Adam Thodey and Rohini Indiresan

Department of Atmospheric, Oceanic, and Space Sciences

University of Michigan

Visible and Infrared Remote Sensing Class Project

4 December 1996

Table of Contents

1.0 Motivation
2.0 History of Discovery and Observations
3.0 Scanning for Near Earth Objects (NEOs)
3.1 Techniques for detecting NEOs
3.1.1 Photographic Technique
3.1.2 Charge Coupled Device (CCD) Technique
3.2 Techniques for Tracking NEOs
4.0 Instrumentation
4.1 Photographic/Telescope based
4.2 CCD systems for NEO survey
4.3 Ground-based Radar for NEO detection
5.0 NEO Observation Programs
5.1 Ground-based Programs
5.1.1 Spacewatch
5.1.2 Near Earth Asteroid Tracking Program (NEAT)
5.2 Spaceborne Programs
5.2.1 Infrared Astronomical Satellite (IRAS)
5.2.2 Infrared Space Observatory (ISO)
5.2.3 Near Earth Asteroid Rendezvous (NEAR)
5.2.4 Small Missions to Asteroids and Comets (SMACS)
6.0 Spectral Properties of NEOs
7.0 Conclusions: Future Hazard Mitigation
8.0 References
Appendix A: List of NEOs
Appendix B: Facts about 433 Eros and 253 Mathilde
Appendix C: Spectral reflectances of some NEOs
Appendix D: List of Asteroids and Comets Tracked by ISO
Abstract

The dangers posed by Near Earth Objects (NEOs) to mankind, as seen by mass extinctions and craters, have been studied extensively over the past few decades. Remote sensing techniques has played an important and useful tool to track and detect these objects. Several ground-based as well as spaceborne programs are actively involved in detecting and tracking potential Earth impactors. This report discusses the various classification of NEOs, the photographic and Charge Coupled Devices (CCDs) techniques used to search for NEOs. Certain observation programs will also be discussed in detail, with a focus on the visible and Infrared instrumentation used to scan for NEOs. Although there is no immediate danger of an impact by NEOs, a few mitigation procedures are also included in the report.

1.0 Motivation

Since its formation, the Earth has been subject to a continuing bombardment by cosmic debris in the form of asteroids and comets striking at speeds of tens of kilometers per second. While the atmosphere protects us from most of the smaller fragments, larger objects (roughly those bigger than 50 meters) are capable of reaching the lower atmosphere or the surface where they explode with an energy greater than that of any but the most powerful nuclear weapons. Impactors larger than a kilometer or so in diameter have the potential for still greater damage through global environmental effects; such impacts could place at risk much of the human population and endanger the survival of civilization. Geologic evidence suggests that occasional rare, very large impacts in the past have led to mass extinctions of living species. The widely observed impacts into Jupiter in July 1994 of the fragments of Comet Shoemaker-Levy 9 released energy measuring in the millions of megatons of TNT and generated fireballs and dark clouds on Jupiter about as large as the Earth (web site on NEO report). These events provided an object lesson on the effects of large impacts.

With the awareness of how these heavenly objects can affect life on Earth, several projects involving the study of Near Earth Objects (NEOs) have been designed to systematically search for these objects and track their orbits. Remote Sensing forms an important and useful tool in achieving these goals.

In the subsequent sections we will provide some background information on NEOs, how they are detected and tracked using visible and IR remote sensing instruments, certain programs involved in the search of NEOs and the spectral characteristics of certain NEOs.

2.0 History of Discovery and Observations of NEOs

The recognition of Earth-crossing comets occurred during the founding period of modern science in the 17th century. Edmond Halley, who carried out the first systematic determination of comet orbits, found that the orbits overlapped the orbit of the Earth. He recognized immediately that comets could collide with the Earth. Over the course of the intervening centuries, hundreds of Earth-crossing comets have been discovered.

Ever since physicist Luis Alvarez and his colleagues at the University of California, Berkeley, discovered a thin layer of Iridium in a geological strata in 1979. The debate over the devastation that asteroids could cause began. It was not until the last 1980s that asteroids were thought to be a real threat due to the discovery of two asteroids that came within 1,100,000 kilometers (1989FC) and 170,000 kilometers (1991BA) of the Earth (Verschuur, 1991).

