ADVANCED NEA TRACKING Justin Manuel Rivera Bergonio Department of Physics and Astronomy University of Hawai i at Mānoa Honolulu, HI 96822 ABSTRACT Asteroids are remnant planetesimals that orbit the Sun. Near Earth Asteroids are asteroids that have closest approaches to the Sun of 1.3 AU or less. Surveying the skies and cataloguing these objects, as well as research in mitigation and defense against potential impacts, has recently been made a priority by NASA (Pasztor, 2013). This research project focused on two things: developing and improving NEA tracking techniques using the 2-meter Faulkes Telescope North, and acquiring data in order to generate a lightcurve for the asteroid Apophis during its close approach. Over the course of a year 52 targets were reported to the Minor Planet Center, of which 6 were Virtual Impactors, 15 were Potentially Hazardous Asteroids, and 7 were on the Minor Planet Center s Near Earth Objects Confirmation Page. For the lightcurve portion of the project, 54 observations were made of Apophis on 11 separate nights from January to April. A total of 41 photometric points were extracted from this data set taken by Faulkes Telescope North, and a preliminary lightcurve was generated using the obtained data set. INTRODUCTION Asteroids that escape the main belt, which lies between the orbits of Mars and Jupiter, and have perihelion of 1.3 AU or less are known as NEAs, or Near Earth Asteroids (Baalke, 2012). A sub-category of NEAs are Potentially Hazardous Asteroids (PHAs), which are asteroids that have absolute magnitudes of 22.0 or brighter and approach the Earth s orbit at a distance of at least 0.05 AU; this distance is known as the minimum orbital intersection distance, or MOID, of an asteroid (Yeomans et al., 2012). The number 0.05 AU is the possible adjustment of an asteroid s orbit that can occur over the span of a century due to the gravitational influence of Jupiter. Another important sub-set of asteroids are Virtual Impactors (VIs), which are asteroids that have Earth impact solutions that satisfy available observations and are within limits of the errors provided by the observer. There are four main orbital types of NEAs: Earthcrossing asteroids known as Apollos and Atens, and Earth-approaching asteroids known as Amors and Apoheles (Baalke, 2012). The Alvarez hypothesis, which suggested the extinction of the dinosaurs was caused by the impact of a 10-mile wide asteroid, first brought to light the potential threat asteroids posed to the destruction of all mankind (Alvarez et al., 1980). Since then, much research has been done to further our understanding of these renegade space rocks. The Minor Planet Center (MPC) is an essential clearinghouse where observations of NEAs and other minor planetary bodies are reported and stored. Along with receiving and storing NEA observations, the MPC also provides services and tools for asteroid hunters, as well as a page known as the Near Earth Objects Confirmation Page (NEOCP) that brings attention to potential NEA candidates. Many surveys have been carried out to catalogue NEAs, one such survey was NASA s Near Earth Object Wide-field Infrared Survey Explorer (NEOWISE) mission that detected 429 NEAs and 107 PHAs in its nine-month mission. Through the 1
NEOWISE study 90% of the NEAs larger than 1km have been found, while only 30% of the projected ~4700 NEAs larger than 100m have been found to date (Mainzer et al., 2012). Along with cataloguing and surveying the skies for NEAs, much work and research has also been done in potential impact mitigation strategies. Some methods of deflecting and mitigating potential impacts that have been suggested are focusing mirrors on the asteroid to produce gas on the surface, solar sails, kinetic energy induced impacts (McInnes, 2003) and even using a gravity tractor to slowly adjust the asteroid s orbit over some period of time (Lovegren, 2005). Other ways of mitigating potential impacts is to collect more observations to better determine asteroid orbits, and to take into account other effects that may cause orbital perturbations such as the Yarkovsky Effect. The Yarkovsky Effect affects asteroids 10km or less in size, the effect becoming greater for smaller asteroids. As an asteroid rotates through space along its orbit around the Sun, the sunlit side receives heat. The heat is then reradiated in all directions, but it is done so nonuniformly such that the afternoon side reradiates more energy than the cooler morning side. Combined with the asteroid s spin orientation, mass, size, and