The best way to estimate the total population of larger size NEAs, and hence the current completion of the survey, is by evaluating the "re-detection ratio" of the survey (fraction of detections that are already-known objects). We have employed this method starting in 2001 (D'Abramo, et al., Icarus 153, 214-217, 2001), and most recently presented updated results at the 2006 Planetary Defense Conference. We have applied a bias-correction to the re-detection ratio to allow for the fact that detection of NEAs is not random, due to some being in orbits that are "harder" to find than others, thus the raw re-detection ratio will always be greater than the actual completion fraction. We use a computer simulation to model the relation between raw re-detection ratio and actual completion, since in the computer model we know the total population and hence the completion at any point in the simulated survey. The relation between model survey completion and re-detection ratio is remarkably stable over a wide range of survey assumptions, leading us to have fair confidence in the derived relation, and hence the real population estimated that way.

Here we have used all NEAs discovered through July, 2010, with the last two years, August 2008 through July 2010 as the "test" interval. Asteroids were binned in half-magnitude intervals of absolute magnitude H, and re-detection ratios computed for each bin, that is, the number of already-known (prior to August, 2008) objects seen in the two year interval divided by the total number detected in that interval. The re-detection ratio so determined was then adjusted to a "completion" estimate, based on the computer model, and from the estimated completion the total population in that size bin is estimated from the already-known population. As a final step, one can add in the new discoveries since the start of the test interval to obtain the current completion rather than completion as of the beginning of the test interval. Our latest results are within error bars of the 2006 population estimates, which is itself a satisfying change from past estimates where the estimated population seemed to be growing almost as fast as the number of discovered objects. We estimate the number of NEAs of diameter >1 km ( H < 17.75) to be 996 +/- 45. The number currently discovered, updated through January 27, 2011, is 821, thus we estimate the current survey completion to D > 1 km to be 82.4% +/- 3.7%. While this completion falls a bit short of the original "Spaceguard Goal" in terms of numbers of objects, it corresponds to having discovered well more than 90% of the impact hazard, since the greatest hazard lies in the largest bodies.

Turning to smaller sizes, the re-detection method works down to a size where the surveys do not re-detect a statistically useful number of objects, currently around H = 21.5 or 22, where about 15 objects were re-detected, leading to ~25% formal uncertainty in the population estimate in those size bins. Beyond, and overlapping, that size range, it is possible to "bootstrap" the population estimate from the computer survey model. For example, the typical survey model predicts that if a survey achieves 25% completion at absolute magnitude H, it should achieve 17% completion at H+0.5, 12% at H+1.0, 7% at H+1.5, and so forth, to 1.0% at H+3.0, and even beyond. This can be done until the model survey (with 100,000 test objects) runs out of "re-detections". Indeed, it can be extended still further using an analytical expression based on "particle in a box" rectilinear motion rather than orbital motion for really-close NEAs, where log(completion) should be proportional to -0.8dH. Using these methods we are able to extend the estimated population down to H = 30.5, or D = 3 m, about the size of 2008 TC3, a size of object expected to enter the Earth's atmosphere about annually. Our population estimate confirms this expected frequency within uncertainties.

Yeomans, Donald; Yeomans, D.K.; Chamberlin, A.B.
JPL/Caltech, UNITED STATES

Long-period comets (LPCs), defined here as those active comets with orbital periods greater than 200 years, are the most difficult objects to mitigate should one be found on an Earth threatening trajectory. The arrival of these objects from the distant Oort cloud cannot be predicted and the impact warning time would be measured in a few months-not years. In general LPCs do not become active, and hence discoverable, until inside the orbit of Jupiter and it takes but nine months for a LPC to travel the distance from Jupiter's orbit to that of the Earth. The mean impact velocity of a LPC is about 51 km/s, three times the 17 km/s value for a typical near-Earth asteroid (NEA) so the impact energy for an LPC would be 9 times that of a NEA of similar mass. However, the bulk density of a comet (~ 0.6 g/cc) is several times less than the density of a stony NEA ((~2.6 g/cc) so for a LPC and a stony NEA of the same size, the LPC’s impact energy would be about twice that of the NEA.

