NATIONAL AERONAUTICS A ND SPACE ADMINIS T RATI ON Near-Earth Object Survey and Deflection Analysis of Alternatives Report to Congress March 2007 SUMMARY Section 321 of the NASA Authorization Act of 2005 (Public Law No. 109-155), also known as the George E. Brown, Jr. Near-Ear th Object Survey Act, directs the NASA Adm i nistra tor to transm it an in itial r e port to Congress not later than one year after the date of enactm e nt that provides: (1) an an alysis of possible alte rnatives that NASA m a y em ploy to carry out the survey program of near-Earth Objects (NEO), including ground- based and sp ace-based alternatives with tec hnical descrip tion s ; (2) a reco mmended option and proposed budget to carry out the surv ey program pursuant to the recommended option; and (3) an analysis of possible altern atives that NASA could em ploy to divert an object on a likely collision course with Earth. The objectives of the George E. Brown, Jr. NEO Survey Pro g ram are to detect, track , catalogue, and characterize the physical characteri stics of NEOs equal to or larger than 140 m e ters in diam eter with a perihelion distance of less than 1.3 AU (Astronom ical Units) from the Sun, achieving 90 percent com p l e tion of the survey w ithin 15 years after enactm e nt of the NASA Authorization Act of 2005. The Act was signed into law by Presiden t Bush on December 30, 2005. A study team, led by NASA’s Office of Progr am Analysis and Evaluation (PA&E), conducted the analysis of alte rnatives with inputs from seve ral other U.S. governm e nt agencies, international organizations, and repr esentatives of private organizations. The team developed a range of possible options from public and private sources and then analyzed their cap abilities a nd levels of perfor m a nce incl uding development schedules and techn i cal risk s. Key Findin g s for the S u rvey Program: • The goal of the Survey P r ogram should be m odifi ed to detect, track, catalogue, and characterize, by the end of 2020, 90 pe rcen t of all Potentially Hazard ous Objects (PH O s) greater than 140 m e t e rs whose orbits pass within 0.05 AU of the Earth’s orbit (as opposed to surveying for all NEOs). • The Agency could ach ieve the specified goal of surveying for 90 percent of the potentially h azardous N E Os by the end of 2020 by partnerin g with other governm e nt agencies on potential future optical ground-based observatories and building a dedicated NEO survey asset assum i ng the partners’ potential ground assets com e online by 2010 and 2014, and a dedicated asset by 2015. • Together, the two observatories potentially to be developed by other governm e nt agencies could com p lete 83 percent of the survey by 2020 if observing tim e at these observ a tories is sh ared with N A SA’s NEO Survey Pro g ram . • New space-based infrared system s, com b ined with shared ground-based assets, could reduce the overall tim e to reach the 90 percent goal by at le ast three years. Space syste m s have addition a l benef its as we ll a s costs and risks com p ared to ground-based alternatives. • Radar system s cannot contribu te to the search for poten tially hazardous o b jects, but m a y be used to rapidly refine tracki ng and to determ ine object sizes for a few 1 NEOs of pot entially high interest. Ex isting radar system s are curren tly oversubscribed by other m i ssions. • Determ ining a NEO’s mass and orbit is required to determ ine whether it repres ents a potential threat and to provide required inform ation for m o st alternatives to m itigate s u ch a threat. Beyond these param e ters, characterization requir e m e nts and capab ilities a r e tie d dire c tly to the m itigatio n stra tegy se lected. Key Findin g s for Diverting a Poten t ially Haz a rdous Object (PHO): The study team assessed a series of approach es that could be us ed to divert a NEO potentially on a collision course with Eart h. Nuclear explosives, as well as non-nuclear options, were assessed. • Nuclear standoff explosions are assessed to be 10-100 tim es more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface o r subsurface use of nuclear explos ives m a y be m o re efficient, but they run an increas ed ris k of fracturing the target NEO. They also carry higher developm ent and operations risks. • Non-nuclear kinetic im pactors are the m o st m a ture approach and could be used in som e def l ection/m itigation scenar ios , especially f o r NEOs that consis t of a single sm all, solid body. • “Slow push” m itigation techniques ar e the m o st expensive, ha ve the lowes t leve l of technical readin ess, and their ability to both travel to and divert a threatening NEO would be lim ited unless m i ssion durat ions of m a ny years to decades are possible. • 30-80 percent of potentially hazardous NE Os are in orbits that are beyond the capability of curren t or p l anned launc h