From: Senate Committee on Commerce, Science, and Transportation
Posted: Wednesday, April 7, 2004
Given at a Senate Science, Technology, and Space Hearing: Near Earth Objects (NEO) Wednesday, April 7 2004 - 2:30 PM - SR -253
The Testimony of Dr. Michael Griffin, Head of the Space Department, John Hopkins University Applied Physics Lab
Mister Chairman and members of the subcommittee, thank you for giving me this opportunity to comment on the greatest natural threat to the long-term survivability of mankind, an asteroid impact with the Earth. Throughout its history, the Earth has continuously been bombarded by objects ranging in size from dust particles to comets or asteroids greater than 10 km in diameter. Although the probability of the Earth being hit by a large object in this century is low, the effects of an impact are so catastrophic that it is essential to prepare a defense against such an occurrence. The first step in that defense is a system to identify and catalog all potential impactors above the threshold of significant damage, approximately 100 meters in diameter. Later, the remainder of a comprehensive Earth-protection system could be assembled so that it would be ready to deflect a potential impactor shortly after it is identified.
In 1998, NASA embraced the goal of finding and cataloging, within 10 years, 90% of all near-Earth objects (NEOs) with diameters greater than 1 km. Impacts by objects of this size and larger could result in worldwide damage, and the possible elimination of the human race. The current system is not sufficient to catalog the population of smaller NEOs. While there are thought to be nearly a thousand objects with diameters greater than 1 km, there are a great many smaller NEOs that could devastate a region or local area. The exact NEO size distribution is not known; however a good current estimate is that there are more than 5 times as many objects with diameters greater than 1/2 km than there are with diameters greater than 1 km. This multiplication of numbers for smaller diameters continues for all sizes at least down to those just large enough to make it through the atmosphere. Thus, if there are about 700 NEOs of 1 km or greater, there are more than 150,000 NEOs with diameters greater than 100 m. The Tunguska event in Siberia in 1908 destroyed an area 50 km in diameter and is believed to have been caused by an impactor less than 50 m in diameter.
The average speed of objects colliding with Earth is about 20 km/s (about 45,000 miles per hour). At these speeds the energy of impact is 44 times the explosive power of the same mass of TNT. Thus, the energy released by the impact of a 100 m object is about equivalent to a 50 megaton bomb. The impacts at Tunguska in 1908, Sikhote-Alin (about 270 miles northeast of Vladivostok) in February 1947, and the recently identified objects that have had near misses with Earth, all show us that impacts with the ability to wipe a large metropolitan area can be expected during the next 100 years.
A great deal has been learned about the nature of the threat in the last decade. It is vital to understand the characteristics of NEOs to know how to defend against a potential impactor. An improved theoretical understanding of the population of NEOs has clarified their evolution through interactions with the planets of our solar system. It has helped us understand their numbers and their distribution in the different classes of orbits. On the practical side, the progress of several space missions has greatly improved our understanding of the physical and chemical characteristics of these objects. A great deal still needs to be done since only a handful of these objects have been observed from sufficiently close distances to see their surface structure, and only one asteroid has been orbited. The Near Earth Asteroid Rendezvous (NEAR) mission orbited and landed on 433-Eros and was able to get the first estimates of the internal structure and composition of a NEO. However, there is still a great deal more that will have to be known about an object if it becomes necessary to deflect it from a collision course with Earth.
In addition to the threat that NEOs represent, they are also potential suppliers of resources for future manned space exploration. In order to use these resources, a much more detailed knowledge of their composition and physical characteristics will be required before the technologies to produce fuels or construction materials from NEOs can be developed.
Current and Future Technologies for Earth Protection
It is estimated that a 30-year advance warning would be required to have a reasonable assurance of deflecting a NEO from a collision with Earth. Thus, if a future impactor were identified today, the time to explore the characteristics of the NEO, develop a deflection system, deliver it to the NEO, and apply the deflection early enough to prevent an impact, requires about a 3-decade lead time.