The Earth impact flux for active comets is less than 1% the NEA impact flux. There are a few lines of evidence that support this claim. Sekanina and Yeomans (1984) plotted the number of Earth encounters in history by all comets within a particular separation distance versus the minimum separation distance itself. For a sample time of 700 years (1300-2000), and taking into account the northern hemisphere observing bias, they found an average impact interval of 43 million years. Despite the improvements in telescopes and orbit determination techniques, the Earth approach rate for LPCs remained relatively constant from 1700 to 2000. Hence, they concluded there is a paucity of small active LPCs. The average Earth impact interval for NEAs with absolute magnitudes brighter than 18 is about 0.5 million years so Earth impacts by LPCs are < 0.5 % the rate for NEAs. Using a technique employed by Marsden (1992), we found that until the end of January 2011, there have been 5320 known NEAs of all sizes that came to perihelion to within 1.05 AU. Their average orbital period is (~2.2 years so that about 2400 NEAs arrived at perihelion (and crossed the Earth’s orbit) each year. During the same interval, if we exclude temporary cometary fragments, 28 Jupiter family comets (JFCs) were discovered (average orbital period (~6 years) that had a perihelion distance less than 1.05 AU. Hence (~5 of these JFCs reached perihelion near Earth's orbit each year. The JFC impact flux compared to the NEA impact flux would be roughly 5/2400 or 0.2% with the flux of LPC and Halley-type impactors being far less.

Finally, we note that during the interval from 1900 through Jan. 2011, 2460 known NEAs of all sizes made 3901 Earth close approaches to within 0.05 AU. During the same interval, only three Jupiter family comets (7P/Pons-Winnecke in June 1927, 1991 R1 SOHO in April 1947 and 1999 J6 SOHO in June 1999) and one LPC (1983 H1 IRAS-Araki-Alcock in May 1983) have come as close. Hence, when compared to NEAs, the hazard of all active comets, including JFCs, Halley-type and LPCs, is well below 1%.

Marsden, B.G. (1992). To hit or not to hit. Proceedings, Near-Earth Objects Interception Workshop. (G.H. Canavan, J.C. Solem, J.D.G. Rather, eds.). Los Alamos National Laboratory, Los Alamos, NM, 67-71.

Sekanina, Z. and Yeomans, D.K. (1984). Close encounters and collisions of comets with the Earth. Astronomical Journal, v. 89, 154-161.

Last year the Catalina Sky Survey discovered 602 Near Earth Objects (NEOs), more than 65 percent of all NEOs found in 2010. Catalina's performance can be attributed to its use of three telescopes with largely non-overlapping capabilities, immediate identification and reporting of NEO candidates, and the use of low detection thresholds and visual validation of proposed new objects. Catalina's observing strategy will be discussed, as will plans to upgrade existing equipment. We will also describe our vision for the development of a next-generation survey completely dedicated to NEO search. Consisting of three Small Binocular Telescopes, CSS-II will have greatly enhanced search and characterization capabilities that will be important for both discovery and follow-up studies of Near Earth Objects.

The Pan-STARRS 1 (PS1) telescope, atop Haleakala in Hawaii, is a 1.8-meter telescope equipped with the largest digital camera in the world. Its 1.4 Gigapixel camera has a field of view of approximately 7 square degrees. PS1 has a wide variety of scientific goals, including searching for NEOs. After a slow start, the PS1 telescope is now regularly discovering Near Earth Asteroids. Some of the early problems will be described, along with a modified survey strategy that is much better optimized for discovery of NEOs. The new survey strategy was implemented in March 2011. On January 30, 2011, an entire night of observing with PS1 was devoted to searching for NEOs. A total of 19 confirmed NEO discoveries were made, including 2 PHAs. One of these, 2011 BM45, is presently one of only a handful of objects classified as Torino Scale 1. The present performance will be described, along with prospects for the future, including likely future discovery rates.