system s. Theref ore, p l anetary gr a v ity assist swingby trajectorie s or on-orbit assem b ly of m odular propulsion system s m a y be needed to augm ent launch vehicle pe rform a nce, if these objects need to be deflected. Alternativ es Considered to Detect, Track, Characteriz e , and Deflect/Mitigate N E Os The following tables provide a sum m a ry of the options considered. Technical descrip tion s of each option, as well as other co m b inations of alternat ives, can be found in subsequent s ection s of this repo rt. Fo r each op tio n, Table 1 s hows the percentage of PHOs that would be found by the survey by the end of 2020 and the year each option would achieve 90 percent com p letion, starti ng with the option of sharing the use of potential g r o und-based o b servato r ies, which will be referred to as the “Reference” architecture through the rest of this docum e nt. Details regardi ng the availability of assets for each option are also found in subsequent se ctions. Table 1 shows that individu ally each of the first three options fall sho r t of m eeting the Congr essional goal. As shown in the last line of Table 1, the m i ni m u m survey arch itecture that achiev es the goal would be a com b ination of the shared ground-based asse ts plus one of two dedicated asset options. 2 Table 1. Detect ion and Tracking C a pability Options & Su mmary Res u lts Option* through 2020 Year 90% Shared ground-based (R eference) 83% 2026 Dedicated ground-based 85% 2024 Dedicated Infrared sensor in Venus-like orbit 89% 2021 Reference + One Dedicated Asset At leas t 90 % Not Later than 2020 * Details of each option are found in a subsequent section of this report. Table 2. Characteriz a tion Options Option* Description s (O1 = Option 1) Option 1 Use Existin g Assets + Detection an d Tracking System s Option 2 O1 + Dedicated Ground System s Option 3 O1 + Dedicated Space-Based Rem o t e Sensing (L 1/L2) Option 4 O1 + Dedicated Space-Based Rem o t e Sensing (Venus-Like Orbit) Option 5 O1+ O2+ O3 + 2 Flyby Missions to 8 Objects Option 6 O1 + O2 + O3 + 8 Orbiter Missions Option 7 O1 + O2 + O3 + Orbite rs at a F i xed Threshold P r obability of I m pact * Details of each option are found in a subsequent section of this report. Table 3. Impulsive Deflection / Mitigation Options Impulsive Technique* Description Conventional Explosive (surface) Detonate on im pact Conventional Explosive (subsurface) Drive explo s ive dev i ce into PHO, detonate Nuclear Explosive (standoff) Detonate on flyby via proxim ity fuse Nuclear Exp l osive (surface) Im p act, detonate via contact fuse Nuclear Exp l osive (delayed) Land on surface, deton a te at optim al tim e Nuclear Exp l osive (subs urface) Drive explo s ive dev i ce into PHO, detonate Kinetic Im pact High velocity im pact *A discussion of these techniques is f ound in a subsequent s ection of this report. 3 Table 4. Slow Push Deflection / Mitigation Options Slow P u sh Technique* Description Focused Solar Use large m i rror to focus solar en erg y on a spot, heat surface, “boil off” m a terial Pulsed Laser Rendezvous, position sp acecraft near PHO, focus laser on surface, m a terial “boiled off” surface provides sm all force Mass Driver Rendezvous, land, attach, m i ne m a terial, eject m a terial from PHO a t high velocity Gravity Tr a c tor Rendezvous with PHO, fly in close proxim ity for extended period, gravitational attrac tion provides sm all force Asteroid Tug Rendezvous with PHO, attach to PHO, push Enhanced Yarkovsky Change albedo of a rotating PHO; radiation from sun- heated m a terial will p r o v ide sm all force as body rotates * A discussion of these techniques is found in a subsequent section of this report. Recommen d ed Survey Program Currently, NASA carries out the “Spaceguard Survey” to find NEOs greater than 1 kilom e ter in diam eter, and this program is currently budgeted at $4.1 million per year for FY 2006 through FY 2012. W e also have benefited from knowledge gained in our Discovery space m i ssion series, such as the Near Earth Asteroid Rendezvous (NEAR), Deep Im pac t , and Stardust m i ssions that ha ve expanded our knowledge of near-Earth astero ids an d com e ts. Participa tion by NASA in international collaborations such as Japan’s Hayabusa m i ssion to the NE O “I tokawa” also greatly benefited our understanding of these objects. NASA’s Da wn m i ssion, expected to launch in June 2007, will in crease our unders tanding of th e two larg est known m a in belt astero ids, Ceres and Vesta, between the planets Ma rs and Jupiter. NASA conducts survey program s on m a ny celestial objects— the existing Spaceguard program for NEOs, surveys for Kuiper Belt Objects, the search fo r extra-solar planets, and other objects of interest such as black holes to understand th e origins of our universe. Our Discovery m i ssion series in planetary science ma y offer