There are two broad categories of NEOs: comets and asteroids. Historically, asteroids and comets have been distinguished by astronomers on the basis of their telescopic appearance. If the object is star-like in appearance, it is called an asteroid. If it has a visible atmosphere or tail, it is a comet. This distinction reflects in part a difference in composition; asteroids are generally rocky or metallic objects without atmospheres, whereas comets are composed in part of volatiles (such as water ice) that evaporate when heated to produce a tenuous and transient atmosphere.

Asteroids with orbits that bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are known as Earth-approaching or near-Earth asteroids (NEAs). It is believed that most NEAs are fragments jarred from the main belt by a combination of asteroid collisions and the gravitational influence of Jupiter. Some NEAs may be the nuclei of dead, short-period comets. The NEA population appears to be representative of most or all asteroid types found in the main belt. On the other hand, an Earth-crossing asteroid (ECA) are those asteroids that have the potential to impact our planet. An ECA is defined as an asteroid moving on a trajectory that is capable of intersecting the capture cross-section of the Earth as a result of on-going long-range gravitational perturbations due to the Earth and other planets.

The near-Earth asteroids are categorized as Amors, Apollos, and Atens, named for famous members of each category:1221 Amor, 1862 Apollo, and 2062 Aten.

Amors: Their orbits lie outside that of the Earth, but cross Mars’ orbit. Eros is a typical example of an Amor.

Figure 1: Orbit of 433 Eros – Amors type asteroid.

Appollos: Their orbits cross that of the Earth with period greater than 1 year. Geographos represents the Apollos.

Figure 2: Orbit of 1566 Icarus – Appollos type asteroid.

Atens: Their orbits cross that of the Earth with a period less than one year. Ra-Shalom is a typical Aten.

Figure 3: Orbit of 2062 Aten – Atens type asteroid.

Asteroids are further classified based on their composition. Thus asteroids are of 3 major types:

C-type: This type includes more than 75% of known asteroids. It is extremely dark with an albedo of 0.03-0.09. C-type asteroids are thought mostly to consist of a type of meteorite called “carbonaceous chondrites,” a mixture of rock and tar that has a composition much like the sun, minus the hydrogen and helium.

S-type: This type includes about 17% of known asteroids. It is relatively bright with an albedo of 0.10-0.22. Its composition is metallic nickel-iron mixed with iron- and magnesium-silicates.

M-type: This type includes most of the rest of the known asteroids. It is also bright with an albedo of 0.10-0.18. Most are thought to consist of nickel-iron with a small amount of iron- and magnesium-silicates.

There are a dozen or so other rare types, but nearly all are variants of the three major ones.

Cometary objects are classed as short-period if their periods are less than 20 years, intermediate-period if their periods are between 20 and 200 years, and as long-period if their periods are greater than 200 years. Active comets can also cross the Earth’s orbit with the potential for collision. At any given size, active short-period and intermediate-period comets contribute only an additional 1 percent or so to the total collision frequency, a value that is small compared to the estimated uncertainty in the ECA population. However, recent evidence indicates that inactive short- and intermediate-period comets may be about 10 to 20 times as numerous as the active, easily discovered comets (Shoemaker et al., 1994).

The first discovered Earth-approaching asteroid was (433) Eros, found in 1898 by G. Witt at Berlin. (1862) Apollo, discovered by K. Reinmuth at Heidelberg in 1932, was the first recognized Earth-crossing asteroid, although (887) Alinda, found by M. Wolf at Heidelberg in 1918, is now recognized as an Earth crosser. In 1973, the first dedicated survey for near-Earth asteroids was initiated by Helin and Shoemaker (1979) at Palomar Observatory, California. This survey used the 46-cm Schmidt telescope for photographing a region near opposition (the direction opposite from the Sun) each month. A parallel effort on the same telescope was carried out by Helin. T. Gehrels began a development effort to utilize CCD detectors to scan the sky for NEOs in 1981. By 1989, a 2048 x 2048 pixel CCD was installed in the Spacewatch telescope at Kitt Peak, Arizona, and the Gehrels team began discovering a substantial number of near-Earth asteroids with this system. Photometric, spectrophotometric, infrared observations all made with optical telescopes, and radar observations are the entire basis for the understanding of the sizes, shapes, rotation states and mineral composition of the near-Earth asteroids. The first radar detection of an NEO was made of the Apollo asteroid (1566) Icarus in 1968 by R. M. Goldstein; since 1975, radar observations have been made of numerous NEOs by S. J. Ostro and his colleagues.

