Surveys, Astrometric Follow-up & Population Statistics Robert Jedicke University of Hawaii Mikael Granvik University of Helsinki Marco Micheli ESA NEO Coordination Centre Eileen Ryan Magdalena Ridge Observatory Timothy Spahr Minor Planet Center Donald K. Yeomans Jet Propulsion Laboratory Asteroid surveys are the backbone of asteroid science, and with this in mind we begin with a broad review of the impact of asteroid surveys on our field. We then provide a brief history of asteroid discoveries so as to place contemporary and future surveys in perspective. Surveys in the United States have discovered the vast majority of the asteroids and this dominance has been consolidated since the publication of Asteroids III. Our descriptions of the asteroid surveys that have been operational since that time are focussed upon those that have contributed the vast majority of asteroid observations and discoveries. We also provide some insight into upcoming next-generation surveys that are sure to alter our understanding of the small bodies in the inner solar system and provide evidence to untangle their complicated dynamical and physical histories. The Minor Planet Center, the nerve center of the asteroid discovery effort, has improved its operations significantly in the past decade so that it can manage the increasing discovery rate, and ensure that it is well-placed to handle the data rates expected in the next decade. We also consider the difficulties associated with astrometric follow-up of newly identified objects. It seems clear that both of these efforts must operate in new modes in order to keep pace with expected discovery rates of next-generation ground- and space-based surveys. 1. INTRODUCTION system. This chapter provides a historical perspective on as- teroid surveys, then focuses on their current capabilities and Without asteroid surveys there would be no asteroid sci- discoveries since Asteroids III, discusses the importance of ence. The cumulative efforts of over 200 years of aster- targeted astrometric follow-up for critical objects, and con- oid surveying has resulted in the discovery of over half a cludes with a speculative forecast on how the next decade million asteroids in the inner solar system that range from of asteroid surveying will unfold. just a tenth of an astronomical unit from the Sun to beyond arXiv:1503.04272v1 [astro-ph.EP] 14 Mar 2015 The benefits of a database containing a large number of Jupiter’s orbit. The surveys have identified asteroids that asteroid orbit elements and basic physical properties have are the targets of spacecraft missions, that are the remnants mostly been achieved over the past couple decades as a re- of larger asteroids that were catastrophically disrupted long sult of the NASA-funded near-Earth object (NEO) surveys. ago, and that allow us to untangle the complicated processes They attempt to optimize their surveying for the discov- that formed our solar system billions of years ago. The sur- ery of unknown NEOs but, in the process, discover and re- veys’ capabilities have improved over the decades as they cover known asteroids throughout the solar system. While took advantage of every new available technology to push it seems obvious that ‘more is better’ it is not necessarily their performance in area coverage, limiting magnitude, and straightforward to justify the argument for different aster- data rates. Their efforts have enabled our community to oid populations — how many asteroids are necessary for advance our understanding of the past, current and future science? Is a complete survey required or is a statistical interactions of both the small and large objects in our solar sample sufficient? 1 The first decade of intensive asteroid surveying at the ume) to constrain the dynamical and collisional evolution of end of the last century was motivated by NASA’s goal of the main belt since its formation (e.g. O’Brien and Green- detecting NEOs that are larger than 1 km in diameter. The berg, 2005; Morbidelli et al., this volume; Bottke et al., this impact on Earth of one of those objects is expected to have volume). global consequences and the set of about 1,000 NEOs of The surveys’ capabilities are now so synoptic that most that size or larger was thought to incorporate 90% of the slow moving objects brighter than about V = 21 are rou- impact risk (e.g. Morrison, 1992; Harris, 2008). Thus, tinely detected multiple times in a lunation, and will be the surveys had a relatively well-defined goal that was mo- re-detected regularly in the future, so that they require no tivated by planetary defense rather than science: identify specific allocation of resources for targeted follow-up ob- most of the largest hazardous asteroids that could threaten servations. Indeed 80% of the known main belt asteroids Earth. Progress towards achieving that goal could be mea- with H < 16:5 are now