thermal inertia a net thermal force arises that perturbs the asteroid s orbit over a long period of time. Detecting this minute change is very important in some cases such as Apophis, a VI discovered in 2004 at Kitt Peak, Arizona, in which the Yarkovsky Effect is the largest remaining uncertainty (Farnocchia, 2013). One of NASA s goals highlighted in its 2011 Strategic Plan is to expand scientific understanding of the Earth and the universe in which we live ( 2011 NASA Strategic Plan ). This year s project had two focuses: (1) tracking of NEAs in order to improve their orbit determination by extending their observational arcs, and (2) generating a lightcurve of asteroid Apophis in hopes of determining its rotational period and the extent of the Yarkovsky Effect on Apophis orbit. Through this research project we were able to expand our understanding of the universe which we live by determining the orbits of these objects and determining if they pose a threat to us or not. METHODS NEA Tracking There were three main steps in tracking NEAs: preparation of a target list, acquisition of observational data, and analysis of the data to obtain asteroid positions. The first step was carried out two days before the reserved telescope time. Time was reserved online a few weeks in advance on my Faulkes Telescope North account. In preparing the target list, we first utilized the MPC s Observational Planning Aid Tool, which allowed me to generate a list of targets. Inputting parameters such as the available sky, the date in which observations would take place, brightness range, and range of uncertainty in a targets position into the tool we were able to generate a start-up list. More targets were obtained by referring to the Space Guard Priority List in which targets are listed in order of priority. After careful consideration of the practicality of each target and after brief meetings with my mentor prior to observations I was able to narrow down the list of targets to less than a dozen choice-targets. The next step was to acquire observational data using the 2-meter Faulkes North Telescope (FTN) atop Haleakala. Operations of the telescope were carried out on my personal 2
computer with internet connection. Operation of the telescope was quite simple in which only three things were required: the expected location of a particular target, the type of filter to be used, and the exposure time. In the case of asteroid tracking, we chose to use the R-filter due to the fact that the camera on the telescope was most sensitive to this particular wavelength. The exposure time used for each target was unique to the objects motion, while the number of exposures taken of a particular object depended on the amount of time needed to detect an object. This procedure was repeated several times for as many targets allowed within the session. Once the session ended the Flexible Image Transport System (FITS) files were downloaded to my computer. The last step was the analysis of the data using Astrometrica, a commercial program that was developed by Herbert Raab of the Joahnnes Kepler University in Austria, which performed astrometric solutions (Raab, 2011). Astrometry in general is the study of the positions of celestial objects. Two methods were used in determining astrometric solutions by Astrometrica: the Data Reduction method and the Stacking method. The Data Reduction method simply extracted the stars and location information from a particular image of a particular target, extracted stars from a star catalogue of our choosing from said-location, and then attempted a pattern match, which resulted in a coordinate system in which to locate the target asteroid. An example of a data reduced image can be seen in Figure 1, where the green circles were stars used in the astrometric solution, also known as reference stars. Figure 1: Data reduction method before (left) and after (right), where green circles indicate reference stars. Pictured: Apophis Figure 2: Progression of stacked images of asteroid 2013 BV15, centered in each frame. From Left: 2- minute exposure, 4-minute stack, and a 6-minute stack The second method was known as the Stacking method. Although essentially the same as the Data Reduction method, the Stacking method added a set of images of a particular target to a reference frame moving with the target asteroid. What resulted was a field with streaked stars and an asteroid with an amplified signal that allowed for greater detectability. An example of the Stacking method can be seen in Figure 2. This method was primarily used for fainter objects and objects that weren t bright enough to be detected in a single exposure. After analysis 3