The well-observed ECAs exhibit a diversity in inferred mineralogy approaching that in the rest of the asteroid population. The majority are similar to the dark C-class asteroids in general properties (presumably moderately low-density, colored black due to the presence of at least several percent of opaques). There are also a large number of S-class asteroids. (S’s are thought to be either stony, chondrite-like objects, stony-iron objects, or a combination of both.) In addition, there are known examples of metallic bodies (probably like nickel-iron alloy meteorites) and rocky, monomineralic bodies. ECAs are often quite irregular in shape; they also tend to have rather rapid spins, but there is a great diversity in such properties. Radar images of several such objects (Castalia, Toutatis, Geographos) show that they have a wide diversity of shapes and possible structure. In the case of Castalia, two fragments appear to be merged to form a dumbbell-shaped double-lobed object.

It is particularly uncertain what the physical properties of comets (extinct or active) might be like. They are brought into the Earth orbit by Jupiter’s gravitational field perturbations from their orbits in the Kuiper belt or the Oort cloud (Weissman 1990). Only one comet has been studied in detail: Comet Halley, which was the target of several flyby spacecraft missions at the time of its last apparition in 1986. The nucleus of Halley is irregular and dark, with an average diameter of about 9 km. Like other comets, it is made of a combination of ices and dust, with much of the atmospheric outgassing near the Sun confined to discrete plumes or jet-like features (“jets”). The non-volatiles include both silicates and organic materials, while the primary ices (with percentages derived for Halley) are water (80%), carbon monoxide (7% – another 8% comes from organic dust, a distributed source in the coma), carbon dioxide (3.5%), plus smaller amounts of methyl alcohol, ammonia, hydrogen cyanide, and hydrogen sulfide. The physical configuration of comets is even less well understood than that of the small asteroids, and many comets have been observed to split under rather modest tidal and thermal forces. A direct estimate of density is derived from the tidal disruption of Comet Shoemaker-Levy 9, yielding a value near 0.5 g/cm.

These properties of NEOs along with the possible hazard to Earth upon impact, has motivated several scientists to conduct a systematic scan of the heavens to detect, track and mitigate impacts due to comets and asteroids.

3.0 Scanning for NEOs:

To study potential impactors, it is not essential to know a great deal about the physical nature of comets and asteroids. The most important properties are simply their mass and impact velocity. However, to be able to intercept and divert an incoming comet or asteroid knowledge of the configuration, density, cohesion, and composition of these objects would be essential. These requirements would thus make ground-based and spacecraft missions to comets and near-Earth asteroids essential for understanding the impact threat.

Some of the programs aimed at discovering potential Earth impactors are: Near-Earth Asteroid Rendezvous (NEAR), Near-Earth Asteroid Tracking (NEAT), Spacewatch, Small Missions to Asteroids and Comets (SMACS), Infra-red Space Observatory (ISO) and Infra-red Astronomical Satellite (IRAS). Most of these focus on the detection and tracking of near-Earth asteroids (NEAs) as against tracking comets since the larger orbits and longer periods of revolution of Earth-crossing comets will require a much longer-term effort and presents a difficult challenge.

3.1 Techniques for Detecting NEOs:

3.1.1 Photographic Technique (Carusi, 1994)

Any detection process of NEOs requires prompt and rapid action and follow-up, since they have angular motions of 1 deg/day and often have directional ambiguity. In the photographic technique, long exposures of the photographic plate are used to enhance the visibility of asteroid images. This gives longer image trails which are easy to spot amidst many diverse images.

Figure 4: Typical discovery photograph which reveals the direction of motion and helps to detect an asteroid using a single plate.

