The Near- Objects hazard and how we deal with it

G. B. Valsecchi IAPS-INAF, Roma (Italy) IFAC-CNR, Sesto Fiorentino (Italy) The problem

NEOs are and on orbits with q ≤ 1.3 au. At any given time, some of them are on Earth intersecting orbits and may collide with it. Asteroids (NEAs) dominate the statistics at most sizes; comets (NECs) dominate at the largest sizes.

The plot of the impactor flux si taken from Brown et al. (2002). The “dinosaur solution”(?!) NEA impacts vs space debris collisions

There is a certain degree of similarity between NEA impacts on Earth and space debris impacts on artificial satellites. There are, however, some important differences: • nature and number of targets to be protected; • time scales involved; • maneuverability of targets; • strong chaoticity of the motion in the NEO case; • chain reaction effect in the debris case. Five steps to decrease the NEA risk

The strategy to deal with the risk associated with Near-Earth Asteroids (NEA) impacts on the Earth can be summarized in five steps (Valsecchi & Milani 2007). We have to: 1) detect them; 2) determine their orbits; 3) compute collision possibilities; 4) exclude collision possibilities with further observations; 5) deflect/destroy, if necessary, those going to collide with our planet, or evacuate the affected region, when appropriate. 1) Detection

NEA detection is difficult because: • their faintness; • their rapid motion in the sky when they are close to the Earth and thus easier to detect; • the geometry of their passages close to the Earth.

NASA funds all the major ground-based and space-based surveys aimed at the discovery of NEAs (e.g., Catalina Sky Survey, Mt. Lemmon Survey, Pan-STARRS). Europe is setting up a ground-based telescopic survey that will complement the US surveys. Other contributions to the detections are minor. 1) Detection: completeness level

Discovered NEAs by decades from 1890 January 1th to 1989 December 31th Discovered NEAs by years from 1990 January 1 to the current date 1600 Credits ESA/NEOCC 80 Credits ESA/NEOCC 1400 70

60 1200

50 1000

800 40 Eros discovery

30 600

20 400

10 200

0 0 1890-1899 1920-1929 1950-1959 1980-1989 1992 1995 1998 2001 2004 2007 2010 2013 2016

Before the late ninenties the yearly discovery rate of NEAs was very low. The rate increased enormously with the advent of CCD-based detectors, and continues to increase nowadays. About 90% of the 1 km or larger NEAs have been discovered so far, but the completeness level at smaller sizes is considerably lower. 2) Orbit determination

The same factors that make NEAs difficult to detect make the initial determination of their orbits problematic: • their faintness implies short observed arcs; • their rapid motion in the sky requires immediate follow-up, that is not always available; • in some cases the geometry of their apparitions makes the observed positions compatible with orbits of very different nature, while in other cases makes the follow-up impossible from the ground. 2) Orbit determination: the players

NASA funds the operations of the Center (MPC) of the International Astronomical Union, where astrometric data are collected, stored and made available to the astronomical community; the MPC computes orbits and gives official designations to all asteroids and comets. Moreover, NASA operates the Planetary Defense Coordination Office at the Jet Propulsion Laboratory (JPL), where independent orbit computations are made. In Europe, a major orbit-computing facility is the University of Pisa (Italy), whose NEODyS and AstDyS websites will migrate, in the next years, to the NEO Coordination Centre of the European Space Agency (ESA). 3) Impact monitoring

The difficulty of impact monitoring lies in the possibility of pre-collision planetary close encounters, that can greatly alter the orbit of the impactor, and originate the keyhole phenomenon. A keyhole is a small region close to the Earth such that, if the NEA passes within it while encountering the planet, it will be put on an orbit that impacts the Earth at a subsequent encounter. The first Impact Monitoring program came online at the University of Pisa, as part of NEODyS, in 1999; a second, independent program became operational at JPL in 2002 (Milani et al. 2000, 2005). The two programs work in parallel since 2002, with the two teams in constant contact in order to improve performances and ensure reliability and redundancy. 3) Impact monitoring: keyholes

5 6 x 10 5 x 10 6 x 10 x 10 2 3σ Left: Line of Variations 2 3σ 1 1 2σ (LoV) and b-plane circle 1 1 2σ correspondingEarth to the 4/5 Earth 0 Earth Earth 0 1σ 0 0 resonance.2186 1σ 2186 2194 2194 −1 2192 −1 −1 2192 2190 Nominal Right: an enlargement,2190 Nominal −1 2191 2191 −2 −2 ζ (km) ζ (km) ζ (km) ζ (km) −2 with LoV, cross-section of −2 1σ 1σ −3 the Earth,2196 and−3 circles 2196

−4 −3 corresponding−4 to the 1/1, −3 2σ 2σ

−5 8/9, 6/7, 5/6,−5 9/11 −4 −4 3σ resonances (Spoto et al. −6 −6 3σ −2 0 2 −1 0 1 −2 0 2 −1 0 1 ξ (km) 6 ξ (km) 5 (km) 6 ξ (km) 5 x 10 2014). x 10 ξ x 10 x 10

Figure 6: The left f gure shows the LOV on the 2185 TP: the VI of 2190Figure is 6: The left f gure shows the LOV on the 2185 TP: the VI of 2190 is markedwith a cross. Theright f gure shows a segment of theLOV on the2185markedwith a cross. Theright f gure shows a segment of theLOV on the2185 TP: theThe keyholesb for-plane impact in (from of thetop to bottom) 2185 2186, Earth 2194, 2192, encounterTP: 2191, the keyholes for impact of (410 in (from 777) top to bottom) 2009 2186, 2194,FD; 2192, 2191, and 2196are markedwith crosses; also shown are the b-planecirclesassociatedand 2196are markedwith crosses; also shown are the b-planecirclesassociated with thekeyholes respective mean are motion located resonances(1/1, near 8/9 6/7, 5/6,the 9/11). intersectionswith the respective mean of motion the resonances(1/1, LoV with 8/9 6/7, 5/6,the 9/11). circles corresponding to the resonances (Valsecchi et al. 2003).

