Physics Research International

Volume 2014, Article ID 561680, 13 pages

http://dx.doi.org/10.1155/2014/561680

## The Potentially Dangerous Asteroid (101955) Bennu

Chorzów Astronomical Observatory, WPKiW, 41-500 Chorzów, Poland

Received 6 June 2014; Accepted 17 November 2014; Published 15 December 2014

Academic Editor: Sergey B. Mirov

Copyright © 2014 I. Włodarczyk. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We computed impact solutions of the potentially dangerous asteroid (101955) Bennu based on 569 optical observations from September 11.40624 UTC, 1999 to January 20.11189 UTC, 2013, and 29 radar observations from September 21, 1999, through September 29, 2011. Using the freely available OrbFit software package, we can follow its orbit forward in the future searching for close approaches with the earth, which can lead to possible impacts up to 2200. With the A2 nongravitational parameter in the motion of the asteroid (101955) Bennu we computed possible impact solutions using different JPL planetary and lunar ephemerides and different number of additional massive perturbed asteroids. The possible impact path of risk for 2175 is presented. Additionally, we computed possible impact solutions using the normal places method of the selection of Bennu’s astrometric observations. Moreover, we computed time evolution of the mean orbital elements and the orbital nodes of Bennu 5 kyr in the backwards and 1 kyr in the future using the Yarkovsky effects. We computed the mean motion and secular orbital resonances of the Bennu. We also computed the influence of the JPL planetary and lunar ephemerides DE403, DE405, DE406, DE414, and DE423 on the close approaches of the asteroid (101955) Bennu with the earth.

#### 1. Introduction

Discovered on September 11, 1999, by the Lincoln Laboratory Near-Earth Asteroid Research Team at Socorro, Bennu was an Egyptian mythological figure associated with Osiris, Atum, and Ra. This minor planet is the target of the OSIRIS-REx sample return mission (http://www.nasa.gov/osiris-rex). OSIRIS-Rex’s Touch-and-Go Sampler evokes Bennu’s image as a heron. The spacecraft is scheduled to launch in 2016, reach 101955 Bennu in 2019, and return samples to earth in 2023.

It is a potential Earth impactor and now, as of March 17, 2014, is listed on the Sentry Risk Table of the JPL NASA with the third highest rating on the Palermo Technical Impact Hazard Scale (PS = −1.71 (cumulative) and −2.32 (maximum)), after the asteroid (29075) 1950 DA (PS = −0.83 (cumulative and maximum)) and 2007 (−1.56 and −1.57), respectively (http://neo.jpl.nasa.gov/risk/). Moreover, the asteroid (101955) Bennu is listed on the second place on the CLOMMON2 site ( (maximum)) provided by the NEODyS (http://newton.dm.unipi.it/neodys/index.php?pc=4.1).

Up to now, there were many methods of computing possible impact solutions of the asteroid (101955) Bennu. In Milani et al. [1] there are computed eight potential earth impacts of the asteroid (101955) Bennu between 2169 and 2199. They found the best fit value of au/My which give the minimum value of the rms for all observations of the asteroid (101955) Bennu. Moreover, they found possible impacts between 2169 and 2199 using different values of in the range au/My.

Vokrouhlický et al. [2] investigate the possibility of detecting the Yarkovsky effect via precise orbit determination of the near-earth asteroids on example of the asteroids 6489 Golevka, 1620 Geographos, 1566 Icarus, and 1998 with different values of the parameter . Their figures present 3 uncertainty ellipsoids with and without the Yarkovsky effects projected onto the range () versus range rate () plane for the close approach of different asteroids.

Farnocchia et al. [3] computed for asteroid (101955) 1999 Bennu au/My and au/My. Tables 2 and 3 list some physical parameters and nongravitational parameters, and , for 37 asteroids.

Vokrouhlický et al.[4] present explicit, analytic formulas for the solar radiation dynamical effects on the orbits of two near-earth asteroids: (1566) Icarus and (6489) Golevka. They plot the 3 confidence ellipses of the Icarus and Golevka orbit uncertainty onto the space of radar observables: the geocentric distance (in km) and the rate of change of the geocentric distance (in km/day) computed for the nominal model, that is, without the radiation effects, and for the extended model, that is, including the radiation effects. These simulated orbit displacements will be useful for the search of the nongravitational parameters in the motion of asteroids.

