Charlie Beeson1,2

Supervisors: Jose Luis Galache3, Martin Elvis2

1University of Southampton, Southampton, SO17 1BJ, UK
2Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA
3IAU Minor Planet Center (MPC), Smithsonian Astrophysical Observatory

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Abstract

An estimated 20,500±3,000 Near Earth Asteroids (NEAs) larger than 100m exist, yet only less than 30% have been discovered and fewer than 5% have well-characterized spectra. At current rates, it will take ~15 years to discover the rest, more than 18 years to characterize orbits, and ~190 years to complete spectroscopy. NEAs are possible hazards through collision with Earth, present an opportunity for obtaining valuable resources, and have been identified as prime targets for human exploration. These goals require that we identify the rest of the population and characterize their orbits, surface compositions, shapes and rotation modes. We have identified the main constraints of observing NEAs with ground-based telescopes. We discuss the best strategies to more efficiently find the undiscovered objects and achieve character parameterization on a larger scale.

1. Introduction

Millions of asteroids orbit the Sun in the main belt between Mars and Jupiter (Petit et al. 2002).  Some have scattered within Mars’ orbit through entering orbital resonances with Mars or Jupiter, and so are defined as NEAs (Greenstreet & Gladman 2012). They range in diameter, D, from dust grains to the largest known NEA, Ganymed, with D=31.7km, (MPC data) following a power law slope with regards to the number of NEAs by size distribution. The recent unexpected impact in Chalyabinsk, Russia, as well as the flyby of 2012 DA14 has brought public attention to an area of astrophysics that some scientists have been trying to highlight for decades.

This preliminary report aims to show the progress so far in creating an improved strategy for finding and characterizing a larger proportion of the NEA population as quickly as possible. I will first highlight four key motivations for improved discovery rates along with the importance of spectroscopy as a means of composition characterization. Key contributors towards NEA discovery will then be discussed, followed by a brief look at programs planned for the future. Detailed aims are then stated followed by an explanation of the computational methods used. Graphical and numerical results are then presented followed by a discussion of findings. I conclude with a summary of the preliminary report and a discussion of future work that will complete the project.

1.1 Why Study NEAs?

Motivations for studying NEAs are numerous: NEAs pose a threat to Earth through collision events, they are plausible targets for human rendezvous missions, they present an opportunity for mining valuable resources and they are also of interest to Space Scientists. For each of these motivations, I will highlight why not only increased discovery rates, but also increased spectroscopic characterization rates, are crucial. Without spectroscopic characterization, we have no knowledge of an NEA’s composition.

1.1.1 Collision Threat

The threat of impact is real and requires further study and funding. Approximately every fortnight, a known NEA passes closer to Earth than the distance to the Moon (Barbee et al. 2010), and yet we have only discovered <30% of the predicted population that are larger than 100m in diameter.

The impact in Chelyabinsk and the flyby of 2012 DA14 have brought public attention to the field; however, other impacts of note include the Tunguska Event in Siberia, in 1908. With a power 1000x that of the Hiroshima bomb, it devastated an area of forest the size of Washington D.C.. It is thought the NEA causing this event was of merely 30-40m in diameter, highlighting the need to expand our surveys to better find and characterize smaller rocks. The Rio Curaça Event in Brazil in 1930 depopulated hundreds of square miles of rainforest. The Planetary and Space Science Center (PASSC) currently has 184 confirmed NEA-Earth impact craters on file at time of writing, a number that is continually rising. These sites can be seen in Fig. 3 where it is clear how selection effects have meant only a tiny proportion of the sites that exist have been found. This is due to difficulties finding impact sites that are covered by ocean (2/3 of the planet), in forest area (covering half of the remaining land area), obscured by weathering or vegetation, and due to the specialist knowledge required to distinguish an impact site from other naturally occurring land features. The covering of craters on the Moon’s surface is much more visible, and so better shows the density of impacts one would expect in our region of the Solar System (Ito & Malhotra 20).

 

Fig. 1: Crater Site Map. There are 184 confirmed asteroid impact sites as of March 2013. Image courtesy of the PASSC.
Fig. 1: Crater Site Map. There are 184 confirmed asteroid impact sites as of March 2013. Image courtesy of the PASSC.

