T HE ThrEAT Impact - National Space Society

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winter 2008TheThreatOfImpactA SpecialReporton NEOsBeyond the ShuttleWhat it means for the ISSShackleton DomeEnvisioning A Lunar CityBooks:Andrew Chaikin’sA Passion for Mars,The Return to Lunashort story contest,and 50 Years of NASA Art

DealingwiththeThreatofImpactDetection, deflection, or deep impact?The near Earth object environment has remained virtually constant for the past three billion years.Our society has been vulnerable to the destructivepower of impact events ranging from the 1908 Tunguska event (in which the impact of an estimated 45meter-diameter object destroyed 2,000 square kilometers of Siberian forest) to the 12-kilometer-diameterobject responsible for the Chicxulub impact 65 millionyears ago, which is thought to have caused the extinction of the dinosaurs and 70 percent of all species aliveat the time. Such cosmic collisions occur infrequentlycompared with a human lifetime, yet when they dohappen, they dwarf the natural disasters that are morecommon in human experience.Surprisingly, in the instance of this most devastatingof natural disasters, humankind is far from helpless.With modern-day telescopic and spaceflight capabilities, we can detect and predict potential impacts, andwith adequate early warning we can deploy space systems capable of altering the orbit of threatening NEOsand force them to pass harmlessly by Earth, avoidingimpact. Even if our discovery of the NEO were insufficient to successfully divert it, a prepared society30By Russell L. Schweickartwould nevertheless be able to mitigate the effects of animpact by evacuation and other disaster preparednessmeasures.Impacts and Impact PrecursorsA NEO impact will occur when the orbits of a NEO andEarth intersect in space and both bodies reach thatintersection at the same time. Most frequently, a NEOthreatening impact with Earth has experienced priorclose passes by Earth which have, in fact, set up thesubsequent impact. Close gravitational encounters withthe Earth can substantially change the orbit of a NEO,and on occasion, cause a precise change which bringsthe NEO back several years later for a direct impact.The small region near the Earth through which a NEOmust pass for such a resonant impact to occur isreferred to as a gravitational keyhole. Whenever a NEOpasses nearby, whether passing in front of or behindthe Earth, it passes through a field of dozens of suchkeyholes. Consequently, NEO deflection is most easilyaccomplished when it can be diverted from a precursorkeyhole rather than from a direct impact. A deflectionoperation to avoid a keyhole target of a kilometer or soad Astra Winter 2008threat.indd 3011/20/08 12:42:05 PM