additional opportunities in the fut u re beyond our current survey efforts. NASA recommends that the p r ogra m continue as curr ently planned, and we will also take advantage of opportunities using potential dual-use telescopes and spacecraft—and partner with other ag encies as f easib le—to attempt to ach ieve the leg i slate d goal with in 15 years. H o wever, due to current budge t constraints, NASA cannot initiate a new program at this tim e. 4 BACKGROUND Asteroids an d com e ts are the two typ e s of pot entially hazardous objects (P HO) discussed in this study. For objects in the inner solar system, as tronomers can distinguish these bodies on the basis of their app earance. Moving objects that app ear as a star-like point of light are known as asteroids. Moving objects that appear diffuse or those that have visible tails are known as com e ts. People have known about com e ts since antiquity. The existence of astero ids, ho wever, was not known until about 20 0 years ago when telescopes b ecam e powe r ful enough to detect th e largest such objects. It has taken several generations of improvem e nts in te lescope design to detect and understand the sm all bodies that orbit near Earth. Differences in the app e arance of co m e ts and as teroids ref l ec t in pa rt a d i f f e rence in their com position. Generally, asteroids are rela tively rocky or m e tallic objects without atm o spheres, while comets are com p osed in part of volatiles s u ch as water ice th at vaporizes when heated to produce a tenuous and transient atmosphere around the solid nucleus. Com e ts that are far from the Sun or those that have lost m o st of their vo latile s can look lik e an asteroid . A volatile-rich ob ject will develop an atm o sphere only wh en heated sufficiently by a relativel y close approach to the Sun. The near-Earth as tero ids are categorized as Apollos, Atens, Amors, and Interior Earth Objects (IE O s), depending on whether their orbi ts cross Earth’s orbit with a period of more than one year, cross Earth’s orbit w ith a period of less th an one year, exist com p letely outside the Earth’s orbit, or ex is t co mpletely within the Ea rth’s orbit, respectively. The distribution of thes e objects in the NEO population is shown in Figure 1. A p ollo Semi ma jor A x i s ≥ 1 . 0 A U Per i h e l i o n ≤ 1 . 02 A U Earth C r os si n g Am o r 1. 02 A U < Peri he l i on ≤ 1. 3 A U 62 % of kno w n astero i ds A p ollo Near-Earth Pop u lat i on Ty p e 6 kno w n asteroids IEO 32 % of kno w n astero i ds Am o r 6 % of kno w n astero i ds At e n 62 % of kno w n astero i ds A p ollo Near-Earth Pop u lat i on Ty p e 6 kno w n asteroids IEO 32 % of kno w n astero i ds Am o r 6 % of kno w n astero i ds At e n At e n S e mima j o r A x i s < 1 . 0 A U A p he l i on ≤ 1.0 1 67 A U E a r t h C r o s s i ng Inner Eart h Obj ects (IE O s ) A p h e lio n < 0 . 98 3 AU A l w a y s i n si de E a rt h’ s o r b i t ( a ka A poh e l e) Figure 1. Near Earth Asteroid Orb i t Types 5 Near-Earth Objects (NE O s) are as teroids and co m e ts in orbits that allow them to enter Earth’s neighborhood, defined by astronom ers as ha ving a perihelion (closest approach to the Sun) of less than 1.3 AU (Astronom i cal Units, 1 AU is approxim a tely 150 m illio n km , the m e an distan ce b e tween the S un and Ea rth). Extinct com e ts m a y m a ke up 5-15 percent of the NEO population, and som e m a y retain volatiles. As of Dece mber 4, 2006, using the Safeguard Survey system descri bed elsewhere in this report, N A SA has identified 701 NEOs greater than 1 km in size and 3,656 NEOs s m aller than 1 km i n size. Of the total num ber of NEOs surv eyed, NASA has found only 63 com e ts. The estim ated population of NEOs greater than 1 km in size is 1,100. The estim ated population of NEOs greater th an 140 m e ters in size is approxim ately 100,000 objects. A constant power law as shown in Figure 2 can be used to estim a te the number of NEOs of a particular size based on our availabl e observations. The figure shows a hundred-f old increase in the num ber of NEOs as the size d ecreases by an order of m a gnitude. In term s of the goals expressed by the George E. Br own, Jr. Near-Earth Object Survey Act, NASA estim ates that the population of NEOs great er than 140 m e t e rs is approxim ately 100,000 objects. Figure 2 also shows the approxim a te absolute m a gnitude (relative brightness) if the objects were placed at a s t andard distan ce of 1 AU from the observer, their averag e im pact interval, and the approxim a te im pact en ergy they would deliver in a collision w ith Earth. Given any size class of NEO, this es tim ate is probab l y accurate to within a factor of two or three, as there are not enough observations in som e classes to form a statistica lly valid s a m p le. Co nstan t P o w e r Law Pro v i d es Go od Fi t to Data ~1 00k NEOs > 1 4 