was completed and at least two astrometric positions of a single target were obtained, the positions were sent to the MPC. Lightcurve of Apophis Photometry is the measurement of the brightness of an object. Brightness s are measured logarithmically in magnitudes, which are unitless. The asteroid Apophis, discovered by R. Tucker, F. Bernardi, and D. J. Tholen in 2004 at Kitt Peak, Arizona, is an Apollo-type asteroid measuring roughly three-football fields in length. Currently Apophis is of particular interest due to the dozen of Earth impact solutions that still exist despite an observational arc spanning nearly 9 years and its theoretical impact energy release equivalent to the energy of eight hydrogen bombs (Baalke, 2013). The Yarkovsky Effect is the remaining uncertainty in Apophis orbit. One component of determining how the Yarkovsky Effect affects Apophis s orbit is the asteroid s rotational period, which can be found through its lightcurve. To obtain a lightcurve one has to obtain photometric information from an asteroid. The program Astrometrica was again utilized to obtain photometry for Apophis. The program used the reference stars obtained from the astrometric solution to create a brightness scale in which to compare the target asteroid, this information was stored and extracted from the chosen star catalogue. Photometric solutions were done simultaneously during the astrometric analysis of an asteroid, where R-magnitudes were reported up to a thousandth of a magnitude. The photometrica data obtained was the observed apparent magnitude of Apophis. We then obtained the expected apparent magnitude from a site similar to MPC known as the Near Earth Object Dynamic Site (NEODyS), took subtracted the expected apparent magnitude from the observed magnitude and obtained a lightcurve point. The error in lightcurve point was simply calculated using the formula: 2.5 ln(10) SNR Once all the photometric data was converted into points, I was able to plot as a function of time in terms of a modified Julian Date: JD 2,456,000. The modified Julian Date used was due to the fact that the observations spanned only four months, so the leading four digits remained the same. The lightcurve generated was then analyzed for instances of periodicity, and then fitted if periodicity existed to find Apophis rotational period. RESULTS & DISCUSSION NEA Tracking A portion of this part of the project was first dedicated to learning the NEA tracking techniques and procedures by focusing on objects with known orbits. These objects were known as numbered objects. A total of 8 numbered objects were used as a control sample to determine whether the techniques and procedures were being carried out correctly. The average residual, or the difference in the observed position from the expected position, was approximately 4 Table 1: Summary of reported targets by orbit type Apollo Aten Amor Misc. FALL 12 11 3 5 4 SPR 13 14 5 4 6 TOTAL 25 8 9 10 Table 2: Summary of reported targets by priority PHA VI NEOCP Other FALL 12 7 0 2 14 SPR 13 8 6 5 10 TOTAL 15 6 7 24
0.12 (arc-seconds), which is well below the overall residual average of numbered objects by other observers of 0.65. This confirmed the methods were being carried out correctly, and thus I was able to move onto higher priority targets for the remainder of the project. After nearly a year of tracking NEAs, a total of 52 targets were reported to the MPC. Of the 52 targets, 6 were VIs, 15 were PHAs, and 7 were NEOCP objects. The first two were given priority in the later half of the project, as can be seen in Figure 3b. Three of the VIs I had gone after and reported were eventually removed from NASA s Sentry Risk Page. If there were no VIs or PHAs that could be done with FTN, NEOCP objects were targeted due to the fact that they were objects that had unusual motions, which suggested they could have been NEAs. However, all of the NEAs gone after this year turned out to either be Main Belt Asteroids, comets, or Mars-crossing asteroids, which, although were interesting, were not the main focus of this project. Going after these objects were not wasteful, however, since we were able to help better determine their orbits through observations and thus remove them from the NEOCP. Focus had shifted during the Spring 2013 semester to higher priority targets such as the VIs, PHAs, and NEOCP objects. This shift was critical in that