The Large Synoptic Survey Telescope (LSST) is a next-generation, ground-based optical survey designed to achieve multiple science goals, including a thorough inventory of the Solar System. The LSST system, with an 8.4m telescope and 3,200 megapixel camera will cover the entire available sky every three or four nights, twice per night to a depth of r=24.5 per visit. Over the proposed survey lifetime of 10 years, each pointing within the 18,000 square degree main survey footprint will be visited about 1000 times, summed over six broad photometric bandpasses. To enhance completeness for Near Earth Objects (NEOs), there will be additional sky coverage to follow the ecliptic into the northern hemisphere. This rapid, deep and wide sky coverage, with extremely well calibrated photometry and astrometry, will result in the largest catalog of Near Earth Objects (NEOs) to date. The catalog will contain well-determined orbits, lightcurves, and colors in ugrizy bandpasses. Detailed modeling of LSST operations, incorporating real historical weather and seeing data from the chosen site (Cerro Pachon, Chile), shows that under the baseline design cadence, LSST could find approximately 75% of the Potentially Hazardous Asteroids (PHAs) with diameters larger than 140m within 10 years. By optimizing about 15% of the total observing time for improved NEO detection, the ongoing simulations suggest that the LSST system could reach 90% completeness for 140m objects within 12 years. Moreover, even under normal baseline operations, LSST will detect much smaller PHAs, as small as 20m at half an AU. The impact warning time for PHAs as small as 45m would be about 1-3 months, depending on their orbits. With its wide sky coverage and the exquisite orbital and photometric information generated for each NEO, LSST will be excellent for identifying potential spacecraft mission targets.

The Gaia mission is an ESA cornestone to be launched in fall 2012. The aim of this mission is to provide, at completion after 5 years, a 3D census of our Galaxy, and besides high precision astrometry and photometry of celestial objects brighter than magnitude Viü20, including a large number of Solar System Objects (SSOs, approx. 300,000, mainly asteroids). The scientific output for solar system sience will consist in physical and dynamical properties for a unprecedented number of bodies: spin properties and shape determination, taxonomy, size, albedo and mass determination, orbits, reference frames and local test of General Relativity. These can be completed in some cases by complementary ground-based observations (e.g. at different time resolution and wavelength). We will present the specificities of the Gaia space-based survey, their general advantages and limitations for astrometry and observing strategy of SSOs. We will focus on the - modest yet specific - potential to discover new NEOs, the huge potential to provide highly accurate astrometry of known objects, and their impact on orbit improvement and orbit propagation.

