Before describing the capability required in a PDS, it is important to define its intended mission. Simply stated, the PDS mission is "to defend the Earth-Moon system against all Earth-crossing-object threats."⁵⁴ At this time, Planetary Defense (detecting, tracking, cataloging, or mitigating ECOs) is not an assigned or approved mission of the Department or Defense or the Air Force.

The capability required of a PDS varies with the scenarios that may occur. Table 3 provides four different scenarios which depend upon the path of the ECO.⁵⁵ The ECO scenario reveals time available for action, nature of action required, probabilities of detection (percentage of currently estimated known ECOs greater than 1 kilometer diameter and percentage of those yet to be detected), distances at which they will likely be detected, deflection velocities (V) required to mitigate, and likely type of ECO (an ECA is an earth-crossing asteroid). An ideal PDS would provide adequate defense for all four scenarios.

One only has to watch Star Trek to imagine the ultimate system to be used to detect and mitigate ECOs-the Enterprise. The Enterprise's on-board detection systems, command, control, communications, and computer, intelligence systems, photon laser systems, and capability to travel at 10 times the speed of light would enable it to protect the earth from all but the least likely of scenarios such as multiple or large ECOs. Limits to advances in technology and spending make it unlikely that such a system would be developed by 2025.

The Enterprise, however, does provide an advanced system model from which we can deduce current or future systems capable of yielding similar results. Such a system can be broken down into three main subsystems: detection, C⁴I, and mitigation.

The earlier an ECO is detected, the more time is available for mitigation action. Thus, of the three subsystems, detection subsystems appear to be the most critical at the present time. It is the first system that should be funded, researched, developed, and deployed. Fortunately, some initial steps in the correct direction already have been taken with regard to detection, The most notably has been initial components of the Spaceguard Detection Network (described in a later section). By 2025 the PDS detection subsystem must be much improved in regards to search (sky coverage), focusing speed, range, and resolution.

Command, control, communications, and computer subsystems are the glue to hold the PDS together. Advanced command, control and computers systems will be necessary to optimize scanning, tracking, and orbit determination for the detection system. Intelligence systems are necessary to determine the composition, strength, and other physical characteristics of ECOs. Advanced command, control, communications and computer systems are required to direct the mitigation systems to their targets and perform their mission. As detection capabilities improve, C⁴I must keep pace with the expanding volume of data that must be shared among globally dispersed observation sites. Present coordination methods using the telephone, fax, and electronic mail for follow-up will be grossly inadequate. Follow-up notification must be immediate, and search data must be updated and shared globally in real-time. Fortunately, communications bandwidth and data storage technologies are expanding at a breathtaking rate even without the concern of planetary defense. Required system capabilities should be available prior to 2025.

Ready-to-go subsystems with ECO mitigation capabilities do not currently exist, though many scientists believe nuclear weapons could provide near-term protection with modification. Many potential nonnuclear defense subsystems have been identified in the past, and we have proposed several more, though we admit they are on the fringe between reality and imagination. Regardless of type, we are not convinced that mitigation subsystems need to be developed in the near term or even prior to 2025. It is perhaps better for us to encourage and wait for technology breakthroughs to drive the direction of these subsystems. If we develop a capable detection subsystem and it detects an ECO of concern, then a timetable for complete mitigation subsystems development and deployment will be necessary and priority for funding will be justified. By 2025 safer, cheaper, and more politically acceptable mitigation systems than the current nuclear systems should be available.

Humanity has observed and often recorded the phenomena of comets, meteors, and meteorites throughout the recorded history, however little was understood. In 616 AD the Chinese reported the crushing of 10 people by a meteorite. The idea that comets might possibly strike the earth was first considered by Jakob Bernoulli a millennium later, in 1682. Fourteen years later, William Whiston predicted that the comet of 1680 would next return in 2255, when it would impact the earth and cause the end of the world. Nearly a century later, in 1777, Anders Lexell showed that the comet observed seven years earlier had made what is still the record confirmed closest approach to earth, little more than 1.2 million miles. And in 1801, Ceres, the first asteroid was discovered.⁵⁶

Little concern with the prospect of an ECO impact seemed evident, however, until the near-earth passage of the asteroid Icarus in 1968. Although the orbit was carefully monitored to bring it no closer than 3.6 million miles from earth, professors at the Massachusetts Institute of Technology challenged 21 students in the Advanced Space Systems Engineering course to propose what could be done if Icarus, the 13th known near-earth asteroid, happened onto a collision course with earth. At least 30 newspapers and other print media published sensationalized and often distorted accounts of the project and the circumstances of the asteroid impact. As a result, many Americans for the first time became aware of both the possibility of an ECO impact and the possibility that something could be done about it.⁵⁷ In 1980, when a new theory explained the extinction of dinosaurs due to a gigantic asteroid impact, the attention of the scientific community was at an all-time high. The concept of planetary defense began to move appreciably forward, at least in the sense of determining the level of an ECO threat.