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in size is far less daunting than a deflection to avoid anEarth-sized target of several thousands of kilometers.Deflection TechniquesThe diversion of a NEO on a path toward an Earthimpact requires, depending on the specific circumstances, either a precise, but modest orbit change,or the combination of a robust orbit change followedby a precise orbit trim. A successful deflection campaign is one in which both an immediate impact andall near-term return impacts are prevented. Theseconditions correlate with the robust and precisedeflection requirements respectively.Robust orbit change, i.e., orbit changes requiringsubstantial total impulse applied to the NEO, can beprovided by either a kinetic impact (KI) or a nuclearstand-off explosion. Both technologies are available;KI was demonstrated conceptually during the 2005Deep Impact mission, albeit that impact was designedfor a different purpose. A nuclear stand-off explosionhas not been demonstrated, but the technology isarguably available for use. Both these techniques,while capable of transferring substantial momentumchange to a NEO, cannot do so with adequate precision to assure a fully successful deflection.A precise NEO orbit adjustment is conceptually available via a number of techniques. The most simpleand readily available concept is the gravitationaltractor (GT), which provides precise adjustmentto the NEO orbit by “hovering” in close proximity to the NEO, using mutual gravity to change theNEO’s velocity. While the non-nuclear combinationof a kinetic impact and gravity tractor will suffice forapproximately 98 percent of the statistical impactthreat, the use of nuclear means cannot be ruled outwithout further technology development. The frequency of NEO impacts which would require the useof nuclear means is approximately 1 in 100,000 yearsbased on the current best estimate of NEO size-frequency distribution.Deflection ImplicationsThe deflection process, regardless of the technologyused, can be understood as an operation which, byslightly adjusting the velocity of a NEO, will cause it toarrive at the impact point slightly earlier or later than itotherwise would have. With a sufficient change in theNEO velocity, the change in the arrival time is enoughthat the impact point has been avoided.A deflection can thus be seen as a process wherebyan impact point is shifted from its original locationto a point ultimately off the Earth’s limb, so that theNEO will fly past the Earth, and not into it. Should,for any reason, the deflection be only partially completed, the NEO’s pathway will change, but it couldstill hit the Earth at a different impact point. Whetherthe deflection is a continuous gravity tractor processor an impulsive kinetic impact or nuclear explosion,a partial completion would therefore result in a newimpact point along the risk corridor. People not nearthe original impact point may then be at risk as aresult of the action taken to deflect the NEO. Riskshifting is an inseparable element of risk eliminationin NEO deflection. Agreeing to deflect a NEO anddeciding which direction the impact point should beshifted are clearly decisions that must be coordinatedamong the international community.Dealing with the Threat of ImpactWhat is needed to match the technical capability forresponding to the NEO impact challenge is an international system of preparation, planning, and timelydecision making. Because NEO impacts can occuranywhere on our planet and affect the entire international community, a collaborative, global responseis necessary. International coordination is necessaryin making any decision regarding a NEO, because ofthe low, but inevitable risk in the process of deflection. Deflecting a NEO from impacting one populationcenter could directly elevate the concern of another.Furthermore, it is highly desirable that a decisionprocess, with agreed criteria, policies, and proce-First StepsNew telescopic resources coming into service within the next decade will dramatically increase the number ofNEOs discovered and tracked. The U.S. Congress has charged NASA with discovering and tracking 90 percentof all NEOs larger than 140 meters in diameter by 2020. While meeting this goal poses a considerable challenge,it is clear that with new telescopes coming online, this goal will be approached in the 2020–2025 timeframe. Inthe process of achieving the 140-meter goal, many smaller but still dangerous NEOs will be discovered with thenumber of such objects likely to exceed 300,000. Based on current empirical experience, the number of potentially damaging NEOs with a non-zero probability of impact within the next 100 years is likely to exceed 10,000by this time. Of these NEOs with at least a small probability of impact over the next 100 years, many are likely toappear threatening enough to necessitate a decision of whether action should be taken to prevent an impact.Above, a computer-enhanced, shadedrelief image of the Yucatan Peninsula inMexico highlights a subtle, semicirculartrough at the Chicxulub impact site,related to the extinction of dinosaurs 65million years ago. Opposite page, a seriesof impact sites that include—clockwisefrom top photo—Meteor Crater inArizona; Wolf Creek crater in westernAustralia; the New Quebec crater inQuebec Canada; and ManicouaganReservoir in Quebec, which is locatedin an eroded ancient impact crater.Preceding pages: An artist’s illustrationof a massive asteroid belt in orbit arounda star that is the same age and size asour Sun.Winter 2008 ad Astrathreat.indd 333311/20/08 12:45:58 PM

