Earth-approaching Asteroids Research Articles

The first steps towards a de minimus, affordable NEA exploration architecture

The impetus for asteroid exploration is scientific, political, and pragmatic. The notion of sending human explorers to asteroids is not new. Piloted missions to these primitive bodies were first discussed in the 1960s, pairing Saturn V rockets with enhanced Apollo spacecraft to explore what were then called “Earth-approaching asteroids.” Two decades ago, NASA's Space Exploration Initiative (SEI) also briefly examined the possibility of visiting these small celestial bodies. Most recently, the US Human Space Flight Review Committee (the second Augustine Commission) suggested that near-Earth objects (NEOs) represent a target-rich environment for exploration via the “Flexible Path” option. However, prior to seriously considering human missions to NEOs, it has become clear that we currently lack a robust catalog of human-accessible targets. The majority of the known NEOs identified by a study team across several NASA centers as “human-accessible” are probably too small and have orbits that are too uncertain to consider mounting piloted expeditions to these small worlds. The first step in developing a comprehensive catalog is, therefore, to complete a space-based NEO survey. The resulting catalog of candidate NEOs would then be transformed into a matrix of opportunities for robotic and human missions for the next several decades and shared with the international community. This initial step of a space-based NEO survey is therefore the linchpin to laying the foundation of a low-risk architecture to venture out and explore these primitive bodies. We suggest such a minimalist framework architecture from (1) extensive ground-based and precursor spacecraft investigations (while applying operational knowledge from science-driven robotic missions), (2) astronaut servicing of spacecraft operating at geosynchronous Earth orbit to retain essential skills and experience, and (3) applying the sum of these skills, knowledge and experience to piloted missions to NEOs.

Divisions I & III WG: on Near Earth Objects

The WGNEO, a Working Group of Divisions I and III, was formed in the early 1990s to coordinate study of Earth-approaching asteroids and comets (NEOs) and provide timely advice to the General Secretary and officers of the IAU on discovery of any objects that threaten collision with the Earth. Since then, the WGNEO has steadily grown, reflecting increasing international interest and concern over impacts, especially from asteroids (which dominate over comets in their risk to Earth). In this triennium, the WGNEO had a membership of 49 (including the Organizing Committee of 17 members), plus 10 consultants. The Chair is David Morrison (USA), Vice-Chair Andrea Milani (Italy), Secretary Richard Binzel (USA), and Past-Chair Andrea Carusi (Italy).

Near-Optimal Deflection of Earth-Approaching Asteroids

Near-Optimal Deflection of Earth-Approaching Asteroids

On the Origins of Earth-Approaching Asteroids

It is generally accepted that Main-Belt asteroids (MBAs) and nuclei of no longer active comets are the sources that replenish the Amor, Apollo, and Aten groups of asteroids, i.e., Earth-approaching asteroids (EAAs). Investigations of the dynamics of EAAs and numerical modeling of their orbital motion have shown that the so-called resonance mechanism of the replenishment of the EAA population with objects from the Main Belt is quite sufficient for its maintenance. In this paper, we compare the physical properties of EAAs and MBAs (and, partly, cometary nuclei) in an effort to gain an understanding of whether the physical properties of EAAs can tell us anything about their origins. The principal result of the performed analysis is the conclusion that the small dimensions of EAAs; their MBA-identical set of taxonomic classes; the identical mineralogy and preponderance of differentiated compositions among EAAs; and their, on average, MBA-identical shape, rotation, optical properties, and surface structure are all convincing proof that the Main Belt is the dominant source of the replenishment of the EAA population and that the share of cometary-origin EAAs does not exceed 10%. The most likely candidates for cometary-origin objects among EAAs are 2100 Ra-Shalom, 2101 Adonis, 2201 Oljato, 2212 Hephaistos, 3200 Phaethon, 3552 Don Quixote, and 4015 Wilson–Harrington.