However the long exposure methodology has several drawbacks like a reduction in the number of exposures leading to a limitation in the area of the sky coverage. They also produce photographs with darker backgrounds, thus making the scanning process harder. These photographs would also depend on factors like moonlight and weather conditions. To overcome these problems, a photographic technique called stereo comparators is used to examine pairs of photographs of the same part of the sky, exposed for short time intervals. The eye detects any moving object since it would stand out above or below the plane of the film, against a background or foreground of stationary objects. The effect of the human brain to see this is used to detect any apparent motion, by looking at the two plates by each eye. Very faint images require long, sky-limited exposures where the background sky is just detectable on the film. This requires that the telescope be able to track the object during the entire exposure. Once the object is detected, its motion vectors are determined from the image movement between exposures or the length of the trail if a long single exposure is used. Thus, the basic detection procedure of a NEO is based on the apparent angular velocity of the object.

The photographic materials used in the detection process are varied, the most popular emulsion being the III-a type emulsions coated on glass from Kodak. Another popular and more recent emulsion is the panchromatic emulsion coated on a film base. This film has high-speed fine grain and good contrast characteristics. However, to use these slow emulsions in photographing faint objects requires soaking/baking them in various dry gases to increase their sensitivity to faint objects. Inspite of all these treatments, the best quantum efficiency of the photographic film is ~ 10% under ideal conditions.

Since absolute speed is essential for the discovery of faint and fast objects, the CCDs are now preferred over the photographic plates. The CCDs also offer the potential to make rapid measurements with ease and can be digitally processed to obtain higher quality images.

However, it should be noted that although the photographic detection procedure is limited in its speed to detect, the photographic telescopes have a large sky coverage area in a single exposure.

3.1.2 Charge-Coupled Device (CCD) Technique

There are two distinct CCD observational techniques, namely, the “stare” and the “scan”. The CCD technique gives analysis in near real-time and with high precision and great detail. In the stare mode, the CCD is used to picture the sky in a series of stare exposures, while in the scan mode the sky is surveyed in a scanning mode. The scanning technique is efficient since it uses less of the telescope time. The CCDs are limited to small fields of view and thus have a smaller sky coverage than that obtained by photographic techniques.

Figure 5: Relationship between focal length, pixel length and sky coverage of a CCD.

This drawback can be overcome by scanning at a rate faster than the sidereal rotation rate. To track the motion in the sky, the charge transfer of the CCD should be in accordance with the drift rate of the image and these should be read out continuously during the detection/observation procedure. The scan rate depends on the cosine of the declination angle delta. The rotation of the Earth with respect to the fixed stars must be taken into account. This scan is typically carried out three times and the position coordinates are compared for each of the images. Objects very close to the Earth will record short streaks in the CCD frame because their motion comes close to matching that of the Earth’s. The optimal interval between successive scans is about 10 mins for the Spacewatch program (which will be discussed in detail in subsequent sections).

3.2 Techniques for Tracking NEOs

In order to make sure that the Earth will not be in danger of being hit by a NEO, these objects must be tracked and their orbits updated. In the case of asteroids, their orbits may be changed due to passing close to a planet, or other heavenly body, and thus could be changed just enough to be on a collision course with the Earth and the destruction of mankind. Comets, also not immune to the gravitational effects of other heavenly bodies, on the other hand, bleed off material as they encounter the solarwind which could slightly change the orbit.

Thus, it is important to keep track of the orbits of NEOs to help detect possible Earth colliding NEOs and when these occurances might take place.

After an asteroid or comet has been detected, its orbit must be determined in order to track it for further study. The most widely used method of determining the orbit of a heavenly object is by taking three consecutive photographs of the object. Then measurements of the location of the object is taken and the orbit determined from these measurements with respect to fixed stars.

In today’s day and age, software is employed to do this work. As with the Near Earth Asteroid Tracking Program (NEAT), which will be discussed later on, an autonomous system is used to scan the sky, detect new objects and calculate its orbit. The orbit is determined by taking the consecutive images and subtracting out the constant background objects, such as the fixed stars of the galaxy. What remains are most likely newly discovered or already known objects. These remaining objects orbits must be calculated, compared to already known objects orbits, and if it is new, then it is recorded as such and reported to the Minor Planet Center. These new findings still must be confirmed.

4.0 Instrumentation

From the previous sections, we see that the photographic or CCD techniques are the most popular methods for searching NEOs. This section will discuss the theoretical aspect of behind these methods.