0 m ~ 1k NEOs > 1k m Figure 2. Frequency of NEOs by Siz e , Impact Energy, and Magnitude 6 More relevant to th is rep o rt is the def i nition of Potentially Hazardous Objects (PHOs), astero ids an d com e ts that have a po te ntia l to even tually im pact the Ea rth. A PHO is an object in our solar sy stem that passes w ithin 0.0 5 AU (about 7.5 m illion km ) of Earth’s orbit and is large enough to pass through Earth’s atm o sphe re and cause significant dam a ge on impact; that is, about 50 m e ters a nd larger. In this report the term PHO wi ll be used to in dicate poten tia l thr eats, with th e understanding that thos e smalle r than 1 km are predom i n antly asteroids. Com e t s do not add substantially to the population below 1 km . Approxim ately 21 percent of the NEOs of any given size class are exp ected to be potentially h azardous. In 2003, NASA chartered a Science Definition T eam (SDT), which reco mmended an NEO survey program to produce a catalog that is 90 percen t c o m p lete f o r PHOs larger than 140 m e ters. The S D T determ ined that im pacts from objects th at are 140 m e ters in size would only produce regional effects, wh ile larger objects woul d have corresponding wider effects such as large sub-global e ffects from i m pacts of a 300- m e ter object and global effects from 1-kilom e ter object im pact s. Im pact of objects 10 kilom e ters and larger are co nsidered an ex tinction-class event for Earth. A survey program that com p letes a 90 percent survey of 140 m e ter or larger PHOs would al so identify virtually all of the global risk from objects greater th an 1 kilom e ter. A survey system could be constructed to catalog hazardous objects dow n to the air blast lim it (about 50 m e ters in size). However, the S c ience Definiti on Team s uggested that cataloging down to 140 m e ters was the m o re prudent approach for the next-generation survey, a system which would also provide warning for 60-90 percent of objects capable of producing potentially dangerous air blasts. Essentially, the 140 m e ter object size is approxim a tely where im pacts transition from causing regional (e.g., a state or seaboard) to m o re localized (e.g. citywide) dam a ge. Since by this definition, object s that do not pass within 0.05 AU of Earth’s orbit are not “ potentially hazardous,” these ob jects are assessed (without n ecessity of discovery ) to be no threat to Earth. Therefore, NASA recomm e nds that the Survey’s goal be m odified to detect, track, catalogue, and characterize 90 perc ent of all PH Os greater than 140 m e t e rs by the end of 2020 rather than 90 percent of a ll N E Os of greater than 140 m e ters that pass within 1.3 AU of the Sun as expressed in the George E. Brown, Jr. Near-Earth Object Survey Act. Lim iting the objects to only PHOs will reduce the required population to be surveyed an d the Survey program wi ll have a m o re realistic g o al. However, the Surv ey will still pro v ide equa lly effective “ w ar ning and m itigation o f the hazard ” and corresponds with the recomm e ndations of the 2003 SDT report. 7 EXEMPLAR SURVEY PROGRAM An exem plar NEO Survey Program , that at a m i nim u m m eets the goals s p ecified in the George E. Brown Jr. Near-Earth Survey Act, is shown in Table 13. This program could achieve the specified goal of surveying 90 percent of the PHOs by the end of 2020 through NASA partnerships with other governm e nt agencies on potential future optical ground-based observatories: the Panoram i c Survey Telescope and Rapid Response System (Pan STARRS-4 or PS4) and the LSST. Following at least one year of program for m ulation, NASA could build or fund the cons truction of a dedicated survey asset. For exam ple, either an add itional LSST, which m a y be operated by other organizations, or a 0.5 m e ter infrared (IR) satellit e in a Venus-like orbit. Ot her options for the dedicated Survey asset would be evaluated during pr ogram for m ulation. All costs shown are antic ipated c o sts to NASA. Both the shared and dedicated assets woul d detect, track, and characterize NEOs. Analyses to date ind i ca te that the es ti m a ted com p letion dates for developm ent and estim ated co sts are sens itive to m odeling erro rs that m a y vary up to three years. Note that these costs are rough ‘arch itecture costs’ that would need more rigorous analysis if a program we re to be assessed for im plem entation. Currently, NASA carries out the “Spaceguard Survey” to find NEOs greater than 1 kilom e ter in diam eter, and this program is currently budgeted at $4.1 million per year for FY 2006 through FY 2012. W e also have benefited from knowledge gained in our Discovery space m i ssion series, such as the Near Earth Asteroid Rendezvous (NEAR), Deep Im pac t , and Stardust m i ssions that ha ve expanded our knowledge of near-Earth astero ids an d com e ts. Participa tion