The D Discriminant and Near-Earth Asteroid Streams

A method of separating an association of orbits, a stream, from the local background using the traditional D discriminant is introduced. Applying this procedure to all 708 asteroids with perihelia less than 1.3 AU known on April 22, 1999 suggests that the orbits of Earth-approaching asteroids may be rather organized. By searching the volume around every orbit in the sample space, 14 associations are found with from 4 to 25 members that statistically stand out from the local background. A more traditional search, where orbits are gathered into strings with every orbit connected to at least one other orbit by D< D c , reveals eight additional three-member strings. Using Poisson statistics seven pairs are identified as having exceptionally similar orbits; three of these pairs are a part of an association or string. On the other hand, comparing the results of a volume search around randomized orbits indicates that perhaps 50 to 75% of these groupings are due to chance, indicating that observational effects may severely affect the background and therefore the reality of the associations that are separated out. The lack of compositional or taxonomic information about most of the 155 asteroids, 22% of the sample, hinders interpretation of the nature of the streams. Of the three associations found in an earlier study (Drummond 1991, Icarus 89, 14–25), only one remains intact as a result of the stricter statistical requirements adopted here. Three of four fireball streams (Halliday et al. 1990, Meteoritics 25, 93–99), previously found to be related to the old associations, can be loosely associated with three of the new associations, but none of the four recovered meteorites with known orbits can be associated with them. Nevertheless, theoretical meteor radiants are listed, all of which are so large and diffuse, and have such slow geocentric velocities, that it is doubtful if visual meteor showers can be linked to the new streams. However, these associations, strings, and pairs of asteroids are good candidates for meteorite sources, especially where organized falls are recognized, since they generally satisfy the immediate dynamical requirements for delivery, namely, gentle geocentric velocities produced by overtaking the Earth around sunset.

A review of comet and asteroid statistics

The statistics of Earth-approaching asteroids are first summarized, and an enhanced frequency of objects smaller than 100 meters is noted. Superposed on these random hazards may be a periodic one of new comets due to galactic tides of the Oort Cloud with a period of 26–36 Myr (Rampino, 1998). New asteroids and comets are being found evermore frequently because new telescope-and-detector systems are coming on line. These are intended primarily for the discovery of dangerous objects, but a beginning has been made with the study of statistics of main-belt asteroids. In addition to trans-Neptunian objects, cis-Neptunian “Centaurs” are recognized, which may be a link in the evolution of short-period comets and thereby contribute to the flux of Earth approachers. With the new equipment coming on line, we are beginning to see that the global hazard will be mostly quantified within a few decades. We do see a shortage in astrometric follow up fainter than about the 20th magnitude.

Size and Orbit Dependent Trends in the Reflectance Colors of Earth-Approaching Asteroids

New observations show that reflectance colors of Earth-approaching asteroids depend on their sizes and orbits. Most bodies larger than ∼2 km have reddish colors similar to common main-belt asteroids of spectral type S. Smaller bodies, and those that may have recently migrated from the main belt, have relatively neutral spectra in the 0.5- to 0.8-μm wavelength range. These trends may result from the action of “space-weathering,” recently proposed to explain similar spectral variations on the surfaces of S-type asteroids and the disparity between the colors of ordinary chondrite meteorites and their probable main-belt parents.

Physical properties of near-Earth asteroids

More than 400 near-Earth asteroids (NEAs) have been discovered and their discovery rate is continuously increasing. The study of the physical properties of these objects is secressary in order to understand their history and relations with comets and meteors, for the analysis and solution of new applied topics, such as the asteroid hazard problem, and the possibility of using NEAs as foture sources of raw materials in near-Earth space. The present review article collects and summaries the most important new data on the physical properties of Earth-approaching asteroids, obtained by photometric, polarimetric, spectroscopic, radar and other techniques. The analysis of all the available data gives clear indications that asteroid main-belt is the main source of NEAs.

Are Main-Belt Asteroids a Sufficient Source for the Earth-Approaching Asteroids?