14 14 4) Orbit improvement cycle

1999 AN10: Impact Plane on 2027 Aug 07 1999 AN10: Impact Plane on 2027 Aug 07 The shrinking 8 6

Orbital solution based on 123-day arc Orbital solution based on 130-day arc 6 uncertainty region 4 Uncertainty Ellipse is 4 1.6 million km long for the 2027 Earth by 1200 km wide 2 Earth Uncertainty Ellipse is 2 1.3 million km long Earth by 1000 km wide encounter of 0 0 km) 0 km) 0 1999 AN10; left:

(10000 (10000 -2 -2 123-day arc, right: Most likely point of passage Most likely point of passage -4 -4 through this plane through this plane 130-day arc -6 -6 Minimum possible miss distance: 37,000 km Minimum possible miss distance: 37,000 km (image credit: -8 -8 -8-6-4-202468-6 -4 -2 0 2 4 6 8 (100000 km) (100000 km) Chodas, JPL).

Almost always collision possibilities are the consequence of our lack of sufficient knowledge about the orbit of a NEA. 4) Orbit improvement cycle: its effects

The solution is 1999 AN10: Keyholes in Impact Plane on 2027 Aug 07 astrometric follow-up, Orbital solution based on 130-day arc

4 until all collision possibilities become

4 3 0 2 incompatible with the

2 Uncertainty ellipse

0 km) observational record. (10000

Earth As the uncertainty 0 4 4 0 region shrinks, collision 2 6 4 0 2 Keyholes that lead to probabilities may possible impacts

-2 increase, before the -6 -4 -2 0 (100000 km) corresponding collisions get out of the region. Image credit: Chodas, JPL. 5) Dealing with an actual impactor

This step falls more in the realm of civil protection than in that of astronomy, and may involve political decisions at the international level. In case the probability of collision of a NEA becomes substantial, possible actions include (but are not limited to): • the deflection or the destruction of the impactor, if sufficient time is available; • the evacuation of the affected regions otherwise. Major current players in the NEO arena

The annual NEO-related NASA budget went from 4M$ in 2010 to 20.4M$ in 2012, then to 40M$ in 2014, and has increased to 50M$ in 2016. It is then not surprising that the greater part of the worldwide NEA-related activities are carried out by NASA-funded programs. Elsewhere the contributions to the field have consisted mostly in the activity of astrometric follow-up done by skilled amateurs. The NEO Segment of the Space Situational Awareness Program of ESA aims at contributing to NEO activities by setting up: • an orbit computation and impact monitoring component, built upon the NEODyS heritage; • a NEO survey component, based on innovative wide field-of-view telescopes. The rˆoleof the United Nations

The United Nations, through the Committee on the Peaceful Uses of Outer Space (COPUOS), have promoted the establishment of: • the International Warning Network (IAWN), aimed at the discovery of hazardous asteroids and comets and the identification of those objects requiring action; • the Space Mission Planning Advisory Group (SMPAG), whose task is to plan for a mitigation campaign for a possible impactor that includes deflection and/or disruption actions as well as civil defense activities. Conferences on NEOs

• 1993, Tucson (USA): “Hazards due to Comets and Asteroids” (scientists); • 1999, Torino (Italy): “International Monitoring Programs for Asteroid and Threat (IMPACT)” (scientists); • 2002, Arlington (USA): ”Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids (scientists); • 2003, Frascati (Italy): “OECD Global Science Forum Workshop on Near Earth Objects: Risks, Policies and Actions” (national delegates, scientists); • 2004, Tenerife (Spain): “Comet/Asteroid Impacts and Human Society” (disaster managers, scientists); • Prague (Czech Republic): “Near Earth Objects, our Celestial Neighbors: Opportunity and Risk”, IAU Symposium 236 (scientists). Conferences on NEOs (contd.)

The series of “Planetary Defense Conferences” (engineers, scientists): • 2004, Orange County, USA; • 2007, Washington, D. C. (USA); • 2009, Granada (Spain); • 2011, Bucharest (Romania); • 2013, Flagstaff (USA); • 2015, Frascati (Italy); • 2017, Tokyo (Japan). References

• Brown, Spalding, ReVelle, Tagliaferri & Worden, 2002, Nature 420, 294 • Milani, Chesley & Valsecchi, 2000, Planet Space Sci. 48, 945 • Milani, Chesley, Sansaturio, Tommei & Valsecchi, 2005, Icarus 173, 362 • Spoto, Milani, Farnocchia, Chesley, Micheli, Valsecchi, Perna & Hainaut, 2014, Astron. Astrophys. 572, A100 • Valsecchi & Milani, 2007, in “Comet/Asteroid Impacts and Human Society”, Eds.: Bobrowsky & Rickman, Springer • Valsecchi, Milani, Gronchi & Chesley, 2003, Astron. Astrophys. 408, 1179