 

Of the 6013 (as of 22 March 2013) known NEAs larger than 100m, roughly a quarter are large enough and travel close enough to Earth to be classified as Potentially Hazardous Asteroids (PHAs). PHAs are asteroids with H<22 (roughly D>140m) and an Earth MOID <0.05AU, where MOID is the minimum distance between the orbits of the earth and the asteroid (Rivkin 2009). PHAs would cause damage on a regional scale if they collided with Earth, and so they are given particular attention to ensure their orbits are known to a high accuracy.

Alan Harris published a report in 2008 named `What Spaceguard Did’1 which highlights the specific risks posed by NEAs of different sizes. He calculated the intrinsic risk of death posed by all NEAs of any size to be 1 in 720,000. This is slightly less than the risk of death due to a firework accident2. He highlights how discovering and characterizing the population dramatically reduces this intrinsic risk, and unlike the risk of death due to a firework accident, the chance of just one highly catastrophic, but unlikely, collision event dramatically pushes this risk level up. The relatively short lifespan of human beings makes the risk of an asteroid impact seem smaller than it is.

The danger a NEA poses is very roughly quantified using the Torino (Binzel 2000) and Palermo scales (Chesley et al. 2002) mainly as a means of conveying the level of threat to the public. A value from 0 to 10 is calculated by combining collision likelihood and kinetic energy, as per Fig. 2.

 

Fig. 2: Torino Impact Scale. Altered version of an image courtesy of Robert W. O’Connell, University of Virginia
Fig. 2: Torino Impact Scale. Altered version of an image courtesy of Robert W. O’Connell, University of Virginia

 

Kinetic energy cannot be calculated without knowledge of composition via spectroscopy; the damage caused by an NEA will greatly depend on whether it is solid nickel-iron or a loosely-gravitated `rubble pile’ of volatiles and gravel, as this affects to what degree the NEA is broken up and scattered by the Earth’s atmosphere. The size of the NEA in question must also be known, which is calculated by combining knowledge of: apparent magnitude V (brightness in the sky), distance from the observer, and estimated albedo, p (reflectivity coefficient). An NEA’s apparent brightness and its distance from the observer is used to calculate its absolute (intrinsic) magnitude, H. H is defined in the Johnson/Cousin V band magnitude centered on a wavelength of 0.55 microns. H represents the theoretical magnitude one would observe if the NEA were located 1AU from the Sun and 1AU from the observer when viewed at full phase (fully illuminated). More simply, the H magnitude is used as a direct measure of an NEA’s diameter, where a larger H represents a smaller D, and vice versa3. A conversion table from H to D values can be found on the MPC website4:

eq1

The accuracy of size estimates also depends on the accuracy of the albedo, which is expected to vary from 0.05-0.25 for NEAs. This variation creates an inherent uncertainty in an NEA’s size by a factor of √5 for optical measurements. A much more accurate size estimate can be calculated using thermal infrared (IR) observations, however this can only be done from space-based telescopes because the Earth’s Atmosphere absorbs a fair amount of the IR light.

Focus has been placed on finding NEAs that could cause damage in a collision event. How an imminent NEA collision event would be avoided is even less understood:

‘In truth, NASA doesn’t really want the job of global savior, and no one else does, either’

Tad Friend5

However, three of the most popular suggestions include: a `gravity tractor’ that would hover near the approaching NEO, using its own gravity to nudge the NEO into a slightly different orbit; `kinetic impactors’ loaded with lead or copper to aim at the target and deflect its trajectory, and nuclear weapons6. One fact that rings true is that the more warning we have, the better chance that we can do anything about it. To move an NEA out of its current orbit would require far less energy, power and money if these processes were applied far in advance of the predicted collision event. This way, a smaller `nudge’ will cause the NEA to slowly but surely spiral out of our way.

1.1.2 Space Science

Space scientists find interest in asteroids because many have a largely unchanged composition since the Solar System formed. They act as `fossils’ of the early Solar System. Knowledge of their composition, and therefore NEA spectroscopy, is crucial to this field of study.