This paper predicts the size distribution of the Earth-approaching asteroids with diameterd= 10 m to 10 km, assuming they originate as the fragments of main-belt asteroids with a cumulative size distribution proportional tod−2.5and that they have self-similar fragmentation properties. The resulting distribution is dominated by “fast-track” bodies originating from parent asteroids with orbits close to the 3:1 mean-motion resonance with Jupiter. Because the dynamical lifetimes of these Earth approachers are shorter than their collisional lifetimes, their size distribution is nearly proportional tod−3.0, the production distribution in the main belt. This prediction, however, is at odds with the Spacewatch observations. The observed distribution is relatively flat ford> ∼100 m, and relatively steep ford< ∼100 m, so that the number of Earth approachers withd∼ 10 m to 0.3 km is overestimated. If these populations are predominantly of main-belt origin, then the size distribution in the main belt is not a simple power law. A nonuniform size distribution with wave-like oscillations, possibly caused by a cutoff at small sizes, would lead to Earth approachers with a size distribution in better agreement with the observations. If such wave-like oscillations are realistic, then the main belt is sufficient to supply the observed number of Earth approachers throughout the observed size range.

Are Main-Belt Asteroids a Sufficient Source for the Earth-Approaching Asteroids?

We predict the orbital distribution of Earth-approaching asteroids with diameter, d, ranging from 10 m to 10 km, assuming they originate as the fragments of main-belt asteroids. The calculation uses Öpik theory to account for the effects of close encounters with the planets and analytic theory to account for the effects of the ν 6secular resonance and the 3:1 mean motion resonance with Jupiter. We use corrections to the Öpik formulas to account for long-range encounters with Jupiter, outside the Jovian sphere of influence. We also model the ongoing effects of fragmentation, subsequent to the initial, collisional events that eject parent fragments from the main belt. Biasing our predicted orbital distributions to account for the selection effects of the Spacewatch search, and comparing these biased distributions to the orbits of the Earth approachers detected by Spacewatch, we find that we are able to predict the total number and distribution of orbits for kilometer-sized Earth approachers of moderate inclination, but we do not predict the observed concentration of low-eccentricity orbits among the small Earth approachers (SEAs) with d< ∼50 m. We therefore conclude that main-belt asteroids are a sufficient source for kilometer-sized Earth approachers, but not for the SEAs. It is plausible that a significant fraction of the SEAs (greater than 5%) are not of main-belt origin.

Origin of the Spacewatch Small Earth-Approaching Asteroids

Recent discoveries of small Earth-approaching asteroids by the 0.9 m Spacewatch telescope (referred to here as S-SEAs) reveal 16 objects which have diameters ∼50 m or smaller. Approximately half of these objects lie in a region where few large near-Earth asteroids are found, with perihelia (q) and aphelia (Q) near 1 AU,e< 0.35, andifrom 0° to ∼30°. Possible origins for these objects are examined by tracking the orbital evolution of test bodies from several possible source regions using an Öpik-type Monte Carlo dynamical evolution code, modified to include (a) impact disruption, based on a map in orbital (a,e,i) space of collision probabilities and mean impact velocities determined using actual main-belt and near-Earth asteroid orbits, (b) fragmentation, and (c) observational selection effects.Amor asteroid fragments evolving from low eccentricity Mars-crossing orbits beyond theq= 1 AU line provide a reasonable fit to S-SEA orbital data. Planetary ejecta from Mars is only consistent with low and moderately inclined S-SEA orbits. Asteroidal fragments from the main-belt via the 3:1 or ν6chaotic resonance zones rarely achieve low-eorbits before planetary impacts, comminution, or ejection remove them from the system. This source could produce the observed moderate-to-high eccentricity S-SEAs. Plantary ejecta from the Earth–Moon system and Venus are only consistent with low-inclination S-SEA orbits. Moreover, constraints set by the planetary cratering record and the meteorite record suggest that the Earth, Moon, and Venus are unlikely to provide many S-SEAs. All of these results are predicated on the observational bias computations (Rabinowitz, D.L. 1994.Icarus111, 364–377) that provide the current definition of the S-SEA population.

The population of near-Earth objects discovered by Spacewatch

In the past three years the Spacewatch program at the University of Arizona's Lunar and Planetary Laboratory has discovered ~45% of the new Earth Approaching asteroids and found evidence for an unheralded population of small (~10m) objects in the inner solar system. This success is due to the automated Moving Object Detection Program (MODP) which searches successive scans over the same region for objects showing consistent motion. Highlights of recent discoveries, an update on research, and the development and potential of the new 1.8m Spacewatch facility will be discussed.