The NEOSSat microsatellite is a dual use spacecraft funded by Defence Research and Development Canada and the Canadian Space Agency; NEOSSat will acquire metric data of high Earth orbital artificial satellites and will search for near-Earth objects. The microsatellite masses ~75 kg, is 1.0 X 0.8 X 0.4 m, and produces ~35 watts with body mounted photocells. The spacecraft reorients and maintains pointing with reaction wheels and dumps momentum using magnetorquers. The critical enabling technology for NEOSSat is an attitude control system that achieves pointing stability within 0.5 arcsecs in long exposures, a first for a spacecraft of this size. The payload is a 15 cm, f/6 Maksutov telescope with a 1 k X 1 k frame transfer CCD yielding a 0.86° square field of view. A 90 cm-long asymmetric baffle allows the telescope to be pointed within 45° of the Sun. In a 100 second-long exposure the instrument will achieve a V limiting magnitude fainter than 20, but its performance will be slightly impaired near the Sun due to the elevated sky background from the zodiacal light. Launch as a secondary payload is planned during 2011 into an ~800 km-altitude, dawn-dusk, Sun-synchronous orbit. NEOSSat’s search strategy is designed to complement the long-standing, ground-based search efforts by continuously imaging (subject to galactic plane avoidance) the ecliptic region of the sky near 45° solar elongation, effectively creating a curved °optical° fence with an arc several tens of degrees length crossing the ecliptic plane. Imaging cadence will be four images taken over two orbits and ~125 minutes constrained by the spacecraft's orbital period and asteroid proper motion rates. Simulations of discovery rates have been conducted using inner solar system asteroid models; the planned observing strategy has been optimized to discover the largest possible number of objects in Atira and Aten-type orbits, but numerous Apollos are also found. Simulation indicates that extending the search strips to 30-40° ecliptic latitude is required to representatively characterize the intrinsically near-Sun asteroid population. NEOSSat is capable of discovering ~1 new NEO in every two days of operation with half-time use of the spacecraft. The discovery rate would be substantially increased with greater sky coverage; in contrast to ground-based instruments the spacecraft spends approximately half of its on-orbit time slewing from field to field.
Two independently developed asteroid search routines are being modified to cope with the complication of the parallax-induced motion of asteroids as imaged by NEOSSat (Currently all images are planned to be searched by these two independent algorithms.). The spacecraft-induced parallax is useful in constraining initial orbital solutions, and simulation is constraining requirements for follow up astrometric imaging to determine orbits for discovered objects. Follow up astrometry is also planned with cooperating ground-based telescopes.

Introduction: Human exploration of near-Earth objects (NEOs) beginning in 2025 is one of the stated objectives of U.S. National Space Policy. Piloted missions to these bodies would further development of deep space mission systems and technologies, obtain better understanding of the origin and evolution of our Solar System, and support research for asteroid deflection and hazard mitigation strategies. As such, mission concepts have received much interest from the exploration, science, and planetary defense communities. One particular system that has been suggested by all three of these communities is a space-based NEO survey telescope. Such an asset is crucial for enabling affordable human missions to NEOs circa 2025 and learning about the primordial population of objects that could present a hazard to the Earth in the future.

NEO mission targets: Human accessibility of known objects larger than 30 m requires exposing astronauts to long duration missions and entails development of many advanced technologies and capabilities. Even if these capabilities are assumed, consideration of NEOs within the current database available for human exploration constrains program flexibility and increases budgetary risk. However, there is excellent evidence to suggest that the current number of known NEOs is only a small fraction of the total population. This evidence motivates discovery of additional mission targets among the NEO population to aid in the development of a robust and affordable exploration strategy. Improved knowledge of the NEO population is not only scientifically valuable, but is crucial for planetary defense since many of the accessible NEOs discovered would be potentially hazardous objects. Affordable mission scenarios involve NEOs that can be visited with the minimum practicable mass launched to low Earth orbit, short mission durations, and reasonable Earth re-entry speeds. The most suitable targets for human missions are NEOs in Earth-like orbits with long synodic periods, but these mission candidates are often not observable from Earth via ground-based telescopes until the timeframe of their most favorable human mission opportunities, and thus preclude appropriate time for mission development. These objects spend much of their orbital periods in day-time sky viewing geometries that are not conducive to their discovery from ground-based systems. However, this same phasing that places these objects in the daytime sky and makes them difficult to observe, also enables round-trip missions to these targets with minimal propulsion and duration requirements.

NEO Survey Telescope: A space-based NEO survey telescope optimized for human mission NEO target selection can efficiently find these targets in a timely, affordable manner. Such a system could be ready to launch within four to five years of project commitment and find most affordable NEO targets within two years of launch. A space-based telescope is not only able to discover new objects, but also follow up (track) and characterize objects of interest for human space flight consideration. Four separate system options have been analyzed with costs less than $500 million and capabilities to find several tens of highly accessible targets suitable for human missions within the 2025-2030 timeframe.

Conclusion: Deploying a space-based NEO survey telescope is a logical and prudent first step for enabling human exploration, understanding our Solar System, and developing planetary defense strategies.