It is thought that Earth, and the other planets, formed through repeated asteroid collision events. Particularly large asteroids gravitationally attracted smaller asteroids until they gained enough mass to become planets (Barbee et al. 2010). A collision with a large Mars-sized rock created the mass that broke loose and became the Moon7. It is these impacts that are considered to have brought us the water, carbon, nitrogen and amino acids crucial for creating life8. The Chicxulub impact crater in Mexico was identified as a source of iridium (a PGM), providing strong evidence that ongoing NEA impacts have been an important factor in the evolution of life on Earth (Larson 2007).

To this cause, two NEAs and one Main-Belt Asteroid have been visited by unmanned spacecraft missions resulting in a returned sample of one milligram of surface material. OSIRIS-REx9 is a planned mission to NEA 1999 RQ36. It will be launched in 2015 and return in 2023 with 60 grams of surface material. Similar study of a Near-Earth Comet (NEC), Wild 2, resulted in a completely revised view of nebular dynamics (Nuth 1999).

1.1.3 Human Rendezvouz

A human mission to an NEA has been suggested as a means of gaining experience in interplanetary travel, specifically in preparation for a larger scale mission to Mars10. The US Augustine Committee in 2009 named NEAs as prime targets for human exploration and resource utilization (Abell et al. 2012) due to the fact that some are easier to reach than the Moon (Elvis et al. 2011).

Delta-v(Δv) is the change in velocity required to transfer between two orbits. It is the measure of the energy required to reach a celestial body from low Earth Orbit. A lower Δv represents a lower energy, and therefore cheaper, target. NEAs can be listed by minimum total Δv of a round-trip rendezvous using Hohman transfer orbits as a means of ranking their accessibility (Shoemaker & Helin 1978).

Rocket maneuver in the close vicinity of an NEA presents a new challenge for rocket design due to the relatively weak and irregular gravitational field surrounding NEAs; in 2003 a Japanese, unmanned mission to asteroid Itokawa inexplicably lost power and veered off into space. Without knowledge of an NEA’s composition, through spectroscopy, the NEA’s gravitational field, in general, cannot be well understood.

1.1.4 Mining

NEAs are considered to be potential sources of both water and valuable precious metals, in particular platinum group metals (PGMs) (Kargel 1994). Two private companies named Planetary Resources11 and Deep Space Industries12 aim to mine valuable resources from NEAs in the near future. By 2020, Planetary Resources plan to mine water from asteroids for use as a fuel depot in space for long missions, such as a mission to Mars (BBC news13).

The bottleneck in prospecting asteroid resources may be due to the low NEA spectroscopic characterization rates, (~100/year) (Elvis et al. 2013) because only a small proportion of NEAs may be well suited for this purpose, and without confirmation of the presence of valuable resources, a mission with the purpose of mining NEAs can not go ahead. For purposes of mining, a more detailed knowledge of the bulk composition will be needed.

NEAs are broken into subsets depending on orbit type: known as Apollos, Amors, Atens and Atiras14, as per Fig. 3. Atiras are often grouped together with Atens. Apollos are the most common orbit type of the discovered NEAs, followed by Amors (MPC data, 2013).

 

Fig. 3: NEA Orbit Categories, where a is the semi-major axis.
Fig. 3: NEA Orbit Categories, where a is the semi-major axis.

 

All NEAs must lie within Mars’ orbit, i.e. with a perihelion (distance of closest approach to the Sun) of q<1.3AU. Asteroids beyond this are not considered ‘near-earth’:

  • Amors lie the furthest from the Sun, with a perihelion between Mars and Earth orbit (1.017<q<1.3AU and a>1AU).
  • Apollos have a perihelion that causes them to cross Earth’s orbit (q<1.017 and a>1AU).
  • Atens also cross Earth’s orbit, however it is their aphelion, Q (furthest distance from the Sun) causing them to do so (Q>0.983AU and a<1AU).
  • Atiras are completely contained within Earth’s orbit (Q<0.983 and a<1AU).

While sub-divisions are useful, they hold no other specific scientific implications. For mining and rendezvous purposes, some orbit groups can be considered more favorable. NEAs with orbits that bring them close to Earth require a lower Δv to reach. For this reason Atens and Apollos may be favorable targets. However, those spending most of their time closer to the Sun than the Earth (i.e. Atira’s and most Atens) are mostly in the Sun-side of the sky and therefore prove very difficult to find with Earth-based telescopes that can only observe at night.