people who do specialize in this field aim to remove VIs and PHAs due to their threatening nature. The fourth bar in Figure 3b listed as Other were objects that either had high-uncertainties in their orbits or were special targets such as main belt asteroids and comets that were chased after in the early stages of the research project as practice. There was an overall domination of Apollo type objects, which are of special interest due to their Earth-crossing nature, and can be seen in Figure 3a. (a) (b) Figure 3: Target summary by orbit type (a) and priority (b). The blue bars indicate targets done in Fall 2012, while the red bars indicate targets done in Spring 2013. Lightcurve of Apophis For this portion of the project 11 nights of observations were taken of Apophis around its close approach, which occurred on January 9, 2013. This was the most ideal time to obtain a lightcurve due to this rare occurrence when Apophis would be at its brightest, hovering around 15.5 V-apparent-magnitude. However, it was also due to this occurrence that telescope time was very difficult to acquire around the close approach. Hence the earliest night to which observations were taken was on January 21, 2013, where I was able to observe it at around 15.0 R-magnitude (or roughly 15.3 V-magnitude). According to NASA s Jet Propulsion Laboratory Small-Body Database, Apophis had a measured rotation period of approximately 30 hours from data acquired in the 2005 apparition. My observations using FTN spanned well over that 5
rotational period, particularly in the first week of March where I was able to obtain 20 images over the span of 4 nearly consecutive nights. The observed apparent magnitude (Obs.) and the expected apparent magnitude (App.) were both measured in R-magnitudes, and can be seen in Table 2. A total of 55 images were taken spanning an observational arc of nearly three months, and a total of 41 lightcurve points were obtained. Table 2: Photometric data extracted from Apophis around close approach. Observations in bold indicate stacking was required to increase SNR (detectability). Date Time (UTC) Obs. (R) App. (R) Δ δ Date Time (UTC) Obs. (R) App. (R) Δ δ 1/21/13 12:01:35 15.01 15.49-0.48 0.02 3/8/13 7:06:09 17.33 17.75-0.42 0.04 12:02:34 15.03 15.49-0.46 0.02 7:07:25 17.34 17.75-0.41 0.04 12:03:38 15.00 15.49-0.49 0.02 3/18/13 8:05:09 18.59 18.18 0.41 0.06 3/4/13 7:32:10 17.17 17.56-0.39 0.04 8:07:07 18.29 18.18 0.11 0.05 7:33:26 17.07 17.56-0.49 0.03 4/4/13 7:03:06 18.03 18.73-0.70 0.04 7:34:34 17.22 17.56-0.34 0.04 7:04:53 18.14 18.73-0.59 0.04 7:35:57 17.20 17.56-0.36 0.04 7:06:37 18.04 18.73-0.69 0.04 7:37:11 17.16 17.56-0.40 0.04 7:08:25 18.09 18.73-0.64 0.04 3/5/13 6:02:16 17.01 17.61-0.60 0.03 7:10:09 18.11 18.73-0.62 0.04 6:03:33 16.94 17.61-0.67 0.03 4/5/13 7:03:44 18.64 18.76-0.12 0.07 6:04:53 16.94 17.61-0.67 0.03 7:05:22 18.65 18.76-0.11 0.07 6:06:07 16.91 17.61-0.70 0.03 7:06:59 18.57 18.76-0.19 0.07 6:07:22 16.94 17.61-0.67 0.03 7:08:35 18.74 18.76-0.02 0.07 3/7/13 5:32:16 16.80 17.70-0.90 0.03 7:10:10 18.59 18.76-0.18 0.07 5:33:36 16.75 17.70-0.95 0.03 4/12/13 7:03:04 18.49 18.93-0.44 0.06 5:34:54 16.81 17.70-0.90 0.03 7:04:33 18.52 18.93-0.41 0.06 5:36:08 16.81 17.70-0.90 0.03 7:06:09 18.54 18.93-0.39 0.06 5:37:24 16.83 17.70-0.87 0.03 7:07:37 18.46 18.93-0.47 0.05 3/8/13 7:02:09 17.35 17.75-0.40 0.04 7:09:09 18.41 18.93-0.52 0.06 7:03:26 17.36 17.75-0.39 0.04 4/19/13 7:05:06 19.18 19.08 0.10 0.10 7:04:47 17.36 17.75-0.39 0.04 The lightcurve points were plotted against time to reveal potential low peaks and high peaks as can be seen in Figure 4. These peaks may be attributed to portions of Apophis rotational period, but with the given data set results were inconclusive in determining periodicity, if any existed. Along with lack of data points, another thing that was suggested by my mentor was the possibility that Apophis may be tumbling due to its size and its slow rotational period of ~30 hours (Harris, 1993). If Apophis were tumbling, this would mean it was not rotating on a principal axis, and hence would not display simple periodic motion as would be expected, which would prove more Figure 4: Apophis lightcurve from data generated by FTN from January 21, 2013 to April 19, 2013 6