Earth-crossing asteroids: The last days before earth impact

view Abstract Citations (10) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Earth-Crossing Asteroids: The Last Days Before Earth Impact Hills, Jack G. ; Leonard, Peter J. T. Abstract We have determined the observational and dynamical properties of Earth-crossing asteriods in the final weeks before their impacts with Earth. An asteroid that is within a few weeks of either Earth impact or of a very close approach to Earth has a large parallax and a small proper motion. The proper motion of such an asteroid has a maximum value of only a few minutes of arc per day, and it vanishes for some approach directions. An asteroid that is 10 days from impact has a parallax of several minutes of arc when viewed from two positions on Earth having a projected separation of 1 R (solar halo). Wide-field cameras can easily detect this parallax. Impacting asteroids greater than 100 m in diameter, which is near the minimum size that produces significant local damage, appear as stars of at least visual magnitude 18 during their final 10 days to impact unless they approach Earth from the vicinity of the Sun in the sky. They are detectable with relatively small telescopes equipped with CCDs. A typical proposed station for detecting them may have 20 telescopes with apertures between 5 and 16 in. and 1 telescope with an aperture of 36 in. (0.9 m). The largest telescope would only be used to search for objects near the sun (near dusk and dawn), and it would be available during the remainder of the night to improve the orbits and physical characterization of objects that may pass near or hit Earth. While the apertures of the required telescopes are relatively modest, such a systematic search for Earth-approaching asteroids will be a computer intensive task and it will require the correlation of the positions of anomalous objects between two or more sites in near real time to determine whether or not they have a measurable parallax. Thermal infrared emission may provide a complementary way of detecting impacting asteroids especially if they approach Earth from near the direction of the Sun. Publication: The Astronomical Journal Pub Date: January 1995 DOI: 10.1086/117285 Bibcode: 1995AJ....109..401H Keywords: Asteroids; Impact Prediction; Parallax; Radial Velocity; Solar Orbits; Charge Coupled Devices; Monte Carlo Method; Telescopes; Astronomy; METEOROIDS full text sources ADS |

Master: A discovery-class mission for the detailed study of large mainbelt asteroids

MASTER (Mainbelt Asteroid Exploration/Rendezvous) focuses on the comprehensive global study of important large mainbelt asteroids. A three-instrument, four-investigation payload can be placed into orbit around 4 Vesta within the cost/complexity envelope of the Discovery program using a Delta-ll launch vehicle. The detailed global study of such an object will provide essential-new data on the present state and past evolution of asteroids, including information not obtainable by fast flybys or by rendezvous with mere asteroidal shards such as the tiny Earth-approaching asteroids.

The Size and Shape of the Near-Earth Asteroid Belt

Evidence was recently reported for the existence of a near-Earth belt of small, Earth-approaching asteroids (SEAs) with diameters less than ∼50 m. This result was based upon observations made with the Spacewatch Telescope of the University of Arizona during the course of an ongoing search for Earth-approaching asteroids. Using a model to describe the effects of observational bias, it was shown that the orbits observed for SEAs are inconsistent with the orbits of Earth approachers larger than ∼1 km, and imply a relatively high fraction of Earth-like orbits among the SEAs. In this paper, new observations are included and the bias model is extended in order to quantify the number of SEAs within the near-Earth belt and to further constrain their orbital distribution. The calculation shows that relative to larger Earth approachers, SEAs are deficient in Aten-type orbits for which the semimajor axis is less than 1.0 AU. Instead, nearly all SEAs with aphelia less than 1.4 AU (5 ± 3% of the total population) have perihelia between 0.9 and 1.1 AU, thus defining a near-Earth belt. Those SEAs with aphelia > 1.4 AU, however, have a distribution of orbits that are indistinguishable from the orbits of larger Earth approachers. Taking the near-Earth belt into account does not significantly alter the previously determined enhancement in the number of SEAs compared to an extrapolation of the numbers of larger Earth approachers. At ∼10 m, the enhancement factor is 40 to within a factor of 2. Also, the RMS impact velocity of SEAs with Earth (17 km/sec) is nearly the same as for larger Earth approachers (18 km/sec).