1.2 History of Discovery

There have been nine main NEA discovery programs (Fig. 4) invigorated by two key US Congressional mandates.

 

Fig. 4: All NEAs. Created using data from the MPC.
Fig. 4: All NEAs. Created using data from the MPC.

 

Spacewatch was set up in 1984, and was the first organized NEA discovery program. It sparked an interest and influenced the first congressional mandate concerning NEAs. In 2002, its contribution increased due to an upgraded Mosaic camera; however, now it mostly contributes through the follow-up of fainter NEAs that have already been discovered, to better characterize their orbits15.

In 1998, US Congress called for 90% of all Near-Earth Objects, NEOs (NEAs plus a much smaller population of Near-Earth Comets) with D>1km (H<17.5) to be discovered by 2008. NEAs of this size would cause billions of deaths worldwide in an Earth-Asteroid collision event (Toon et al. 1997). US, EU, Japanese and other authorities cooperatively organized an effort to identify, track and study NEOs, resulting in the creation of the non-profit `Spaceguard Foundation’. The effect of the `Spaceguard’ mandate and the George E. Brown, Jr. Near-Earth Object Survey (the response to the mandate following Spaceguard) can be clearly seen in (Fig. 5). The Lincoln Near-Earth Asteroid Research program (LINEAR) was set up in response to `Spaceguard’ and dramatically increased the number of NEAs, particularly those with D>100km, discovered each year.

 

Fig. 5: NEAs with D>140m (H≤22) are plotted. An explanation of H to D conversion is in section 1.1.1 and a conversion chart is available on the MPC website. Created using data from the Minor Planet Center (MPC).
Fig. 5: NEAs with D>140m (H≤22) are plotted. An explanation of H to D conversion is in section 1.1.1 and a conversion chart is available on the MPC website. Created using data from the Minor Planet Center (MPC).

 

LINEAR is more proficient at finding these larger NEAs than it is at finding smaller, less bright asteroids. For this reason, as the D$>$1km population neared completeness, the contribution from LINEAR reduced. Despite this, LINEAR remains the largest overall contributor to NEO discovery, possibly due to the fact that it is the only survey that regularly searches in the galactic plane and at high north ecliptic latitudes16. The `Spaceguard’ goal was achieved on time, resulting in Congress averting its attention towards smaller NEAs.

In 2005, NASA was called to detect 90% of NEOs with D>140m by 2020, due to the fact that these could cause regional scale damage in a collision event  (National Research Council 2010).  This aim has been acted upon by the George E. Brown, Jr. Near-Earth Object Survey. The largest contributors to this population of NEAs have been the Catalina Sky Survey, since being upgraded with thinned, more sensitive CCD’s in 2004, and even more so, the Mt. Lemmon Survey, both of which are based at the University of Arizona. Despite significant efforts, a report by the National Research Council in 2010 found that this current D>140m goal cannot be met in time by current NEO surveys. This report hopes to highlight the main reasons as to why this goal cannot currently be met and how we can alter or improve our observational plans to meet this goal as soon as possible.

Over 900 NEAs are now discovered each year, 400 of which have D>100m. However, with a predicted population of 20,500±3,000 (D>100M) (Mainzer et al. 2011), it will take ~15 years at these rates  before they have all been found and >180 years to characterize their spectra. Data from the Minor Planet Center (MPC) has been examined with the aim of designing a better, more practical observation and follow-up strategy.

1.3 Future Programmes

The Asteroid Terrestrial-impact Last Alert System (ATLAS), going live in 2015, will consist of a global array of small telescopes able to give one month’s warning prior to an impact from a 300m NEA and one week’s warning of a 50m impact (Elvis et al. 2013).

The Large Synoptic Survey Telescope (LSST) will scan more deeply than ATLAS, but will not see first light until 2021.

Future NEA discovery programmes are also going to include space-based projects, which offer many benefits. Ground based surveys currently effectively observe for only 210 nights of the year (Elvis et al. 2013). This is because of bad weather, telescope maintenance and failures, and effects of the Moon and the Milky Way. Depending on the lunar phase, the moon outshines NEAs in varying amounts of sky area. NEAs crossing the bright and densely populated Milky Way can also be hard to find. As the visibility of the Milky Way varies with the seasonal tilt of the Earth on its axis, a seasonal variation in the rate of NEA discovery is observed. Observations from space will avoid effects of bad weather and seasonal tilt, and crucially, can observe at wavelengths that are absorbed by the Earth’s atmosphere, such as infrared (IR).