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dures be established prior to the development ofa specific threat in order to assure that minimization of risk to life and property prevail over competing national self-interests. A global, coordinatedresponse should ensure that three logical, necessary functions are performed.First, a warning network should be established. Thisnetwork would operate a global system of groundand space-based telescopes to detect and trackpotentially hazardous NEOs. The network should alsoestablish criteria for issuing NEO impact warnings.Next, an operations group, drawing on the expertise of the spacefaring nations, should outline themost likely options for NEO deflection missions. Thisgroup should assess the current, global capacity todeflect a hazardous NEO by gathering necessaryNEO information, identifying required technologies, and surveying the NEO-related capabilities ofinterested space agencies. In response to a specificwarning, the group should use these mission plansto prepare for a deflection campaign to prevent thethreatened impact.Finally, an oversight group should develop the policies and guidelines that represent the internationalwill to respond to the global impact hazard. Thisgroup should establish impact-risk thresholds andcriteria to determine when to execute a NEO deflection campaign.Preventing an ImpactWhenever a sufficiently threatening NEO is discovered early enough to mount a deflection campaign,an impact can be averted. For such a scenario to berealized, three essential elements have to be in place:a capable early warning system, a deflection capability, and an institutional process capable of makingtimely decisions.In order to provide the time required for preparation anddeployment of a deflection campaign, the early warningsystem will have to provide the information required tomake a deflection decision at least 10–15 years aheadof impact. On top of that, deflection systems should bedesigned and flight-tested well in advance of their useto ensure a successful deflection capability is availableand well understood.Most critical, however, is the requirement that theinternational community be prepared to authorize adeflection campaign in a timely manner. Failing to provide a decision-making framework before a threatening NEO situation is discovered will risk lengthy argument, political delays, and collective paralysis. Suchavoidable inaction will preclude a deflection and forcethe world to absorb a damaging—and preventable—impact. With the lead time for an authorization decision typically needed 10–15 years ahead of a potential impact, a program to develop that vital decisionprocess must begin now.Take ActionNow that humankind has the scientific, technical,and operational capabilities both to predict whetheran asteroid will come too close for comfort, and tolaunch operational missions to divert a potentialimpact, it is time for the international community toidentify the decision-making institutions and beginthe development of a coordinated decision-makingprocess. Scientific knowledge and existing international institutions, if harnessed today, offer society themeans to avoid such a catastrophe. We cannot affordto shirk that responsibility.An adequate global action program must includedeflection criteria and campaign plans which, whenconditions warrant, can be implemented rapidly andwith little debate by the international community. In theabsence of an agreed-upon decision-making process,we may lose the opportunity to take action to preventimpact, leaving evacuation and disaster managementas our only response to a pending impact. The international community must begin work now on forging itswarning, technology, and decision-making capacitiesinto an effective shield against a future collision.A Coming Wave of DiscoveryThe current NEO search program (Spaceguard Survey), initiated in 1998, has resulted in the discovery andtracking of over 5,700 NEOs in the past 10 years. These NEOs, of all sizes, have been discovered in the process of achieving the goal of discovering 90 percent of all NEOs greater than one kilometer in diameter bythe end of 2008. So far, approximately 80 percent of the statistical population of these large NEOs has beendiscovered, and the search continues.In 2005, the U.S. Congress established a new goal for the Spaceguard Survey: to discover 90 percent of allNEOs greater than 140 meters in diameter over the next 15 years. It is estimated that within the next 15 years,over 500,000 NEOs will be discovered and enter the tracking database, and 200,000–400,000 of these willbe of a size capable of doing substantial damage to the Earth’s surface on impact. About 3 percent of thesenew NEO discoveries will likely have some, generally small, probability of impact in that timeframe. Within thesethousands of potential Earth impactors, there will likely be dozens which will appear threatening enough thatthey will require proactive decisions regarding mitigation or deflection.Opposite page: An image from spaceshows the Vredefort Dome in SouthAfrica, site of an extreme impact eventthat took place more than 2 million yearsago—the oldest and largest known siteof its kind.Winter 2008 ad Astrathreat.indd 353511/20/08 12:51:32 PM

Removingthe BlindersOne of thegreatestthreats tocivilizationremainsunaccounted forFragments of a short-period comet, discovered byEugene and Carolyn Shoemaker and David Levy,enroute to their collisions with Jupiter at 60 km/sec.From July 16 to July 22, 21 distinct impacts wereobserved, the largest of which released an energyequivalent to 6,000,000 Megatons of TNT (600 timesthe world’s nuclear arsenal), a fireball plume risingto a height of 3,000 km, and left a scar 12,000 kmacross (Earth’s diameter).blinders.indd 36Photo: NASA/JPL-CaltechBy Peter Garretson11/20/08 12:59:45 PM