by NASA in international collaborations such as Japan’s Hayabusa m i ssion to the NE O “I tokawa” also greatly benefited our understanding of these objects. NASA’s Da wn m i ssion, expected to launch in June 2007, will in crease our unders tanding of th e two larg est known m a in belt astero ids, Ceres and Vesta, between the planets Ma rs and Jupiter. NASA conducts survey program s on m a ny celestial objects— the existing Spaceguard program for NEOs, surveys for Kuiper Belt Objects, the search fo r extra-solar planets, and other objects of interest such as black holes to understand th e origins of our universe. Our Discovery m i ssion series in planetary science ma y offer additional opportunities in the fut u re beyond our current survey efforts. NASA recommends that the p r ogra m continue as curr ently planned, and we will also take advantage of opportunities using potential dual-use telescopes and spacecraft—and partner with other ag encies as f easib le—to attempt to ach ieve the leg i slate d goal with in 15 years. H o wever, due to current budge t constraints, NASA cannot initiate a new program at this tim e. 18 Table 1. Exemplar NEO Survey Program to Detect, Track, and Characteriz e Detect, Track, & Characteriz e : ≥ 140 meter PHOs Total Architecture Costs* ($M) (thru the year to reach 90% ) Exemplar Survey Program Percent com p leted through 2020 Year to reach 90% $FY06 $RY Reference (Ground) Survey Assets (Shared PS-4 & Shared LSST) 8 3 % 2 0 2 6 $469.0 (thru 2026) $693.5 (thru 2026) Reference p l us a Dedicated L SST 9 0 % 2 0 2 0 $835.5 (thru 2020) $1076.2 (thru 2020) Two of the Options for one additional, dedicated Survey Asset Reference p l us a Dedicated 0.5-m e ter IR in Venus-like orbit 9 7 % 2 0 1 7 $1005.9 (thru 2017) $1239.9 (thru 2017) * Total Arch itecture Cos t s include da ta management and prog ram office costs. STUDY AP PROACH AND ANALYSIS OF ALT E RNATIVES FO R NEO DEFL ECTION PROGRAM The study considered a wide ra nge of techniques to divert a threatening object. These alternatives were broad l y class i fied as “im pul sive” if they acted nearly instantaneously or “slow push” if they acted over an extended period of tim e. Launch, orbit transfer, technology developm ent, and object characteri zation requirem e nts were assessed for each of these alternatives. Th ey we re applied to a set of five scenarios rep r esen ting the likely range of threats. A representative set of potential PH O defl ection approaches was presented during a public workshop NASA held in the course of this study. This study exam ined a num be r of techniques for deflecting a PHO, a nd th e m e thods considered viable have been catego r ized as either impulsive or slow push techniques. Table 14 provides an overview of the im pul sive m e thods. Likewise, Tabl e 15 shows the slow push techniques, where the velocity change results from the c ontinuous application of a sm all force. 19 Table 14. Impulsive Mitigation Alternatives Impulsive Technique Desc ription Conventional Explosive (s urface) Detonate on im p act Conventio nal Explosive (subsurface) Drive explosive device into PHO, detonate Nuclear Explosive (standoff) Detona te on flyby via proxi m ity fuse Nuclear Explosive (surface ) I m pact, detonate via contac t fuse Nuclear Explosive (delay e d ) Land on surfa ce, detonate at optim al time Nuclear Explosive (subsurface) Drive e xplosive device into PHO, detonate Kinetic Im pact High velocit y im pact Table 15. Slow Push Mitigation Alternatives Slow Push Technique Desc ription Focused Solar Use large mir r or to focus s o lar energy on a spot, heat surface, “boil off” material Pulsed Laser Rendezvous, position spac ecraft near PHO and focus laser on surfa ce, m a t e rial “ boiled off” surface provides sm al l force Mass Driver Rendezvous, land, attach, mine m a terial and eject material fro m PHO at high velocity Gravity Tractor Rendezvous with PHO and fl y in close proxim ity for extended period, gravitatio nal attraction provides sm all force Asteroid Tug Rendezvous with PHO, attach to PHO, push Enhanced Yarkovsk y Effe ct Change albedo of a rotatin g PHO; radiation from sun- heated m a terial will provide small force as body rotates In the im pul sive category, the use of a nuclear device was found to be the most effective m eans to deflect a PHO. Because of th e large am ount of energy delivered , nuclear devices would require the leas t am ount of detailed inform ation about the threatening object, reducing the need for detailed charac terization. W h ile detonation of a nuclear device on o r below the surface of a threatening o b ject was fo und to be 10 -100 tim es more efficient than detonating a nuclear devi ce above the surface, the standoff detonation would be less likely to fragm ent the target. A nuclear standoff m i ssion could be designed knowing only the orbit and approxim a te m a ss of the threat, and m i ssions could be carried out increm e n tally to reach the required am ount of deflection. Additional infor m ation about the object’s m a ss and physical properties w ould perhaps increase the effectiveness, but likely w ould not be required to accom p lish the goal. It should be noted that because of restrictions found in Article IV of the Treaty on Principles Governing the Activities of States in the Exploration and Use of Ou ter Space, including the Moon and Other Celestial Bo dies , use of a nuclear device would li kely require prior inte rn ationa l coordination. The study team also exam in ed conventional explosives, but found they were ineffective ag ains t most threats. 