The Sentinel mission, as a part of the B612 Foundation, will be placed in a Venus-like orbit. From here, the effects of the Moon will also be irrelevant, as well as avoiding other NEA discovery selection effects that will be described in Section 3.

NEOCam, as part of the NEOWISE discovery program, will be placed at the Sun-Earth L1 Lagrange point17 (roughly 106km Sunward of Earth) (Elvis et al. 2013). This is a cheaper orbit to reach than that of the B612 mission, however from here, effects of the Moon will still be significant.

We can also look forward to a tripled field of view for the Catalina Sky Survey in 2013, and an improvement to Pan-STARRS 1 named Pan-STARRS 4 (PS4) of twice the sensitivity, to see light in 2015 (National Research Council 2010).

2. Method and Aims

2.1 Aims

Having highlighted the importance of NEA discovery and spectroscopy, I aim to highlight some recommended changes and improvements to current and proposed NEA observational plans. Analysis of the known population in comparison to the predicted population will be shown as well as analysis of just the known population with discussion of how observational biases may affect discoveries.

Less than 30% (MPC data and (Mainzer et al. 2011)) of the predicted population of NEAs larger than 100m have been discovered and less than 5% have well-characterized spectra. To assess why these percentages are so low (are NEAs too dim, or too fast moving for our telescopes to detect? etc.) would require analysis of the entire population (discovered and undiscovered). Instead, we use a dataset mimicking the entire population by projecting data of known NEAs for the upcoming two years. In two years time, the current distribution of the known asteroids‚ proper motion, for example, will be more spread out, as will the distributions of apparent magnitude, and not just confined to the range that we can observe. Therefore, due to the short periods of NEAs, we can mimic the whole population.

2.2 Computational Methods

The MPC provides an observation tool18 to obtain ephemerides (positional coordinates) and other orbital elements for the upcoming two years for known NEAs. The MPC also keeps an updated list of known NEAs along with their orbit category, whether they are a PHA, and for how many oppositions they have been observed (observations taken opposite the Sun from the Earth).

Mechanize19 and BeautifulSoup420 modules were used with Python2.4.3 to extract these values for NEAs that may be observable in 2013. We discarded NEAs at an unobservable solar elongation (<60˚), too fast a sky motion (>9”/min), too dim a V-band magnitude (V), or with more than one variant orbit solution. Individual files were created for the remaining NEAs by combining all data on each object into one file per object, including ephemerides for three times per night for the upcoming two years.

From these files, plots of a number of parameters of the observable population were created to enable an observation plan to be formulated. We investigated sky motion, the effect of moon phase, the brightest magnitude achieved and how long each NEA remains close to this magnitude, or brighter than a cut-off magnitude suitable to the instrumentation available. Values for semi-major axis, a, inclination (angle to the ecliptic),i, and eccentricity, e, were also investigated.

The format of data on the MPC website can be hard to parse with a computer. This heterogeneity of format, combined with the numerous changes in NEA naming designations that have taken place, caused the project to present computational difficulties. One NEA may be listed by up to three different designations. The Python script was written to recognize all three, and combine the data under any name to one file.

3. Preliminary Results

3.1 Orbital Parameters

Figures 6 and 7 show the values of a, e and i plotted by orbit type of the known population. As expected, the MPC data for a correlates with the categorization into orbit type (Atens with a<1 and Apollos with a nearer to 1 than Amors). Atens would make the best targets for exploration and mining due to their proximity to Earth (a~1) but, unfortunately, they are the least populated category.

Both more circular orbits (e~0) and low i values require a lower Δv for a rendezvous mission, and so these targets are more preferable for exploration (Shoemaker & Helin 1978) (Elvis et al. 2011).

 

Fig. 6: (Left) Semi-major axis and (Right) Eccentricity by orbit type (both MPC data)
Fig. 6: (Left) Semi-major axis and (Right) Eccentricity by orbit type (both MPC data)

 

Fig. 7: Inclination by orbit type (MPC data)
Fig. 7: Inclination by orbit type (MPC data)

 

3.2 Brightness

V magnitude at discovery (Fig.8-Right) shows an expected bias towards brighter NEAs.