Current Detection Programs

By 1982 the discovery rate of NEOs reached 10 each year as several systematic photographic search programs were established. The greatest leap forward thus far in the area of ECO detection occurred in 1989, when the Spacewatch program began operation. Conceived and directed by Tom Gehrels at the University of Arizona, Spacewatch incorporates modern electron charge-coupled detectors (CCD) and computers to automate much of the discovery process. Digital intensity information is read from a 2,048 x 2,048 pixel array and is used to build an exhaustive catalogue of all objects, including stars, galaxies, belt asteroids, comets, and NEOs in the image. The data is stored magnetically, and later the same night, the computer directs the 36-inch telescope back to the same area for a second image. The computer instantly compares the objects in the second image with the first, checking off each object against what is stored in the catalogue and notes any feature that only appears in one image. Finally, the computer takes a third image to verify that objects that seem to move between the first two images, continue to do so.⁵⁸ On a good clear night, as many as 600 new asteroids are discovered, and on average, one in 900 of these is a NEO.⁵⁹

With planned improvements to the Spacewatch network including a new 1.8-meter mirror telescope at Kitt Peak and electronics upgrades in Australia and in France, Mr Gehrels estimates that if there are any 1 kilometer or larger asteroids on a collision course with the EMS, we should know of them by the year 2008. Unfortunately, though, Spacewatch will not be sufficient to entirely rule out the threat of smaller but still dangerous asteroids and of long period comets.⁶⁰

Figure 3-1 shows the locations worldwide of the four current ECO search programs. (At Palomar, California: the Palomar Asteroid and Comet Survey and Palomar Planet-Crossing Asteroid Survey surveys; at Kitt Peak, Arizona: Spacewatch; in Western Australia: Anglo-Australian Near-Earth Asteroid Survey.⁶¹)

Note that only one survey is currently operational in the Southern Hemisphere. The 1991 Spaceguard Survey Workshop recommended a $50 million up-front and $10-15 million per-year program.⁶² With six globally dispersed Spacewatch-type telescopes, scientists expect to achieve a discovery rate of one object for every two seconds of observation time.⁶³ (In addition to Kitt Peak and Palomar, other Northern Hemisphere observatories would be located, possibly in India and France. In the Southern Hemisphere, in addition to Australia, Chile would be an ideal site.⁶⁴)

Source: Hazards, 129-136.

Detection, Tracking, and Homing

The detection subsystem of the 2025 PDS is comprised of three, broad functional roles, each of which can be further subdivided into several discrete tasks. The roles, in sequence include detection, tracking, and homing.

The detection role is comprised of two tasks: discovery and discrimination. The PDS detection subsystem detects all potential ECOs at a maximum distance from the EMS. Long-range detection equates to advance warning time. Advance notice of a potential impactor is the single most important variable in the PDS problem. The earlier an ECO is discovered, the more options are available to mitigate the threat. The detection system or systems should continuously search the total volume of space for all asteroids and comets that exceed a size and mass that can be assured to burn up during descent through the earth's atmosphere.

It is of great importance also to quickly determine whether the just-detected object is a true, first-time discovery, or whether it has been previously discovered, catalogued, and then lost for a period of time because of obstructions or excessive distance from earth.

The problems of long range and discrimination are not the only major detection obstacles to overcome. The volume of sky is perhaps the greatest obstacle. Present telescope capabilities only can search approximately 6,000 square degrees of the night sky each month. Total sky coverage is 41,000 square degrees.⁶⁵

For 2025 we have specified a required capability to search the entire volume of space on a daily basis, to detect an object of a minimum size of 100 meters in diameter at a minimum distance of 2.5 astronomical units (AU) (slightly more than the average distance to the main asteroid belt between Mars and Jupiter of 2.2 AU from Earth), and to confirm within seconds whether the object is a new discovery or is an object that is already cataloged.⁶⁶ Current Spacewatch capabilities require 150 telescopes to discover all 200,000 (or more) 250 meter ECOs within 20 years and orders of magnitude more of them to discover 100 meter objects.⁶⁷ Obviously, this is not the solution. Computers must be harnessed to modern telescopes in a way to dramatically reduce the time it takes to make initial and follow-up observations.