Farnocchia and Chesley [5] collect equations for two nongravitational parameters, , that is, nongravitational radial acceleration parameter, and , that is, nongravitational transverse acceleration parameter. They also computed value of for the asteroid (29075) 1950 DA from theory and using astrometric observations.

Farnocchia et al. [3] concluded that the Yarkovsky effect for the asteroid (99942) Apophis with two adopted values of : au/ and au/ cannot be detectable with the radar apparition in 2013. They also list 25 main belt asteroid perturbers and associated values which we used in our orbital computations with the OrbFit software. denotes mass expressed as a product of the mass () and gravitational constant () in units km^{3}/s^{2}.

Chesley et al. [6] contain a detailed analysis of the relevance of the different components of the dynamical model, a determination of Bennu’s density and mass, and a rigorous statistical analysis of the potential impacts with associated impact probabilities. We used their nongravitational parameter au/d^{2} in our computations of possible impact solution for the asteroid (101955) Bennu.

Moreover, to compute possible impact solution of the asteroid (101955) Bennu with the earth we take into account the OrbFit software with different JPL planetary and lunar ephemerides, perturbations of different number of additional massive asteroids, weighting and selection of observations according to the NEODyS, the error model based on Chesley et al. [7], the Yarkovsky effects, and the normal places method.

There are no differences in observational material between us and Chesley et al. [6]. But in some case, when the observational material is dense, they used new weights. Similarly, in our work we applied additionally the normal places method; see Section 3.1.

Chesley et al. [6] present computations based on two JPL ephemerides: DE405 and DE424.

We computed possible impacts with many different JPL ephemerides: JPL DE403, DE405, DE406, DE414, and DE423. Moreover, we added 25 perturbing asteroids when Chesley et al. [6] used 16 asteroids. Also our computations of the earliest possible impacts were made for 0, 4, 16, and 25 massive perturbing asteroids—see Table 2.

We also draw the path of risk of the asteroid Bennu in 2175.

We show that errors of propagated orbital elements of the asteroid Bennu are great, mainly in 2175, after CA with the earth in 2135. We computed these errors and present in Table 6. Hence impact hazard after 2135 can only be explored through statistical means, as Chesley et al. [6] state.

Our computations of CAs of the asteroid Bennu with the earth till 2135 give almost the same results as Chesley et al. [6].

Moreover, we extend orbital evolution of the asteroid Bennu from 2000 to 2140 as was explored in Chesley et al. [6] to time-span 5000 yr backwards and 1000 yr forward from the osculating epoch 2014 May 23.0 = JDT 2456800.5. Position of the asteroid Bennu in its orbit is chaotic but the orbit in the space is stable over several thousands of years as was depicted in Section 6 and in Figure 2 where the stable mean values of orbital elements are presented.

#### 2. The Initial Orbital Elements of Asteroid (101955) Bennu

Currently, March 17, 2014, there are 10708 near-earth asteroids ( au): 832 Atens with orbits similar to that of 2062 Aten ( au; au), 5345 Apollos with orbits crossing the earth’s orbit similar to that of 1862 Apollo ( au; au), and 4531 Amors with orbits similar to that of 1221 Amor (1.017 au au). They are listed at the Minor Planet Center (MPC) (http://www.minorplanetcenter.net/iau/lists/Unusual.html) and at the JPL NASA (http://neo.jpl.nasa.gov/stats/).

The MPC and the JPL NASA classified asteroid (101955) Bennu as an Apollo-class object and additionally as a potentially hazardous asteroid. Bennu belongs to one of 1457 potentially hazardous asteroids (PHA) (http://www.minorplanetcenter.net/iau/lists/Dangerous.html). According to the Minor Planet Center (MPC), the PHA are objects with brighter than and an earth MOID less than 0.05 au. MOID, Minimum Orbit Intersection Distance, is the minimum distance between the orbit of the earth and the minor planet. According to the JPL NASA (http://neo.jpl.nasa.gov/risk/a101955.html), its absolute magnitude is 20.9, with a diameter of about 0.560 km and a rotation period equal to 4.288 h. According to Chesley et al. [6] the mass of Bennu is equal to kg and the bulk density is kg/m^{3}.

Table 1 lists the computed orbit of the asteroid (101955) Bennu published by different sources: the MPC in M.P.O. 251037 (http://www.minorplanetcenter.net/iau/ECS/MPCArchive/2013/MPO_20130127.pdf), the NEODyS-2 (http://newton.dm.unipi.it/neodys/index.php?pc=1.1.0&n=101955), the JPL Small-Body Database (http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=101955;orb=1), and the author.