Those at a brighter (lower) V show the expected bell shape curve, but the number of NEAs at dimmer magnitudes drops off uncharacteristically due to the limits of brightness to which telescopes can achieve. Without telescope limitations, we would expect the curve shown by the brighter NEAs to continue on past V=19 up to billions of numbers of tiny, very dim NEAs.

 

Fig. 8: (Left) H magnitude of observable NEAs in 2013, (Right) V magnitude when discovered - clear bias towards brighter NEAs
Fig. 8: (Left) H magnitude of observable NEAs in 2013, (Right) V magnitude when discovered – clear bias towards brighter NEAs

 

As for spectroscopy, it was found ~7\% of NEAs remain within half a magnitude of their brightest V for more than the 30 days required for planning spectroscopy (Fig.9-Left) and that mostly, we discover NEAs close to their brightest point (Fig.9-Right). The number of days for which an NEA remains brighter than specific V values (e.g. V<20 for Magellan21 and V<17.5 for IRTF/SpeX22) also showed promising results, with many NEAs remaining visible for almost the entire year (Fig.10).

 

Fig. 9: (Left) How long NEAs stay bright , (Right) Difference in brightest V magnitude to discovery V magnitude.
Fig. 9: (Left) How long NEAs stay bright , (Right) Difference in brightest V magnitude to discovery V magnitude.

 

Fig. 10: How many days NEAs are visible for specific telescope limitations.
Fig. 10: How many days NEAs are visible for specific telescope limitations.

 

3.3 Sky Motion

Finally, the speed at which observable NEAs will be traversing the sky when at the brightest magnitude in their orbit was also plotted (Fig.11).

The Magellan telescope can track up to a speed of 9″min-1 (a typical value for the telescopes in questions) so, promisingly, only ~5% of NEAs will be missed due to motion.

 

Fig. 11: Sky Motion of ‘observable’ NEAs when at their brightest point throughout 2013. ‘Observable’ meaning they are at a visible position in the sky (elongation > 60 degrees).
Fig. 11: Sky Motion of ‘observable’ NEAs when at their brightest point throughout 2013. ‘Observable’ meaning they are at a visible position in the sky (elongation > 60 degrees).

 

3.4 Discussion

To increase the rate of discovery, more time or sky area covered by ground-based optical telescopes will be needed. The Catalina Sky Survey plans to upgrade its sky detection area by a factor of three, which will improve results (Larson et al. 2001). PTF-2 surveys an even larger area per exposure, however it requires optimizing for NEA use.

The B612 Foundation is planning its ‘Sentinel Mission’23 dedicated to providing vital information about PHAs through placing an Infrared telescope in a similar orbit to Venus’ around the Sun by 2018. This would avoid ground-based observational problems such as daytime, latitude, moon time and atmospheric affects. From this orbit it would traverse the sky faster, mimicking Venus’ rapid orbit to create a map of the inner solar system.  Sentinel is expected to find all D>100m NEAs in a 5.5 year mission, however it is privately funded and so relies on philanthropic donations.

The low spectroscopy rate is harder to overcome. B612 will not carry a spectrograph. Without faster tracking capabilities or a 10m ground-based spectrograph dedicated to NEA characterization, we cannot expect to see an improvement. However, we can decrease the number of NEAs that require spectroscopic follow-up, hence reducing the size of the task and the time in which it can be completed. The taking of `colors’, using distinct wavelength filters can be used as a cruder and simpler means of ascertaining an NEA’s composition on a very basic level. This could be enough, however, to down-size the population that requires a thorough investigation of composition with spectroscopy. The taking of `colors’ could be completed with much smaller telescopes through the coordination of a network of amateur astronomers. This way only NEAs, shown through the taking of `colors’, to have an iron-like composition need to be followed up for many of our purposes.

Emphasis should also be placed on finding NEAs smaller than 100m. NEAs as small as 20m (which are much more common, but harder to find) could devastate cities in a collision event. A 20m NEA will be 4 orders of magnitude fainter than a 140m NEA, which is, at present, impractical to find.