Tracking, the second broad role, begins as soon as it is determined that an object has the potential to impact the EMS. The tracking role encompasses the follow-up functions of astrometric analysis and the constant awareness of the object's whereabouts. Astrometric analysis refers to the precise calculation of position and velocity. These aspects are discussed in detail in the later C⁴I section. The tracking subsystem should strive to use an independent means of orbit calculation to confirm the initial diagnosis of an earth-crossing orbit or dangerously close passage. Calculation of an EMS threatening orbit must be completed with sufficient advance notice to still permit selection of the most benign and most cost-effective approach to mitigate the threat.

For 2025 our tracking requirements are that astrometric analysis be completed within hours of discovery, the ability to know an ECO's whereabouts at all times regardless of whether it may be visually blocked by other celestial objects in the foreground or background, the ability to track an ECO regardless of meteorological conditions and the effects of daylight and moonlight, and, the ability to feed targeting information in realtime, or near real-time, to the mitigation system throughout application.

The last broad role of detection is homing/results assessment. In one sense it can be thought of as targeting and battle damage assessment (BDA). However, in planetary defense, destruction of an ECO is only one possible response to the situation.

Specific 2025 tasks and requirements encompass the ability to accurately guide a spacecraft to the ECO, to observe on earth the mitigation actions as they are applied, immediate feedback of the success or failure of the mitigation action, and, if mitigation is unsuccessful or only partially successful, continued observation until successful hand-off to the detection or tracking subsystem.

In summary, detection is currently the most advanced portion of the PDS by far. The seven-year-old Spacewatch program is currently searching space for 1 kilometer and larger ECOs, and all earth-crossing asteroids should be known by 2008. However, several major shortfalls exist with Spacewatch. First, the Spacewatch ECO size cut-off at 1 kilometer and greater is an order of magnitude larger than we feel can be safely ignored. Secondly, the current rate of discoveries is barely acceptable at the 1 kilometer size cut-off (given a total estimated population of approximately 2,000). To search for all objects greater than 100 meters the estimated population climbs to several hundred thousands, thus a significantly faster detection rate must be achieved.

Detection Concepts for 2025

So, how can the greater rates of discovery necessary in 2025 be achieved? One way of substantially increasing ECO discovery rates is by using the current capability of the USAF's Ground-Based Electro-Optical Deep Space Surveillance System assets. It is estimated that a single GEODSS telescope could improve upon the Spacewatch program's discovery rate by a factor of 20.⁶⁸ To speed tracking solutions, increased access to the large planetary radars at Puerto Rico and California is also recommended.⁶⁹

One 2025 concept is to employ change detection sensors. Rather than scrutinizing all objects in space, the sensors would search only for movement ("change") in space. With movement sensitivity properly gauged to eliminate distant bodies, observation devices could concentrate only on near-earth and thus potentially earth-crossing objects.⁷⁰

How also will daily total sky coverage and constant, real-time tracking occur? Use of only ground-based optical assets is insufficient to search the total sky. While ground-based optical can currently detect 100 meter ECOs in opposition (on the side of the Earth opposite from the Sun), they are blinded when objects are in conjunction (sun side). Emerging technologies available in 2025 should be better able to handle this problem.

Use of space for basing space observation platforms makes good sense for 2025. While it is currently much more expensive to use a space-based platform rather than a ground-based one, the cost difference should be less pronounced in 2025, particularly when effectiveness and lack of downtime are factored in. Space-based systems will not have to deal with clouds, weather, and pollution, for example.

Figure 3-2 shows one detection concept suggested by the Lawrence Livermore National Laboratory. By placing sensors in space, operational time is substantially increased, surface weather conditions are eliminated as an obstacle to viewing faint objects, and a larger unobstructed field of view is possible.

Source: L. L. Wood, et al., "Cosmic Bombardment IV: Averting Catastrophe in the Here-and-Now," Presentation to Problems of Earth Protection Against the Impact with Near-Earth Objects (SPE-94) (Chelybinsk, Russia: Russian Federal Nuclear Center, 26-30 September 1994).

Table 4 summarizes potential detection technologies and systems with respect to their technology availability, ECO scenario applicability, risk level, problems, maintenance requirements, and cost.

Borrowing from the New World Vistas study, distributed constellations of lightweight and relatively inexpensive sensing satellites could be deployed and linked to each other by laser data links.⁷¹

^* ECO Scenarios 1-4 are described in Table 3.

Active sensing systems on these satellites would potentially use infrared, light detection and ranging (LIDAR), radar, laser detection and ranging (LADAR), and radio array to detect the radiation and low-frequency radio emissions caused by object movement in the solar winds.

Satellite constellations might best be placed in orbits other than around the earth. For example, Aten asteroids, which threaten the earth from the sunward side, could be detected by satellites in orbit around Venus, Mars, or Jupiter or by satellites in a halo orbit around the Lagrangian point between the Earth and the Sun, or in solar orbit above the main asteroid belt between Mars or Jupiter.⁷²

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