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Near Earth Asteroids Long Term Impact Monitoring: Difficult but Necessary

F. Bernardi 1,2 , A. Milani ² , G. B. Valsecchi ¹ , S. R. Chesley ³ , M. E. Sansaturio 4 , O. Arratia 4 ¹ IASF-Roma, INAF, Italy ² University of Pisa, Italy ³ Jet Propulsion Laboratory, U.S.A. 4 University of Valladolid, Spain.

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Near Earth Asteroids Long Term Impact Monitoring: Difficult but Necessary

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  1. F. Bernardi1,2, A. Milani², G. B. Valsecchi¹, S. R. Chesley³, M. E. Sansaturio4, O. Arratia4 ¹IASF-Roma, INAF, Italy ²University of Pisa, Italy ³Jet Propulsion Laboratory, U.S.A. 4University of Valladolid, Spain Near Earth Asteroids Long Term Impact Monitoring: Difficult but Necessary

  2. Current impact monitoring • It is about 15 years since NASA and the astronomical community are involved into the search and monitoring of Near Earth Asteroids (NEA) and comets that can potentially hit the Earth • In all world there are two independent impact monitoring systems: • CLOMON2 of NEODyS in Pisa and a duplicate in Spain • Sentry of JPL in Pasadena, CA, USA • The two systems monitor the impact hazard for our planet for the next 80-90 years, determining the Impact Probability (IP) for each NEA http://newton.dm.unipi.it/neodys http://neo.jpl.nasa.gov/risk/

  3. Current impact monitoring:direct hits • Direct hits, i.e. collisions taking place without intervening planetary approaches, are easier to spot than resonant and non-resonant returns • Probabilities are comparatively higher • Deflections are more expensive in terms of Δv • Deflection decisions are comparatively easier to take (if time allows)‏

  4. Current impact monitoring:resonant returns • Resonant and non-resonant returns are less easy to spot than direct hits • Probabilities are comparatively lower • Deflections are (generally) less expensive in terms of Δv • Deflection decisions are less easy to take, as much better orbital information is needed (see Apophis), but more time may be available

  5. Current impact monitoring:resonant returns • Collision regions (keyholes) are found at peculiar locations on the b-plane (the plane normal to the unperturbed geocentric velocity of the impactor) of a pre-impact encounter • They tend to be small, whence the gain in Δv in case of deflection, and the greater difficulty in deciding a deflection

  6. Future impact monitoring • For large impactors, current impact monitoring may not give sufficient advance time for deflection • If Δt is the number of years between deflection and collision, Ahrens and Harris (1992, 1994) estimate that, to avoid a direct hit, the needed Δv in m/s is 0.07/Δt • So, the longer our predictability horizon, the easier the deflections • How far could/should we push the predictability horizon?

  7. The problems • We have basically two problems: chaos and dynamical model • Chaos: encounter sequences lead to multiplicative accumulation of the along-track separation from each encounter (i.e., to exponential divergence and chaos) with maximum Lyapounov exponent proportional to encounter frequency • So, no matter how precisely determined is a NEA orbit, after sufficient time we are led to a situation resembling that of a newly discovered NEA, with all the uncertainty concentrated along-track

  8. The problems • Dynamical model: given the sensitivity of encounter outcomes to small changes in the initial conditions, the possibility to extend the predictability horizon for a NEA, beyond a certain date, depends from the accurate modelling of so-far neglected non-gravitational perturbations like the Yarkovsky effect • We have selected (101955) 1999 RQ36,a NEA with a very well determined orbit, to test the possibility of extending the predictability horizon (see Milani et al. 2009 for details)‏

  9. The problems • Note: The non-gravitational effect called Yarkovsky effect is due to the solar radiation that is re-emitted through thermal radiation in a different direction, because the object is rotating and because the material has thermal inertia

  10. (101955) 1999 RQ36 • (101955) 1999 RQ36 has the lowest formal uncertainty in semimajor axis at epoch of any asteroid: 5 m • The Yarkovsky effect causes a change of the semimajor axis of the order of 200 m/y for this asteroid • Orbits determined accounting for the Yarkovsky effect differ from the purely gravitational solution by about 40 times more than the formal uncertainty • Very long propagations could be within the predictability horizon in a purely gravitational model, but non-gravitational effects make this horizon closer

  11. (101955) 1999 RQ36 • Time evolution of the Minimum Orbit Intersection Distance (MOID) for (101955) 1999 RQ36 • The MOID is less than the effective radius of the Earth (horizontal lines) many times, between about 2100 and 2230

  12. Monte Carlo runs • We have run Monte Carlo (MC) simulations with the method of Chodas & Yeomans (1999): 500000 MC samples drawn from a 7-dimensional space of initial conditions (6 elements and the Yarkovsky-induced da/dt), and followed until 2200 • All MC samples remain close together up to a pair of Earth encounters in 2060 and 2080, after which they become widely scattered • 461 MC samples collide with the Earth, giving an Impact Probability (IP) of about 0.00092 • 272 MC samples collide in 2182; of these, 268 have similar dynamical histories: thus, there is a dynamical route leading to impact in 2182 with an IP of about 0.00054

  13. The 2182 impact • In 2182, the IP is high because the LoV portion close to the Earth is folding, crossing our planet twice close to the fold tip • This means that the along-track separation is very small: something in the previous evolution has refocused the impactors

  14. The 2162 b-plane • The double keyhole for impact in 2182 seen on the b-plane of the 2162 encounter • The fold tip is marked in red • The green circle shows the size of the lunar orbit • The along-track size of the two keyholes are 187 and 118 Earth radii: the keyhole is larger than the door!

  15. The 2182 impact • Time evolution of the stretching (essentially, the along-track separation) for a typical 2182 impactor • In 2182 the stretching has come back near the value it had almost a century before • Consequence: an along-track deflection would have to be attempted no later than 2080

  16. Conclusions • Long-term impact monitoring is doable, provided we determine the Yarkovsky drift in individual cases • NEAs whose MOIDs are small and decreasing in the next few centuries have to be carefully monitored • Question: how typical is 1999 RQ36? ....2001WN5.... see “Long term impact risk for (101955) 1999 RQ36” Milani et al. 2009, submitted to Icarus

  17. Extra slides

  18. Yarkovsky parameters • The χ2 of the least squares fit to the value of da/dt • The quadratic approximation for the χ2 is obtained by a parabola fit to 5 data points (the asterisks) • The horizontal line corresponds to a 90% confidence level for the fit

  19. Monte Carlo runs • Above: a subset of our MC samples plotted in the plane da/dt in AU/Myr vs a-1.1 AU at epoch 2081-Jan-01.0 • Below: the orbits leading to an impact • The star represents the purely gravitational solution

  20. The folds of the 2162 encounter • The relevant LoV portion 5 months after the September 2162 encounter with Earth (blue sphere)‏ • The yellow sphere marks the LoV segment where the 2182 impactors are located • The nearby fold reaches the yellow sphere exactly in time for the 2182 impact

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