4. Summary

Only ~30% of the predicted population of NEAs larger than 100m have been discovered (MPC data and Mainzer et al. 2011). At current rates it will take 15 years to discover the rest, 18 years to characterize orbits, and $\sim$190 years to complete spectroscopy. Ways in which we can improve the characterization rates were discussed. NEAs that have not been discovered include those that are only visible during daytime (mostly Atiras and Atens), and/or those that have long, slow orbits that do not pass through the areas of sky that we have been searching. A Python script was developed to analyze the data of known NEAs in the MPC database and to make use of the MPC’s capability to predict future ephemerides as a way of mimicking the entire (discovered and undiscovered) population. Finding undiscovered NEAs will require more time on ground-based optical telescopes, searching in a larger area of sky or observing from a different orbit, e.g. B612 Foundation. We must also focus on finding NEAs smaller than D=100m. Increasing rates of spectroscopy will require faster tracking capabilities and preferably a 10m spectrograph for NEAs alone.

5. Future Work

The `NEA\_makefile’ script will be adapted for use on the MPC website as an observation planning aid.

Values of V magnitude for the past ten years for all known NEAs will be further assessed to find if NEAs are generally discovered before or after the brightest point in their orbit.

Delta-v will be calculated and used for ranking NEAs by accessibility using code in C from Shoemaker Helin, 1978 and then compared to values calculated using models from Barbee et al., 2010. Delta-v for each orbit type will also be compared to assess if Atens are indeed more energetically favorable than other orbit types.

The effect of the Moon on observations will be studied by combining data of the apparent closeness of each NEA and the Moon with the brightness of the Moon at that stage in the lunar cycle.

The extent of the bias in the inclination and eccentricity data for known NEAs will be compared to predicted values of the entire population from William Bottke’s model. William Bottke’s model, however, predicted ~90,000 NEAs, so the model must be downsized to match recent corrections to the size of the predicted population of ~20,500 (Mainzer et al. 2011).

Recent analysis of the range of periods of known NEAs suggests that projecting the known population further than two years in the future will improve the degree to which the known population can mimic the unknown population. The computational method will need to be altered to ensure data used is ~5 years since discovery.

Analysis of how many days NEAs remain at observable magnitudes will be altered to consider specific size populations. It is expected that the smaller and therefore dimmer NEAs will not be observable for as many days as is shown in Fig. 10 and therefore a new approach for attaining spectra of these smaller NEAs may be required. Spectra may need to be taken at the moment of discovery.

 6. References

  • Abell, Sanders, G. B., Mazanek, D. D., et al. 2012, CASI, 1667, 6449, 6449
  • Barbee, B. W., Esposito, T., Pinon, E., et al. 2010, AIAA 2010-8368, arXiv:
  • Binzel, R. P. 2000, Planet. Space Sci., 48, 297, 297
  • Chesley, S. R., Chodas, P. W., Milani, A., Valsecchi, G. B., & Yeomans, D. K. 2002, Icarus, 159, 423, 423
  • Elvis, M., Beeson, C. L., & Galache, J. L. 2013, in One chapter in the book to be called ‘Asteroids – Prospective Energy and Material Resources’
  • Elvis, M., McDowell, J., Ho man, J. A., & Binzel, R. P. 2011, Planet. Space Sci., 59, 1408, 1408
  • Greenstreet, S., & Gladman, B. 2012, in AAS/Division for Planetary Sciences Meeting Abstracts, Vol. 44, AAS/Division for Planetary Sciences Meeting Abstracts, 305.05
  • Ito, T., & Malhotra, R. 2010, A&A, 519, A63, A63
  • Kargel, J. S. 1994, J. Geophys. Res., 99, 21129, 21129
  • Larson, S. 2007, in IAU Symposium, Vol. 236, IAU Symposium, ed. G. B. Valsecchi, D. Vokrouhlicky, & A. Milani, 323{328
  • Larson, S. M., Hergenrother, C., Whiteley, R., et al. 2001, in International Workshop on Collaboration and Coordination among NEO Observers and Orbital Computers held at Kurshiki City Art Museum, Japan from October 23 to 26, 2001 organized by Japan Spaceguard Association; edited by Syuzo Isobe and Yoshifusa Asakuro, pp.13-17, ed. S. Isobe & Y. Asakuro, 13{17
  • Mainzer, A., Grav, T., Bauer, J., et al. 2011, ApJ, 743, 156, 156
  • National Research Council, N. C. 2010, Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies FINAL REPORT
  • Nuth, III, J. A. 1999, in Lunar and Planetary Inst. Technical Report, Vol. 30, Lunar and Planetary Institute Science Conference Abstracts, 1726
  • Petit, J.-M., Chambers, J., Franklin, F., & Nagasawa, M. 2002, Asteroids III, 711, 711
  • Rivkin, A. S. 2009, Asteroids, Comets, and Dwarf Planets
  • Shoemaker, E. M., & Helin, E. F. 1978, in NASA Conference Publication, Vol. 2053, NASA Conference Publication, ed. D. Morrison & W. C. Wells, 245{256
  • Toon, O. B., Zahnle, K., Morrison, D., Turco, R. P., & Covey, C. 1997, Reviews of Geo-physics, 35, 41, 41