20 Non-nuclear kinetic im pact alternatives are the most effective non-nuclear option, transferring 10-100 tim es less m o m e ntum than nuclear options for a fixed launch m a ss. Im pact velocities, varying from 10-50 km /s , produced a factor-of-three variation in deflection perform a nce. In addition, kineti c im pacts a r e a l so sensitive to the poros ity, elasticity, and com position of the target and m a y require large perform a nce m a rgins if these ch aracteristics are not well determ ined. Slow push techniques analyzed in this study included a gravity tractor, which could alter the cours e o f an object u s ing the g r avitationa l attraction of a m a ssive spacecraft flyin g in close prox im ity, and a space tug, wh ich could at tach itself to a PHO and move it usin g high-efficiency propulsion system s. An a ttached space tug h a s generally 10-100 tim es more perform a nce than the gravity tractor, bu t it requir e s m o re deta iled ch arac ter i za tio n data and m o re robust guidance and control a nd surface attachm e nt techn o logies. S l o w push techniques were determ ined to be useful in relatively rare cases (fewer than 1 percen t of expected threat scen ario s). This techn i que could b e effective in instan ces where sm all increm ents of velocity (less th an 1 mm/s) could be appl ied to rela tive l y sm all objects (less than 200 meters in diam eter) ove r m a ny decades. In general, the slow push system s were found to be at a very low t echnology readiness leve l and would require significant developm ent efforts. Deflection Performance Analysis Figures 4 and 5 graphically represent deflec tion capabilities. Th e system perform a nce required to deflect any object on a given trajectory m a y be described as the velocity change nece ssary to cha nge its p a th multiplied b y its m a ss. The “ef f ective m o mentum change” perfor m ance param e ter allo ws m a ny different scenarios to be plotted sim u ltaneously across a wide range of astero id m a sses and required deflection velocities ( ∆ V). I t is d i splay e d log a rithm i cally on the Y-axis of these figures. The logarithm i c X- axis rep r es ents laun ch p e rform a nce to place the d e flection p a yload on an intercept tra j ecto r y. A key para m e ter that matches la unc h capability with a ce rta i n payload a t a certain tim e (f light tim e) to intercep t an as te roid is C3, which is equa l to twice the specific (per unit m a ss) orbital energ y and is represented in units of km 2 /s 2 . The launch C3 corresponding to payload cap abilities of the two launch sy stem s considered (Delta IV Heavy and Ares V) are at the top of each figure. The lines to the right of each figure m a y be us ed to translate effective m o m e ntum change to the design param e ters of PH O m a s s (and size) and deflection ∆ V. Lines of constant object m a ss (and size) sp aced logarithm i cally run diagonally across vertical lines representing a logarithm i c range of deflection ∆ V. As an example, following the diagonal line representing a m a ss of 10 10 kg (approxim a tely 200 m ) to its extrem e lower left at the vertical 1 cm /s ∆ V line, this corresponds to an effective m o m e ntum change of 10 8 kg m / s on the far left. The lines p l otted repres ent the p e rform a nce of the deflection alte rnatives. If an alternative h a s a higher effec tive m o m e ntum cha nge capability than is required, it is considered “feasible” for a single-launch de flection. Therefore, using the previous exam ple of an effective mom e ntum change of 10 8 kg m / s and assum i ng that a Delta IV Heavy launch vehicle is used and that C3 = 25 km 2 /s 2 is requ ired to interc ept, all but the 21 10 km /sec kinetic interceptor and the conven tional explosives would m e et perform a nce requirem e nts. None of the slow push techni ques could m eet this hypothetical scenario. Figure 4 shows that im pulsive techniques us ing proxim a l nuclear explosives generally were found to provide greater potential for m o m e ntum transfer per kilogram of payload weight delivered to the thr eat than any other option c onsidered. Standoff nuclear concepts, such as those