challenging in determining the effect of the Yarkovsky acceleration on Apophis orbit. Collection and processing of more data points around Apophis close approach, careful analysis of each image to ensure artifacts such as faint stars aren t being taken into consideration in the photometric analysis, and proper error analysis may bring about a more improved lightcurve, but due to time constraints only a preliminary lightcurve from data collected using FTN was obtained. The FTN data will, however, may contribute as a small piece to the giant puzzle of generating a useful lightcurve of Apophis, and perhaps may reveal whether it is truly tumbling, which would contribute in determining the Yarkovsky Effect on Apophis orbit. CONCLUSION This semester I focused on tracking NEAs, ultimately focusing on high-priority targets, and acquiring observational data in hopes of aiding in the generation of a lightcurve for Apophis. For the first, and main, portion of the project I had reported up to 52 targets total, of which 6 were VIs, 15 were PHAs, and 7 were NEOCP objects. Of the 6 VIs reported, three were eventually removed from NASA s Sentry Risk Page. For the second portion of the project I was able to obtain 55 images of Apophis during its close approach. From the 55 images I was able to extract 41 lightcurve points using Astrometrica. I was able to generate a preliminary lightcurve with the FTN data, but results in Apophis rotational period proved inconclusive due to the small data set. Analysis, collection, and processing of more data points, a more careful treatment of the images, and proper error analysis may be carried out to generate a lightcurve for Apophis in the coming future. ACKNOWLEDGEMENTS I would also like to acknowledge the Las Cumbres Observatory Global Telescope Network, Minor Planet Center, SpaceGuard, and Herbert Raab of the Johannes Kepler University of Austria for providing the tools used in this project. I would also like to extend my gratitude to the Hawai i Space Grant Consortium for giving me this opportunity to gain a unique experience and insight into the world of research in astronomy, graduate student at the Institute for Astronomy (IfA) Marco Micheli for helping me out throughout this project, and J.D. Armstrong for providing me with what seemed like an infinite amount of telescope time. Finally, I would like to thank Dr. David Tholen of the IfA for going above and beyond as a mentor, and for teaching me about asteroids, astrometry, and, ultimately, about life. 7
REFERENCES "2011 NASA Strategic Plan." 2011 NASA Strategic Plan. National Aeronautics and Space Administration, n.d. Web. 28 Nov. 2012. <http://www.nasa.gov/pdf/516579main_nasa2011strategicplan.pdf>. Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel. "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction." Science 208.4448 (1980): 1095-108. Print Baalke, R. "99942 Apophis (2004 MN4) Impact Risk." 99942 Apophis (2004 MN4) Impact Risk. N.p., n.d. Web. 28 May 2013. <http://neo.jpl.nasa.gov/risk/a99942.html>. Baalke, R. "Near-Earth Object Groups." Near-Earth Object Groups. National Aeronautics and Space Administration, n.d. Web. 27 Nov. 2012. <http://neo.jpl.nasa.gov/neo/groups.html>. Farnocchia, D., S. R. Chesley, P. W. Chodas, M. Micheli, D. J. Tholen, A. Milani, G. T. Elliot, and F. Bernardi. "Yarkovsky-driven Impact Risk Analysis for Asteroid (99942) Apophis." ArXiv.org. N.p., 20 Feb. 2013. Web. 27 May 2013. <http://lanl.arxiv.org/pdf/1301.1607.pdf>. Harris, Alan W. "Tumbling Asteroids." Icarus (1993): 209-11. Academic Press, Inc. Web. 29 May 2013. Lovgren, Stefan. ""Gravity Tractor" Could Deflect Earth-Bound Asteroids." National Geographic. National Geographic Society, 9 Nov. 2005. Web. 14 June 2012. <http://news.nationalgeographic.com/news/2005/11/1109_051109_asteroid_tug.html>. Mainzer, A., T. Grav, J. Masiero, J. Bauer, R.S. McMillan, J. Giorgini, T. Spahr, R.M. Cutri, D.J. Tholen, R. Jedicke, R. Walker, E. Wright, and C.R. Nugent. "Characterizing Subpopulations within the Near Earth Objects with NEOWISE: Preliminary Results." ArXiv.org (2012): 5, 20. Web. 13 June 2012. <http://lanl.arxiv.org/pdf/1205.3568.pdf>. McInnes, C.R. "Deflection of near-earth asteroids by kinetic energy impacts from retrograde orbits." Planetary and Space Science. 52. (2003): 587. Print. Pasztor, Andy. "NASA Budget Priority: Asteroid Defense." The Wall Street Journal. Dow Jones & Company, 10 Apr. 2013. Web. 27 May 2013. <http://online.wsj.com/article/sb10001424127887324010704578415021066954816.html>. Raab, H. "About the Author of Astrometrica." Welcome to the Astrometrica Homepage. N.p., 2011. Web. 27 Nov. 2012. <http://www.astrometrica.at/default.html?/author.html>. Yeomans, D., and R. Baalke. "Near Earth Object Program." Nasa. Nasa, 2012. Web. 14 Jun 2012. <http://neo.jpl.nasa.gov/neo/groups.html>. 8