7. Appendix

7.1 Summary of Acronyms and Definitions

AU – Astronomical Unit. The mean Earth-Sun distance.

LEO – Low Earth Orbit. All human space flights (except Apollo program lunar flights) took place in LEO)

LSST – Large Synoptic Survey Telescope, large-scale survey telescope planned for 2021

MOID – Minimum Orbit Intersection Distance

MPC – Minor Planet Center

NEA – Near Earth Asteroid: periastron, q<1.3AU

NEO – Near Earth Object: encompassing Asteroids and Comets

PHA – Potentially Hazardous Asteroid: MOID< 0.05AU of Earths orbit and have a diameter, D>100m

PGM – Platinum Group Metal

VAs – Virtual asteroids – asteroid solutions that fill the uncertainty region around the nominal orbital solution

VI – Virtual Impactor – VAs that will travel within one Earth radius of Earth.

WISE/NEOWISE – (Near Earth Object) wide- field infrared survey explorer

Apastron – aphelion of an asteroid

Periastron – perihelion of an asteroid

Apollos – semi-major axis, a> Earth a, and periastron inside Earths orbit

Amors – a Earth < a < a Mars, and periastron to the sun outside Earths orbit

Atens – a < a Earth, and apastron outside Earths orbit

Atiras – orbit completely within Earth orbit. Atiras are often categorized with Atens.

Albedo – reflective coefficient: a value from 0 (perfect black, no reflecting power) to 1 (perfect white)

Ecliptic – the plane of the Suns apparent path across the celestial sphere

Parsec – 3.26ly, the distance corresponding to the parallax of 1 second

Solar Elongation – angle between the line connecting the Earth and the Sun and the line connecting the Earth and the NEA. If el<60 then the NEA is not practically observable.

7.2 Notes

1http://www.readcube.com/articles/10.1038/4531178

2The risk of death due to a firework accident is ~1 in 600,000

3www.physics.sfasu.edu/astro/asteroids/sizemagnitude.html

4http://www.minorplanetcenter.net/iau/Sizes.html

6http://www.newyorker.com/reporting/2011/02/28/110228fa fact friend

7http://www.newyorker.com/reporting/2011/02/28/110228fa fact friend

8http://www.newyorker.com/reporting/2011/02/28/110228fa fact friend

9http://osiris-rex.lpl.arizona.edu

10http://www./ostp.gov/galleries/press release files/Augustineforweb.pdf

11http://planetaryresources.com

12http://deepspaceindustries.com

13http://www.bbc.co.uk/news/science-environment-17827347

14http://neo.jpl.nasa.gov/neo/groups.html

15http://spacewatch.lpl.arizona.edu/

16www.ll.mit.edu/LINEAR

17L1 is one of five orbital positions chosen for minimal energy usage.

18http://www.minorplanetcenter.net/iau/MPEph/MPEph.html

19http://wwwsearch.sourceforge.net/mechanize/download.html

20http://pypi.python.org/pypi/beautifulsoup4

21A 6.5m primary mirror telescope at Las Campanas Observatory in Chile: http://obs.carnegiescience.edu/Magellan/

22An IR spectrograph: http://irtfweb.ifa.hawaii.edu/

23http://b612foundation.org/media/sentinelmission/

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