4.1 Photographic/Telescope based

The main objectives of a search for NEAs (which present very little population density per unit area of the sky, short period of observability after discovery and rare additional observations) are:

1 to obtain sufficient sky coverage to increase the probability of a discovery;

2 to look at data in near real-time to confirm a discovery;

3 intensive collaboration to follow-up the discovery and deduce sufficient orbital information to aid in their tracking and recovery at a later time.

Figure 6: Viewing geometries for the spacecraft camera for the acquisition of consecutive frames.

Detection depends on the population of asteroids for a limiting magnitude, and is proportional to the primary sky coverage obtained. The population is modeled as an exponential function given by:

N = k exp [bV(1,0)] Eqn 1

Using a simple model, the average discovery rate is given by: (Dunbar et al., 1984)

Eqn 2

where D = discovery rate

S = sky area covered

V_lim = limiting apparent magnitude of the search instrument

b = population index, when b = 0.9 gives exp(b) ~ 2.5

For a telescope, the rate of sky coverage depends on the telescope field of view and the efficiency of the detector. The latter is dependent on the speed of the optics and the intrinsic properties of the recording medium. This is expressed as the minimum exposure time required to reach a certain search depth. Thus, the sky coverage rate is given by:

Eqn 3

where FOV = field of view in solid angular measure

t_exp = exposure time

t_dead = time interval between exposures

n = number of exposures required, typically 2 for just detecting.

Another aspect that needs attention is the telescope/detector combination. To sample fainter, fast-moving objects, it is better to have a fainter threshold for detection. This increases the search volume. This means that the telescope should have a larger aperture and/or a more sensitive detector, both typically associated with a smaller field of view and, hence, reduces the sky coverage. This arrangement also introduces the problem of being able to discriminate potential “target” images from the “noise” represented by the vast number of main-belt asteroid images that are also detected. In the detection process, the only distinguishing features of the asteroids are their apparent motions. NEAs have apparent motions well in excess of 0.5 degrees per day, often steeply inclined to the ecliptic due to their proximity to the Earth. In contrast, main-belt asteroids have apparent motions of less than 0.3 degrees/day.

Although the detection process based on the apparent motions is very simple and fairly reliable, it can sometimes lead to misinterpretations. This is especially true if the main-belt asteroids have highly eccentric or highly inclined orbits or if the NEOs have a high line of sight motion component and a low tangential (apparent) component. With these and other limitations of photographic techniques to detect asteroids, electronic detectors (especially Charge Coupled Devices – CCDs) are becoming more and more popular. These have high quantum efficiency combined with the power to handle large digital data output. Thus they offer the potential for semi-automated searches and near real-time data reduction, greatly increasing the productivity of the search.

The disadvantages of a photographic plate is that the limiting magnitude of a fast-moving object is not increased appreciably by increasing the exposure time, but is determined by the amount of time an image spends on a particular grain of the plate i.e., it is limited by the apparent motion of the object. Thus the best quantum efficiency of a photographic plate is only a few percent as against 80% for a good CCD. In addition, the CCD has a larger spectral bandwidth of 400 – 800 nm. In any case, regardless of the technique used, all detection processes are dependent on the population of the NEOs and sky coverage of the detector system.

As mentioned in the preceding sections, both detection and tracking rely on various kinds of instrumentations. The most popular sensors in the visible and IR regions are the CCDs and the telescope. However, photographic techniques have several limitations and therefore CCDs are becoming more and more popular and versatile. A brief theory of the CCD sensor will be presented below, and subsequent sections will discuss in detail the specific characteristics of instrumentations used in particular projects that detect NEOs.

4.2 CCD Systems for Near-Earth Object Surveys (Alan W. Harris):

Several automated systems using CCD detectors and computer searching algorithms are used to detect NEOs of ~ 1 Km or greater in a time scale that is shorter than that obtained using photographic plates. The following are some examples:

1. The Spacewatch (SW) Telescope on Kitt Peak, Arizona. It consists of a 1.2m telescope with a single CCD detector and a second telescope (SW-II) of 1.8 m aperture.
2. The Lowell Observatory Near-Earth Object Survey (LONEOS) telescope is a modified Schmidt telescope of 0.58 m aperture, at Lowell Observatory.
3. The USAF Space Command currently operates a network of 1 m, f/2 wide-field telescopes, the Ground-based Electro-Optical Deep Space Surveillance (GEODSS) system, for tracking Earth satellites. With only minor modifications and changes to the computer software, it can be used for NEO surveys.