An international program to protect the Earth from impact catastrophe: Initial steps

NASA has recently completed a study of initial steps to deal with possible cosmic impact hazards. It is generally accepted that impacts by Earth-approaching asteroids and comets pose a significant hazard to life and property, with the greatest risk associated with objects large enough to disturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Various studies have suggested that the minimum mass impacting body to produce such global consequences is tens of billions of tons, resulting in an explosion with energy approaching a million megatons of TNT. The corresponding threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km. Smaller objects (down to 100 m) can cause severe local damage but pose no global threat; below 100 m most projectiles disintegrate high in the atmosphere. Current technology permits us to discover and track nearly all Earth-crossing asteroids or short-period comets large enough to threaten global catastrophe. As the initial step in any program to deal with this hazard, we require a long-term telescopic search that reaches stellar magnitude 22 in order to achieve a nearly complete census of objects 1 km or larger. Such a program can be carried out with existing technology using an international network of six telescopes of 2–3 m aperture equipped with modern CCD detectors and automatic signal processing capability. If any object threatens impact with the Earth during the next century or so, we can expect it to be identified with a lead time of at least several decades, sufficient to plan an international campaign to deflect or destroy it.

Evidence for a near-Earth asteroid belt

IN January 1991, the 0.9-m Spacewatch telescope made the first observation1 of an asteroid outside Earth's atmosphere but in the neighbourhood of the Earth–Moon system. Since then, more than 40 Earth-approaching asteroids (defined as objects with perihelia of less than 1.3 AU) have been discovered, including 13 smaller than 50 m. Using these data, one of us (D.L.R.) has shown2 that there is an excess of Earth-approaching asteroids with diameters less than 50 m, relative to the population inferred from the distribution of larger objects. Here we argue that these smaller objects — characterized by low eccentricities, widely ranging inclinations and unusual spectral properties — form a previously undetected asteroid belt concentrated near Earth. The recent discovery of additional small Earth-approaching asteroids supports this conclusion.

The size distribution of the earth-approaching asteroids

The discovery circumstances of the first asteroids ever observed outside the earth's atmosphere but within the neighborhood of the earth-moon system are described. Four natural objects with diameters in the range 5-50 m were detected during a search for earth-approaching asteroids conducted each month at the 0.91-m Spacewatch Telescope at Kitt Peak. An additional 19 earth approachers with sizes in the range 50 m to 5 km were discovered. These obervations determine the cumulative flux of asteroids near earth as a function of absolute magnitude. For asteroids larger than about 100 m, a power-law dependence with exponent of about 0.9 is observed, consistent with their evolution from the main-belt population. At about 10 m, the flux is more than two orders of magnitude greater than this power-law extrapolation.

Evolution of Earth-crossing binary asteroids due to gravitational encounters with the Earth

During close encounters with the Earth, binary Earth-approaching asteroids have their orbital energy changed by Earth tidal forces. The total amount of this change, over the binary's lifetime, is of the order of the energy itself. As a consequence, an initial population of binaries with separations of a few times the sum of the radii of the components evolves to a broader orbital distribution, which includes both increased separations, leading to formation of “loose binaries” and possibly to escape of the components, and smaller orbital distances, eventually creating contact binary systems. The existence of well-separated systems is suggested by the terrestrial cratering record, while contact systems have been recently observed by radar.

Near miss of the Earth by a small asteroid

THE 0.91-m Spacewatch Telescope, on Kitt Peak in Arizona, is being used to search for new Earth-approaching asteroids. On the night of 18 January 1991 a streaked image extending over more than 161 arcseconds was recorded. Further observations over the next 4.6 hours showed that the images were of an object travelling in an independent orbit around the Sun; it was given the asteroidal designation 1991 BA. Orbital computations, presented here, indicate that 1991 BA was only 0.0053 AU from the Earth at the time of discovery, closing to 0.0033 AU at the last detection. Extrapolation of the calculated orbit shows that the object passed only 0.0011 AU (170,000km) from the Earth 12 hours after it was found. The observed brightness translates into an absolute visual magnitude 28.9, corresponding to a diameter of only 5–10 m. 1991 BA is the closest and smallest asteroid yet observed outside the Earth's atmosphere.