kilograms. Think what it would take to deflect a mass of 50,000Boeing 747s traveling at 36,000 kilometers per hour! If it were toimpact Earth, its kinetic energy would exceed five million timesthat of 9/11. It is estimated that developing a capability to deflectan asteroid would require decades of advance warning to successfully respond.Here are the facts: Our solar system is a cluttered shootinggallery of objects that threaten our lives and the existence oflife on Earth. We know that objects have struck the Earth in thepast, causing huge devastation and mass extinction. Today, wedon’t have the capability to know with certainty where and whenthe next impactor will strike. Yet virtually zero space or defensedollars are going toward creating such a capability. It is time tomove the topic of Earth-colliding asteroids and comets from discussions in the academic halls of astronomy and astrophysicsand onto the agenda of national security policymaking.What is the Threat?Fortunately, we now have a very developed scientific and operational knowledge about the very real threat that asteroid andcomet impactors could pose to our national and internationalsecurity. To provide some perspective on the difficulty, the asteroid Apophis (previously 2004 MN4), scheduled to pass Earthclosely in 2029 and 2036, has an estimated mass of 20 billionblinders.indd 37On March 18, 2004, an asteroid came within 3.4 Earth radii—inside the orbits of our geostationary satellites—but it wasn’tdiscovered decades in advance. In fact, it was detected onlytwo days in advance. More recently, in July of 2008, a binarynear Earth object consisting of one 600-meter object and one200-meter object cruised past us at a distance only six timesfarther than the orbit of the Moon. It was discovered only sevenmonths in advance. Apophis, the most threatening asteroid, wasdiscovered only four years ago. As I was editing this, 2008 TC3,a mere 5-meter asteroid, created an atmospheric airburst overSudan, releasing 1–2 kilotons of energy. It was discovered 20hours in advance. There must be significant improvement in ouradvance-warning capabilities because time is the most important factor in preventing such a catastrophe.What are the Odds?Our space situational awareness capabilities as yet are verymodest. After 13 years of searching, we are only now approaching the goal of finding 90 percent of the near Earth asteroidsgreater than one kilometer in size and capable of global destruction (742 have been discovered out of an expected populationof approximately 1,000). At the time of this writing, we know of5,443 NEAs (we discover between 3–5 new large NEAs permonth), of which 957 are classified as potentially hazardous.All are larger than 150 meters and capable of causing at leastregional destruction. Recent modeling at Sandia National Labsuggests that asteroids as small as 30 meters can penetrateand airburst, completely devastating an area the size of theWashington, D.C., metropolitan area. Estimates of the numberof near Earth asteroids in this size category suggest a populationof 400,000 to 750,000.11/20/08 1:00:13 PM

This illustration shows the devastation that would accompany a 50-meter nickel-iron impact,such as the asteroid that excavated the 1.2 kilometer-wide, 170 meter-deep Barringer Crater nearWinslow Arizona 50,000 years ago. Impacting in excess of 12.8 km/sec with a mass of 300,000tons, it released 2.5 megatons of TNT (150 times the bombs used on Hiroshima and Nagasaki),excavating 175 million tons of rock, a fireball flash-incinerating anything within 10 km, a 2,000km/sec shockwave leveling everything within a 24 km radius, and producing hurricane-force windsas far out as 40 km. Below: In 1908, an object, perhaps only 30 m in diameter air-bursted 5–10 kmabove a remote region in Tuguska, Siberia releasing the equivalent of 5 to 30 megatons TNT.planetary size objects, some larger than Earth. Periodically, passing starsknock an Oort comet loose and send it hurtling toward the Sun. A comethitting the Earth would be unpredictable and deadly. Comet threats arenot likely to give us the courtesy of a multi-decade warning.Protecting the PlanetThe Comet DangerUnfortunately, the asteroids are the easy problem when compared to acomet collision. A comet would actually hit the Earth eight times fasterthan a near Earth asteroid—at speeds of up to 288,000 kilometers perhour. A comet is now thought to have impacted North America only13,000 years ago and killed off most of its human inhabitants, the Clovis culture, and large mammals such as the woolly mammoth.The potential population of comets is staggering. More than 1,000 KuiperBelt objects have been detected, and astronomers say there might be 50–100,000 more, mostly small but some rivaling Pluto in size. The Oort Cloudis thought to contain as many as a trillion comets with perhaps 1,00038Planetary defense is greeted with a certain amount of “not my job”syndrome. It is a sort of tragedy of the commons—it is everybody’s joband nobody’s job. Defense agencies, science groups, emergency institutions, and Congress each know a threat exists, but nobody seems towant the responsibility. Who will have the nerve to take it on?Time to Take ActionAs space advocates armed with knowledge of the threat of near Earthobjects, we share in the responsibility of these agencies. We should notbe tolerant of polite giggles when the topic is raised. We need to projectthe deadly serious gravity of this issue. We must act to ensure that thedecision-makers follow through with policy to uphold what we havenow confirmed about this threat and what must be done about it.As space advocates, we also recognize that the technologies thataddress planetary defense also empower space exploration, open thespace frontier, and enable space settlement. We should be asking ourlegislators what their stance on the issue is—and if their answer is uneducated, unacceptable, or negligent, we should not be silent. We shouldput pressure on our elected officials and the organs of national security.As space advocates, we must take action to inspire the decision-makersin our nation to use our capabilities in space to provide for the commondefense of our planet and the prosperity of the human race.Illustration courtesy of www-th.bo.infn.it/tunguska; Photo: NASA/Leonid KulikIn fact, the Association of Space Explorers (ASE) has stated that whilewe are tracking perhaps 79 percent of potentially civilization-endingNEAs, we are tracking less than 1 percent of the Tunguska-sized NEAsthat could cause enormous destruction on the planet. Today, 206 ofthe discovered NEAs have some probability of impact. ASE estimatesthat within a little over a decade we are likely to be tracking perhapsas many as 1,000,000 NEOs, of which 10,000 may have some probability of impacting Earth in the next 100 years. In that time, perhaps50–100 will appear threatening enough to warrant active monitoringand/or deflection. Statistically, that leaves 75–150 asteroids that will bethreatening enough to warrant action, but will remain undetected byour detection and tracking programs.It is now past the time for this to move out of the realm of science andinto the realm of policy and national security. So why hasn’t actionbeen taken?ad Astra Winter 2008blinders.indd 3811/20/08 1:01:11 PM