producing highly concen trated and directionally focused x-rays or neutrons, were shown to present a generall y lower risk of fragm e nting a PHO than im pulsive techniques involving direct cont act, but also produce a lower effective mom e ntum change than surface or su bsurface nuclear explos ives. Perform a nce m a y vary significantly, depending on the t ype of nuclear device used and whether it is “off-the- shelf” as op posed to optim ized for the PHO defl ection m i ssion. Additio nally, the perform a nce of kinetic impactors was found to be som e what less robust than any of the nuclear explosions. However, their effectiv eness depends strongly on the structure of the PHO. Kinetic im pactors m a y also be signi fican tly less effective for objects which are essentially loose rubble piles. Conventional explosives we re found to have the lowest perform a nce am ong the impulsive techniques due to their relatively low-energy density. Figure 5 illustrates that slow push techniques m a y be useful for im parting mom e ntum changes sm aller than 10 9 kg m / s. The asteroid tug appear s to have sign if icantly g r ea ter perform a nce than the g r avity tractor for a given launch m a ss, even accounting for pulsed operation on a rotating P HO. The disadvantag e of the aste ro id t ug is the additional com p lexity required to anchor the tug to the NE O, particu l ar ly if the PHO structu r e has not been well characterized or th e target is rotating very rapidly. These figures show that nuclear explosives and kinetic im pactors were generally found to provide greater potential fo r m o m e nt um transfer per ki logram of payload weight delive r ed to the NEO than other a lte r n ative s . Addition a lly, th ese f i gures illustr a te how the alternatives m i ght be applied to hypotheti cal deflection scenarios. The inclusion of actual objects in thes e scenarios was chosen not b ecause they repres ent actual im pact threats, but because they are both p ublicly kno wn and are represen tativ e of classes of potential threats. The hypothetical scenarios in clude missions to deflect: A. The 330-m e ter asteroid, Apophis, before its close approach to Earth in 2029. This scenario was divided into two design points: A1. For the first, knowing th e asteroid’s orbit is assu m e d and a relatively large mom e ntum change is required to deflect the object with the required certainty. Apophis m u st be deflected by at least one Earth radius or about 6,400 km to achieve a probability of collision of less than 10 -6 . A2. For the second, very accur ate information about the object’s orbit is assum e d and the im petus necessary to divert the as te roid with cer tain ty is substantially reduced. A pophis m u st be deflected by at least five km to achieve a probability of collis ion of less than 10 -6 . B. Apophis after the close approach and be fore the 2036 Earth encounter, assum i ng a predicted co llis ion. C. The 500-m e ter asteroid (VD17) that could be a threat in the year 2102. 22 D. A hypothetical 200-m e ter asteroid, represen tative of 100- m e ter-class asteroids. E. A hypothetical asteroid larger than one km in diam eter. F. A hypothetical long-period com e t wi th a very short tim e (9-24 m onths) to im pact. The approxim a te perform a nce requirem e nts for each of the s c enarios are overlaid on Figure 4 for the im pulsive techniques a nd Figure 5 for the slow push m e thods. Figure 4. Deflection Performanc e of Impuls i v e Alternativ es 23 Figure 5. Deflection Performance of Slow Pus h Alternatives POTENTI AL BENEF I TS TO SCIENCE NEOs are prim itive bodies, prim arily astero ids th at probab l y represen t alm o st the full range of m a t e rial contained in the m a in aste roid belt of our solar system . The population also con t ain s the nuc lei of extinct co m e ts, which like l y still re f l ect the co mposition of all but the m o st volatile species and still c ontain a significant inventory of organic substances. The m o st recent National Academ y of Sciences’ Decadal Survey for solar system exploration summ arizes the key scien ce issues with respect to prim itive bodies as f o llows: • Where in th e solar system are the prim itiv e bodies found, and what range of sizes, com positions, and other physical char acteristics do they represent? • What processes led to the for m ation of these objects? • Since their f o rm ation, what processe s have altered the prim itive bodies ? • How did prim itive bodies m a ke planets ? • How have they affected the planets since the epo c h of for m ation? Characte r iz a tion will ce rtainly p r ovid e ne w inf o rm ation on the sizes, composition s , an d other physical characteristics of asteroids and com e t nuclei. Inform ation on the m a terial of these objects will also provide da ta to understand alte ration processes. A wide area search, su ch as that bein