* Astronomy

Title: Numerical predictions of surface effects during the 2029 close approach of asteroid 99942 Apophis
Author: Yang Yu, Derek C. Richardson, Patrick Michel, Stephen R. Schwartz, Ronald-Louis Ballouz

Asteroid (99942) Apophis' close approach in 2029 will be one of the most significant small-body encounter events in the near future and offers a good opportunity for in situ exploration to determine the asteroid's surface properties and measure any tidal effects that might alter its regolith configuration. Resurfacing mechanics has become a new focus for asteroid researchers due to its important implications for interpreting surface observations, including space weathering effects. This paper provides a prediction for the tidal effects during the 2029 encounter, with an emphasis on whether surface refreshing due to regolith movement will occur. The potential shape modification of the object due to the tidal encounter is first confirmed to be negligibly small with systematic simulations, thus only the external perturbations are taken into account for this work (despite this, seismic shaking induced by shifting blocks might still play a weak role and we will look into this mechanism in future work). A two-stage approach is developed to model the responses of asteroid surface particles (the regolith) based on the soft-sphere implementation of the parallel N-body gravity tree code pkdgrav. A full-body model of Apophis is sent past the Earth on the predicted trajectory to generate the data of all forces acting at a target point on the surface. A sandpile constructed in the local frame is then used to approximate the regolith materials; all the forces the sandpile feels during the encounter are imposed as external perturbations to mimic the regolith's behavior in the full scenario. The local mechanical environment on the asteroid surface is represented in detail, leading to an estimation of the change in global surface environment due to the encounter. Typical patterns of perturbation are presented that depend on the asteroid orientation and sense of rotation at perigee.

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Asteroid (99942) Apophis makes its closest approach to the Earth (0.376 AU) on the 5th April 2014

Ephemeris

Date       UT      R.A. (J2000) Decl.    Delta     r     El.    Ph.   Mag
2014 04 04 000000 20 07 08.2 -18 27 59   0.374   0.966   74.0  84.2  19.9
2014 04 05 000000 20 13 14.8 -18 00 17   0.374   0.963   73.4  84.7  19.9
2014 04 06 000000 20 19 20.6 -17 31 48   0.374   0.960   72.9  85.2  20.0
2014 04 07 000000 20 25 25.4 -17 02 35   0.374   0.956   72.4  85.7  20.0
2014 04 08 000000 20 31 29.1 -16 32 40   0.375   0.953   71.8  86.3  20.0
2014 04 09 000000 20 37 31.4 -16 02 06   0.375   0.950   71.3  86.8  20.0
2014 04 10 000000 20 43 32.4 -15 30 54   0.376   0.947   70.7  87.3  20.0
2014 04 11 000000 20 49 31.7 -14 59 07   0.377   0.943   70.2  87.7  20.0
2014 04 12 000000 20 55 29.4 -14 26 48   0.378   0.940   69.6  88.2  20.1
2014 04 13 000000 21 01 25.3 -13 53 59   0.379   0.937   69.1  88.7  20.1
2014 04 14 000000 21 07 19.2 -13 20 43   0.381   0.933   68.5  89.2  20.1
2014 04 15 000000 21 13 11.2 -12 47 03   0.382   0.930   68.0  89.6  20.1
2014 04 16 000000 21 19 01.0 -12 13 00   0.384   0.926   67.4  90.1  20.1
2014 04 17 000000 21 24 48.7 -11 38 38   0.386   0.923   66.9  90.5  20.2
2014 04 18 000000 21 30 34.1 -11 04 00   0.388   0.920   66.3  90.9  20.2
2014 04 19 000000 21 36 17.3 -10 29 07   0.390   0.916   65.8  91.3  20.2
2014 04 20 000000 21 41 58.0 -09 54 03   0.393   0.913   65.3  91.7  20.2

Title: Yarkovsky-driven impact risk analysis for asteroid (99942) Apophis
Authors: D. Farnocchia, S. R. Chesley, P. W. Chodas, M. Micheli, D. J. Tholen, A. Milani, G. T. Elliott, F. Bernardi
(version, v2)

We assess the risk of an Earth impact for asteroid (99942) Apophis by means of a statistical analysis accounting for the uncertainty of both the orbital solution and the Yarkovsky effect. We select those observations with either rigorous uncertainty information provided by the observer or a high established accuracy. For the Yarkovsky effect we perform a Monte Carlo simulation that fully accounts for the uncertainty in the physical characterisation, especially for the unknown spin orientation. By mapping the uncertainty information onto the 2029 b-plane and identifying the keyholes corresponding to subsequent impacts we assess the impact risk for future encounters. In particular, we find an impact probability greater than 10^-6 for an impact in 2068. We analyse the stability of the impact probability with respect to the assumptions on Apophis' physical characterisation and consider the possible effect of the early 2013 radar apparition.

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Herschel intercepts asteroid Apophis

ESA's Herschel space observatory made new observations of asteroid Apophis as it approached Earth this weekend. The data shows the asteroid to be bigger than first estimated, and less reflective.
Catalogued as asteroid (99942) Apophis (previously 2004 MN4), it is often nicknamed 'the doomsday asteroid' in popular media, after initial observations made after its discovery in 2004 gave it a 2.7% chance of striking Earth in April 2029.
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