numbered, meaning that their or- sured against the derived population size or by the redis- bits are good enough to predict their future ephemerides to covery rate (e.g. Jedicke et al., 2003; Harris, 2007). It is with 200 for at least the next decade. On the other hand, possible that there remain undiscovered large objects on an rapidly moving, nearby NEOs that may pose an Earth im- impact trajectory with Earth, but the probability is small. pact hazard in the future usually require rapid follow-up to The last decade of asteroid discovery, roughly since As- measure their astrometric positions (Ticha et al., 2002) over teroids III, has mostly retired the risk of an unanticipated as long an arc as possible during their discovery opposi- impact of a globally devastating asteroid (e.g. Harris, 2008; tion to 1) accurately assess their Earth impact hazard (e.g. Mainzer et al., 2011b). Having achieved that goal, the sur- Milani et al., 2005; Harris et al., this volume) and 2) in- veys are now focussing upon detecting even smaller aster- crease the probability that detections of the object in future oids with the goal of discovering > 90% of the potentially apparitions can be associated with the discovery apparition hazardous objects (PHOs) larger than 140 m diameter be- observations (e.g. Milani et al., 2012; Farnocchia et al., this cause they 1) represent the lower limit of those that can volume). cause serious regional ground destruction and 2) contribute We think that continued asteroid survey efforts in the roughly 90% of the residual hazard to the Earth from un- next decades are justified in order to expand upon the rich known impactors. The discovery rate for objects of less asteroid science yield of the past decades, to provide excit- than 140 m diameter is currently about 400 per year so that ing new discoveries that will generate unexpected insights Asteroids VII could be published before the goal is reached into our solar system’s formation and continuing evolution, unless new technologies are brought to bear in the coming and to further reduce the NEO impact hazard risk. The re- decades. mainder of this chapter provides an introduction to the his- Thus, for PHOs, there are well-defined and clearly- tory of asteroid surveys before Asteroids III, the continuing motivated practical goals for discovering a specific number survey improvements and their current status as of publica- of objects but the situation is not so clear for other aster- tion of Asteroids IV, the importance and state-of-the-art of oid populations in the inner solar system. How many main follow-up efforts, and our perspective on upcoming surveys belt asteroids are necessary? To what absolute magnitude and technologies as we look forward to Asteroids V. should we strive to be complete for Jupiter’s Trojan aster- oids? Without the NEO surveys driving the discovery of as- 2. A BRIEF HISTORY OF ASTEROID DISCOVERY teroids to fainter apparent magnitudes and smaller sizes we (PRIOR TO ASTEROIDS III) would not have discovered, for example, extremely young asteroid families that can be traced back in time to their col- The first asteroid to be discovered, (1) Ceres, was vi- lision origin (e.g. Nesvorny´ et al., 2006; Nesvorny´ et al., this sually identified by Father Giuseppe Piazzi, director of the volume), widely-separated asteroid pairs on nearly identi- Palermo Observatory, on the morning of 1801 January 1, cal orbits that point to YORP spin-up and tidal disruptions the first day of the nineteenth century. Piazzi was iden- of large asteroids (e.g. Vokrouhlicky´ and Nesvorny´, 2008; tifying and correcting the positions of stars in an existing Walsh et al., this volume; Vokrouhlicky´ et al., this vol- catalog when he noticed that one ‘star’ in Taurus shifted ume), main belt comets that provide evidence of a wa- position from night to night. He followed the object un- ter reservoir that may have allowed life to thrive on Earth til 1801 February 11 but his discovery was not announced (e.g. Hsieh and Jewitt, 2006; Jewitt, 2012; Jewitt et al., until the summer of that year. It was recovered on 1801 this volume), and contemporary catastrophic disruptions of December 7 only after Karl Gauss, with characteristic ge- main belt asteroids that suggest that YORP driven rota- nius, provided an elliptic orbit for (1) Ceres that allowed tional spin-up might be the dominant cause of their breakup an accurate-enough ephemeris prediction. Three more as- (e.g. Jewitt et al., 2010; Denneau et al., 2015). In addi- teroids were discovered shortly thereafter: Heinrich Olbers tion, we would not have been able to measure the ages of identified the second and fourth asteroids, (2) Pallas and main belt asteroid families through fitting the characteris- (4) Vesta, in 1802 and 1807 respectively while Karl Hard- tic Yarkovsky/YORP-induced ‘V’ shape of the families’ ab- ing discovered (3) Juno in 1804.