g proposed for NEOs, will also subs tantially incre a se th e identif i catio n of Kuiper Belt Obje cts (KBOs). For exam ple, if 10 percent of 24 the observing tim e on the proposed Dedicated L SST was spent in a KBO search m ode, roughly 100,000 faint KBOs should be discove red. An expanded KBO da tabase will allow the study of dyna mical di stributions, further resonances, the existence of a KBO dem a rcation beyond 50 AU, high-eccentricity/h i gh- inclinatio n orbits, size distribution s , frequency of binary objects and collisi on rates, chem ical compositions and the relationship of objects to dust disks around other stars. The survey will also provide a rich database of targets for future space m i ssions. Detection surveys such as the proposed Pa n-STARRS and LSST provide unique solar- system scien ce becaus e they are designed to detect and perform follow-up studies of moving objects. Centaurs, Jupiter Fam ily Co m e t s , and certain extinct com e ts m a y be rela ted th rou gh a common origin in the Kuiper Belt. Dedicated assets will assure that appropriate follow-up is carried out over the annual tim e fr a m e s that are required to produce orbits for the slower-m oving objects found in the outer solar system . Thus, a colla ter a l r e sult of the N E O survey program c ould be both the delineat ion of the structure of the Kuiper Belt and the discove ry of m a ny ne w m i nor planets. It also is importan t to un derstand wh at a vigo rou s characterization effort will not do. Characte r iz a tion to inf o r m def l ection m i ssions has not identified a need for sam p le return f r om either an aste roid o r com e t. Astero id and com e t sam p le-return m i ssions are high priorities in the Decadal Survey, but they are not included in the trad e space of this stu dy. However, a vigorous survey program would iden tif y likely ca ndidate s f o r scien tif ic vis its for the sam p le return m i ssions iden tif ied as a p r io rity in the D ecadal Su rv ey. Rem o te charac ter i za tion will allo w the m o st interes ting o b jects to be selec t ed f o r f u rther sc ien tif ic investigatio n and will a l low the ins t r u m e nts and experim e nts of these m i ssions to be tailored in ways that otherw ise would not have been considered. NEOs are generally am ong the easiest asteroids to visit, and the design of a spacecraft to work in the relatively benign environm ent near one AU offers less co st and risk th an a m i ssion to the m a in belt. A sam p le return m i ssion to a NEO charac ter i zed f o r a def l ec tion m i ssion will carry sub s ta ntia lly lowe r risk th an a m i ssi on to an object about which m u ch less is known. POTENTI AL BENEF I TS TO EXPLORATI O N There m a y be future longer-term options fo r system -level demonstrations that could contribute to PHO deflec tion de m onstrations. These system s, currently in developm ent, have considerable m a ss, precision rendez vous and docking capability, as well as considerable perform a nc e m a rgin. Near-Earth Object Resources The study team noted the connection between the goals of the Vision for Space Exploration and a program to survey the popul ation of NEOs. Disc overing and exploring resources that exist on N E Os m a y lead to future utilization. Human Visits to Astero id s The Vision for Exploration di rected NASA to extend hum an presence across the so lar system , starting with a hum a n return to th e Moon by the year 2020, in preparation for hum a n exploration of Mars and other destin ations. NEOs are one of those potential 25 "other d e stin ations. " NASA is curren tly de veloping a new launch system , the Ares I and V launch vehicles, and a new crew exploration vehicle, the O r i on. It is possible that the system s used to return hum a ns to the Moon could be used to also visit a N E O. 26 Appendix: Acronyms and Definition of Terms Acronym Description AU Astronom ical Unit D Diam eter ESA European Space Agency FY Fiscal Year HQ Headquarters IEO Interior Ea rth Object IOC Initial Ope r a tiona l Capab ility IR Infrared energy band IRTF Inf r aRed Telescope Fa cility Isp Specific Im pulse JPL Jet Propulsion Laboratory KBO Kuiper Belt Objects L1 First Sun-Earth Lagrange Point LEO Low-Earth Orbit LINEAR Lincoln Near Ea rth Aste roid Resea r c h LONE OS Lowell Observatory Near-Earth-Object Search LSST Large Synoptic Survey Telescope MOID Minim a l Orbita l Interse c tion Dis t an ce MPC Minor Planet Center NASA National Aeronautics an d Space Adm i nistration NEAT Near-Earth Asteroid Tracking NEO Near-Earth Object NVO National Virtual Observatory PA&E Office of Program Anal ysis and Evaluation Pan STARRS Panoram ic Survey Telescope & Rapid Response System PHO Potentially Hazardous Object PS Pan STARRS Vis Visible light band 27
Posted on: Sat, 01 Jun 2013 09:34:25 +0000
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