Asteroids and comets represent both an opportunity and a risk. They can help us unravel the mysteries of the formation of the solar system, and future generations might even be able to mine them for precious materials. Yet a single impact could cause significant damage on earth and potentially lead to the mass extinction of life.
The vast majority of known asteroids are collected in the asteroid belt, a ring located between the orbit of Mars and the orbit of Jupiter. Among all the asteroids in the solar system, a large fraction belong to the class of so-called “Near Earth Objects” (NEOs). As of March 2012, 8758 NEOs have been detected. Of those, 848 are estimated to have an effective diameter larger than one kilometer. 154 are considered to be potentially hazardous asteroids, and 1298 are categorized as potentially hazardous. It is estimated that there are between 30,000 to 300,000 NEOs with diameters around 100 meters, meaning a large number of NEOs are still undetected.
The danger they pose varies by size: Impacts from asteroids over one kilometer in diameter are expected to release over 100,000 megatons of energy with global consequences for our planet, while those with an average diameter of 100 meters are expected to release over 100 megatons of energy, potentially causing significant tsunamis and/or destruction of land or a large city.
Near misses in the past
Several instances of impacts have been recorded over the past century: One early impact occurred over Tunguska Russia, in 1908. The asteroid, estimated to have a diameter between 40 to 70 meters, exploded 8.5 kilometers above the ground with an equivalent energy to 10 megatons of TNT. More recently, a small 10 meter object exploded over the Mediterranean Sea in June 2002 with a measured explosion equivalent to 26 kilotons of TNT. In October 2009, another small asteroid was observed exploding over Indonesia, thought to be around 10 meters in diameter, with an estimated explosive energy equal to 50 kilotons TNT.
Other objects routinely miss the Earth by small distances – “small” at least when compared to the scale of the solar system. What came to be known as ‘the Great Daylight Fireball’ in October 1972 was a meteor that came within 57 kilometers of the Earth’s surface, clearly visible over the northern part of the US and Canada. In March 1989, a potential asteroid impact was publicized, with a 300-meter-asteroid missing by the Earth by 700,000 kilometers. Due its diameter and relative velocity, if the asteroid had impacted, it would have created an explosion 12 times as powerful as the most powerful nuclear bomb ever exploded. In March 2004, a relatively small asteroid was detected near the Earth, a 30-meter-asteroid that passed within 42,600 kilometers of Earth (or one tenth the distance to the Moon, or equal to the distance from earth’s surface to the geostationary ring of communication satellites). There have been numerous other close encounters, at least one per year in this decade.
The effect of an asteroid impact is often compared against either natural phenomena such as earthquakes or volcanoes, or against large nuclear explosions. Objects 50 to 150 meters in diameter are thought to result primarily in airbursts while those around 300 meters in diameter can destroy a small European country. How likely are those impacts? Experts disagree. According to a NASA report published in 2003, the expected frequency of impact of bodies between 50 and 100 meters in diameter is about one every thousand years. According to another NASA report, impacts of slightly larger asteroids – those with a diameter between 200 and 300 meters – might occur once every 63,000 years.
Besides monitoring the sky and cataloguing all potentially hazardous objects, a considerable effort has been devoted to devising mitigation techniques that could prevent an impact: From nuclear explosions to spacecraft ramming into the asteroid to low-thrust engines attached to the NEO, and to lasers: a variety of concepts and techniques have been proposed and partially developed. Up to now though, no space mission has tested any of the proposed techniques.
Contact low-thrust techniques include attaching low-thrust engines, or solar sails to the asteroid to deflect it from its original course. Impulsive techniques include ramming into the asteroid at hypervelocity with a spacecraft, or exploding a nuclear warhead in the proximity of the asteroid, or impacting the asteroid with a cloud of particles creating artificial drag with the aim of deflecting it.
Low-thrust contactless techniques include the use of lasers or solar concentrators to sublimate the surface of the asteroid and create a jet of gas to blow the asteroid away from its original trajectory. Other proposals include changing the optical properties of the asteroid.
Some new developments and techniques have been demonstrated to be more effective than others. For example, the use of ion beaming appears to be more effective than the gravity tractor, while the use of solar pumped lasers flying in formation with the asteroid seems to be more practical than directly focusing the light of the sun. On the other hand, no single technique seems to be ideal for all sizes and types of asteroid or is fully scalable. Nuclear explosions carry the highest energy per kilogram of mass of the spacecraft, and kinetic impacts are the simplest solution, but they pose some questions about the control of the deflection and on their effectiveness against highly porous asteroids.
By comparison, low-thrust techniques can provide a more controlled deflection but might require more massive systems in space and a longer deflection time.
Safety isn’t free
Europe is currently investing substantially into the development of techniques to avoid an impact. In 2011 about four million euros were awarded to a consortium known as NEOShield. In 2012 a second grant of over four million euros was awarded to the University of Strathclyde in Scotland to initiate a four-year research and training program called “Stardust.” The mandate of Stardust is to train the next generation of scientists, engineers, and decision makers that will investigate new and advanced techniques to counter the asteroid threat. Stardust will also cover other threats as well, including space debris. Man-made objects in the earth’s orbit (like abandoned satellites) can also pose a threat to space missions and humans on the surface. They are, in a way, like little asteroids.
Current technological developments and research programs like NEOShield and Stardust will bring us closer to being able to prevent catastrophic impacts. But protecting our planet from an impact is not simply a technological problem. Decision makers need to commit resources to this endeavor, or all developments will remain only on paper.