Space legislation: HR 6063This year, a major legislative step forwardwas taken in planetary defense. On October15, 2008, the president signed HR 6063,the 2008 NASA Authorization Act into law.Excerpted below are the sections that specifically deal with near Earth objects. Recognizing the severity of the threat and the possibilityof avoidance, it requests basic information forsound policy making, including a mission torendezvous and characterize the near Earthasteroid Apophis as well as a larger spacemission to find asteroids as small as 140 m indiameter. Most significantly, it tasks the President’s Office of Science and Technology Policy (OSTP) to establish a policy for notificationof federal agencies and emergency responseinstitutions of an impending NEO threat andrecommend a lead agency for deflection.TITLE VIII—NEAR EARTH OBJECTSIN GENERALThe Congress reaffirms the policy directionestablished in the National Aeronautics andSpace Administration Authorization Act of2005 (Public Law 109-155) for NASA to detect,track, catalog, and characterize the physicalcharacteristics of near Earth objects equalto or greater than 140 meters in diameter.NASA’s Near Earth Object Program activitieswill also provide benefits to NASA’s scientificand exploration activities.FINDINGSCongress makes the following findings:1) Near Earth objects pose a serious andcredible threat to humankind, as many scientists believe that a major asteroid or cometwas responsible for the mass extinction of themajority of the Earth’s species, including thedinosaurs, nearly 65,000,000 years ago.2) Several such near Earth objects have onlybeen discovered within days of the objects’closest approach to Earth and recent discoveries of such large objects indicate thatmany large near Earth objects remain undiscovered.Photo: NASA3) Asteroid and comet collisions rank as oneof the most costly natural disasters that canoccur.4) The time needed to eliminate or mitigate thethreat of a collision of a potentially hazardousnear Earth object with Earth is measured indecades.Ida, a 15.7 kilometer stony-iron asteroid and its tiny satellite Dactyl (1.5 km diameter) as photograhed by the Galileo spacecraft in1993. It was the first binary asteroid of any kind ever discovered. At present, there are 29 known binary near Earth objects.5) Unlike earthquakes and hurricanes, asteroids and comets can provide adequate collision information, enabling the United States toinclude both asteroid-collision and comet-collision disaster recovery and disaster avoidancein its public-safety structure.6) Basic information is needed for technicaland policy decision making for the UnitedStates to create a comprehensive programin order to be ready to eliminate and mitigatethe serious and credible threats to humankindposed by potentially hazardous near Earthasteroids and comets.7) As a first step to eliminate and to mitigatethe risk of such collisions, situation and decision analysis processes, as well as proceduresand system resources, must be in place wellbefore a collision threat becomes known.REQUESTS FOR INFORMATIONThe Administrator shall issue requests forinformation on—1) a low-cost space mission with the purposeof rendezvousing with and characterizing theApophis asteroid, which scientists estimatewill in 2029 pass at a distance from Earth thatis closer than geostationary satellites; and2) a medium-sized space mission with thepurpose of detecting near Earth objects equalto or greater than 140 meters in diameter.ESTABLISHMENT OF POLICYThe Director of OSTP shall—1) develop a policy for notifying federal agencies and relevant emergency response institutions of an impending near Earth object threat,if near-term public safety is at stake; and2) recommend a federal agency or agencies tobe responsible for protecting the nation froma near Earth object that is anticipated to collide with Earth and implementing a deflectioncampaign, in consultation with internationalbodies, should one be required.PLANETARY RADAR CAPABILITYThe Administrator shall maintain a planetaryradar that is, at minimum, comparable to thecapability provided through the NASA DeepSpace Network Goldstone facility.ARECIBO OBSERVATORYCongress reiterates its support for the use ofthe Arecibo Observatory for NASA-fundednear Earth object-related activities. TheAdministrator shall ensure the availability ofthe Arecibo Observatory’s planetary radarto support these activities until the NationalAcademies’ review of NASA’s approachfor the survey and deflection of near Earthobjects, including a determination of the roleof Arecibo, that was directed to be undertaken by the fiscal year 2008 OmnibusAppropriations Act, is completed.Winter 2008 ad Astrablinders.indd 393911/20/08 1:01:40 PM

