care much about our statistics; if we are using erroneous data to make our predictions, it is very possible that our current level of risk is higher than we think. There is also always the fact that some people do win the lottery despite the odds, and that if humanity is planning on sticking around for a long time, we’ll need to face the problem eventually — better sooner rather than later.
I am not in a position to precisely tell if our current defense system is adequate, but from the little I know, it doesn’t seem to be. I don’t think that we are allocating enough resources to track as many Near Earth Objects as our technology allows us to, but most importantly, we don’t have a system that would allow us to do something if we detected a threatening object.
The Lifeboat Foundation has a page about Asteroid Impacts and the necessity of creating an “Asteroid Shield”:
If we don’t do something, sooner or later Earth will be hit by an asteroid large enough to kill all or most of us. That includes the plants and animals, not just people. Maybe this won’t happen for millions of years. Maybe in 15 minutes. We don’t know. For example, on 23 March 1989 asteroid 1989FC with the potential impact energy of over 1,000 megatons (roughly the equivalent a thousand of the most powerful nuclear bombs) missed Earth by about six hours . We first saw this fellow after closest approach. If 1989FC had come in six hours later most of us would have been killed with zero warning.
The method they suggest is very elegant:
Picture credit: Dan Durda
The goal is to first detect the asteroid and then to alter its orbit . If you attempt to destroy an asteroid as they often do in Hollywood movies, you will likely change the situation from a single impact situation to a many impact situation. [...]
We support the proposal by the B612 Foundation to significantly alter the orbit of an asteroid in a controlled manner by 2015. They propose use of the gravity deflection approach, where you station a spacecraft a short distance from the asteroid, and use the gravitational attraction between spacecraft and asteroid to pull the asteroid off course.
This method would be less likely to break up an asteroid than alternative methods since there would be no physical contact with the asteroid. You do not want the asteroid to break into multiple pieces as you will then have multiple problems instead of one problem . Also note that a large asteroid could be blown apart by a nuclear device detonated in its core only to have gravity draw the asteroid back together, essentially nullifying the effect of the explosion.
On the detection front, one interesting (but still at a very early stage) project is Orbit@Home, a distributed computing project running on the Berkeley Open Infrastructure for Network Computing (BOINC — See note at the end** for my comments on this). The project, when running, would calculate the orbit of as many NEOs as possible and report, as fast as possible, the result so that – if need be – we can act.
The lead developer of the project answers some questions about it in this discussion forum thread:
[T]he data relative to the asteroids is already collected by the Minor Planet Center. Every observer is asked to send his observations there, and then MPC processes them, and publishes the Minor Planet Electronic Circulars, or MPECs. The latest MPECs are available here. So as a beginning, orbit@home will monitor these files. Every time a new MPEC is published, the data is collected and processed by [orbit@home], creating [work units], waiting for results, and then updating the local database. Orbit@home doesn’t need to recruit astronomers, or have any particular connection with observatories. All the data available will be processed by o@h, without limits on country or team. Backyard astronomers are welcome, but before they should go trough the process of getting an MPC code, that also certifies the quality of the data (see MPC website).
Orbit@Home is not currently active, but on February 26, 2007, they have announced that they finally got enough funding to allow them to release a public beta and have “new work units generated on a daily basis, and a graphical screen saver.” When? That’s not clear yet, but you can sign up for the project and have you BOINC client on standby (you can join more than on project, so in the meantime you can crunch other kinds of scientific data).
Well, that’s it for now. I’ve learned a lot writing this post, and I hope that you had fun reading it.
- Asteroids, Tsunamis, and Knowing When To Shout
- Torino 2, and Counting
- Life in the Shooting Gallery
- Gravity Tugboat for Dangerous Asteroids
- Earth Impact Effects Program
- Possibility of an Earth Impact in 2029 Ruled Out for Asteroid 2004 MN4
- Tunguska event, a.k.a., the Great Siberian Explosion
Note: I am no astronomer. I’m not even an amateur astronomer. If you find errors in this post, please let me know in the comments and I will correct them and give you credit. My goal is just to share what I have learned about Near Earth Objects in the hope that others will find it interesting. I don’t claim to be an expert on the subject.
**BOINC: The best projects to run right now, in my opinion, are Rosetta@Home and ClimatePrediction.net; respectively, they use the spare cycles of your computer’s CPU to crunch scientific data related to protein structure prediction (a holy grail of biotechnology) and run very complex climate models to better understand global warming.