The deflection technologies available today, which are chemical rockets and nuclear weapons, both have limited abilities to slow down or speed up an asteroid. A 100 m object has a mass of the order of 1 million tons, and a 1 km object has a mass of the order of 1 billion tons. To prevent an object from colliding with Earth, it must be sped up or slowed down by about 7 cm/s (about 1/6 of an mile per hour) divided by the number of years in advance that the change is applied. The fuel that can be contained in a medium-sized scientific spacecraft could successfully deflect a 100 m body if it were pushed about 15 years in advance. The Space Shuttle's main engines and the fuel contained in the large external tank could successfully deflect a 1 km diameter object if it were applied about 20 years in advance. Nuclear weapons carry much greater impulse for their mass. However, they deliver that impulse so quickly that they are more likely to break up the body than to deflect it. Because NEOs are in their own orbits around the Sun, the pieces of a disrupted object will tend to come together one half of an orbital period later. Therefore, the successful use of nuclear weapons for deflection will require the development of techniques for slowing the delivery of the impulse to the NEO and will probably also require many small weapons to be used to deflect a single NEO.
The orbital mechanics required to approach a potential impactor also require it to be identified early. It may take 5 years or more for any deflector mission to rendezvous with a NEO in an arbitrary Earth-crossing orbit.
What Remains to be Done
An overall Earth protection system must have three components. First, a search system is needed to identify any potential NEO impactors. Second, a series of detailed investigation missions are needed to understand the structure, composition, rotational state, and other physical properties of potential impactors. And finally, deflection technologies are needed to change the speed of a NEO to ensure that it will not impact Earth.
The United States and other countries around the world have concentrated on the first part of the Earth-protection system. At the current rate of discovery, the group of observatories that are finding and cataloging NEOs will come close to achieving their goal of identifying 90% of the greater than 1-km diameter NEO population by 2008. More than 50% of the expected population has already been discovered and discoveries continue to be made each month. While this effort will retire most of the risk of a global catastrophe, the size distribution of NEOs shows us that there are a great many more small objects than larger ones. Their numbers increase by a factor of about 220 for a diameter that is reduced by a factor of 10. This very large number of small-to-modest sized objects represents the greatest remaining threat to regional safety that is not being addressed. The equipment used by current NEO surveys is sized to find the largest objects. Some sub-kilometer objects are found serendipitously; however, these telescope systems are not optimized to find the smaller objects.
A NASA NEO Science Definition Team recently examined the requirements for extending the NEO search to smaller diameters and showed that a system to accomplish the discovery and cataloging of 90% of all NEO greater than 100 m diameter within 10 years could be accomplished with a single Discovery-class spacecraft in a heliocentric orbit at about 0.7 AU. This modestly priced system (the Discovery class is about $300 million full mission cost) could be constructed and put on-station in four to five years.
Detailed Examination of NEOs
Several space missions that are contributing to the detailed investigations of NEOs and comets have been launched and others are currently in development. As stated above, the NEAR mission provided the first detailed information on the mass, shape, structure, and composition of an asteroid. However, we know from ground-based spectroscopic data that there is a great deal of variability among these objects.
The Giotto and Deep Space 1 missions took close images of comets Halley and Borelli. The Stardust mission will be returning with dust particles from comet Wild 2 in January of 2006. The Deep Impact mission will create a crater in comet Tempel 1 to learn about the internal composition of comets. And the DAWN mission will examine the composition of asteroids 1 Ceres and 4 Vesta, two of the largest planetoids in the solar system. These missions are making steady progress in our understanding of the formation of the solar system and the characteristics of the small bodies within it. Continuation of this series of investigations is vital to our future ability to deal with the threat and opportunities of NEOs.
While there has been a great deal of theoretical examination of deflection techniques, no practical systems exist at this time. As the search systems and detailed examination missions progress, it is important to continue the development of deflection system technologies so that a full Earth-protection system could be deployed rapidly if a future impactor is discovered by the search systems.
The threat to life on Earth from NEOs is real even though the likelihood of a severe impact during the next few years is low. The most important thing that is needed in order to deal with this risk is an improved search system. Recent studies have shown that a search spacecraft that can catalog 90% of the remaining NEOs larger than 100 m in diameter over 10 years of operation can be launched within 4 or 5 years at the cost of a NASA Discovery-class mission. In addition, the pace of mission developments for detailed examination of small solar system bodies should continue undiminished. This is clearly summarized by the cartoon below, originally published in New Yorker magazine in 1998.
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