- . . ."' . Illustration: NASA/JPL-Caltech/T. Pyle (SSC) 40footprints asteroid.indd 4011/20/08 1:02:49 PM

Footprintson NEOsPossibly more accessible than the Moon,could a near Earth asteroid be the nextstop for manned space exploration?By Mark Williamson41footprints asteroid.indd 4111/20/08 1:03:25 PM

s recently as October 6, 2008, an asteroid measuring a few meters across was discovered bythe Catalina Sky Survey from the group’s observatory near Tucson, Arizona. The Jet Propulsion Laboratory quickly confirmed that an atmospheric impactwould occur early the next morning over northernSudan, northeastern Africa. And at around 2:45a.m. local time, a “brilliant fireball”—in the words ofthe NASA/JPL Near Earth Object Program Office—impacted the Earth’s atmosphere with an estimatedenergy equivalent to a kiloton of TNT.space community is shown by the fact that theworld-renowned International Academy of Astronautics and the European Space Agency are sponsoring the 1st IAA Planetary Defense Conference(subtitled “Protecting the Earth from Asteroids”),in Granada, Spain, in April 2009. “More and moreevidence confirms that impacts by asteroids andcomets are not uncommon and that even relativelysmall objects can cause local and regional disasters,” says William Ailor of The Aerospace Corporation and co-chair of the meeting.According to JPL’s Don Yeomans, “The follow-upastrometric observations from professional andsophisticated amateur astronomers alike wererather extraordinary, with 570 observations from 26observatories being reported between the time ofdiscovery to just before the object entered Earth’sshadow less than 19 hours!”Conference delegates will discuss everything fromdetection, tracking, and characterization of objectsto deflection techniques and the political, legal, andpolicy issues that the organizers say “should beconsidered as part of an overall mitigation strategy.”Of course, this begs the question of what a “mitigation strategy” might entail.Luckily, asteroid 2008 TC3 was too small for the restof us to lose sleep over. In fact, according to Yeomans, objects of this size enter Earth’s atmosphere“every few months on average.” However, as JPL’sSentry System for impact risks has shown, thereare much bigger near Earth asteroids (NEAs) thathave our planet potentially in their sights. Additionally, more than 900 asteroids and comets are designated potentially hazardous objects.Most authorities believe it is necessary to understand the composition of near Earth objects—a termthat includes comets as well as asteroids—beforecontemplating their deflection. The first phase hasalready begun, most notably with NASA’s NearEarth Asteroid Rendezvous probe, NEAR-Shoemaker (which became the first spacecraft to orbitthe asteroid Eros in 2000 and the first to make alanding the following year), and Japan’s Hayabusasample-return spacecraft, which landed on asteroid Itokawa in 2005 and is due to return a samplein 2010.Currently, the Sentry System flags only one NEA,designated 2007 VK184, that registers on the Torinoimpact hazard scale (and only at the lowest level),but that could change without warning as otherscome into view.Serious IssueThat the issue is taken seriously by the professional42Nor should we forget NASA’s Deep Space 1, whichcrashed into Comet Tempel 1, and Dawn, a cooperative effort between the U.S. and Europe that waslaunched to asteroids Vesta and Ceres in 2007. Butnote the difference between these real-life productsof spacecraft engineering and the spaceships weIllustration: NASA/Goddard Space Flight Center Scientific Visualization StudioAad Astra Winter 2008footprints asteroid.indd 4211/20/

trough at the Chicxulub impact site, related to the extinction of dinosaurs 65 million years ago. Opposite page, a series of impact sites that include—clockwise from top photo—Meteor Crater in Arizona; Wolf Creek crater in western Australia; the New Quebec crater in Quebec Cana

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