As mentioned in the case of a photographic technique, the limiting magnitude expected for a given system, as a function of rate of motion in the sky needs to be determined. A moving object will record as a streak, and thus the limiting magnitude decreases with increasing motion. Since several passes (detections) are required to confirm a moving object, a crude rate of motion is implicitly included, but full orbit determination is not.

The limiting visual magnitude for an untrailed image, m_1 , is scaled from the empirical limit derived for Spacewatch:

Eqn 4

where A is the effective light collecting area of the telescope, that is, the area of the primary mirror, less obstructed area, times the transmission factor of the optics.

Q is the quantum efficiency of the CCD detector. For unthinned, front-illuminated detectors (Original Spacewatch and LONEOS) this is taken to be an average of 0.25 over the spectral band 0.50 to 0.85 m. For thinned, anti-reflection coated, back-illuminated detectors (Current Spacewatch, Spacewatch II and GEODSS), Q has an average value of 0.75 over the spectral band 0.45 to 0.80 m.

t is the integration time. For sidereal scanning (with the telescope clamped down, as with Spacewatch) this is fixed by the drift rate. Scanning can also be accomplished at a faster rate by driving the telescope in declination and clocking the CCD chip at a faster rate, e.g. Spacewatch II or LONEOS. In the “stare mode”, with the telescope tracking at sidereal rate, the exposure time may be chosen to be almost any value.

a is the sky area (in arcsec²) which contains the image of a point source. Generally this is the area of a square array of pixels needed to span the image width plus some margin. for the Spacewatch cameras, it is 3×3 pixels; for LONEOS and GEODSS it is 2×2 pixels.

(S/N) is the limiting signal-to-noise level that is deemed detectable.

r₁= is the rate of motion in the sky at which a point image trails by the width of the image during the exposure time t.

The limiting magnitude for a short trailed image of an object moving at rate r > r₁ is:

m_lim = m₁ – 1.25 log(r/r₁) Eqn 5

r₂ is the rate of motion at which the trail becomes too long to reconstruct as a single signal. This limit occurs for a variety of reasons, including limitations of computer capability, long trails crossing confusing images, or just the physical dimensions of the CCD chip. Typically, r_2 corresponds to a trail length of 100 pixels for all systems.

For r > r₂ , the limiting magnitude is given by:

m_lim = m₂ x -2.5 log(r/r₂) Eqn 6

where m₂ = m₁ x -1.25 log(r₂/r₁) is the limiting magnitude at the rate r₂.

The rate of sky coverage is just the FOV of the instrument, divided by the exposure time, divided by the number of passes per area. The best strategy is to cover all available sky each month. Also, it is not practical to observe too close to the galactic plane, because the number of confusing stars becomes so great that images of asteroids have an increasing probability of being obscured by such stars. This limit is of course magnitude dependent: the fainter the limiting magnitude, the farther from the galactic plane one must stay. With these limitations, the net rate of sky coverage of ~130 sq. deg./hour is necessary to cover all available sky each month. Figure 5 shows the relationship between pixel size, telescope focal length, and pixel angular coverage (Douglas George). Matching a CCDs pixel size to the imaging task is essential for getting the best results.

The following section discusses some of the programs involved in tracking and detecting NEOs.

4.3 Ground-based Radar for NEOs detection

Although this is a more non-intuitive technique, it is still a key discovery technique and is likely to play a major role in the identification of NEOs in the future. The basics strategy of radar observation is to transmit an intense, coherent signal with very well known polarization state and time/frequency structure. This is then compared to measure properties of an echo to deduce the properties of the target. This information will depend on the echo strength which must be several times than the rms fluctuation in the receivers thermal noise.

Delay Doppler measurements are extremely useful for refining orbits and predicting ephemerides. These measurements can also provide two-dimensional images with resolution of decameters. Imaging data sets with adequate coverage in subradar longitude/latitude can be used to determine the targets shape and spin vector. Radar uses wavelengths that are sensitive to near-surface bulk density and structural scales larger than a few centimeters. They can penetrate the optically opaque comas of comets and reveal large particle clouds.

The two continuously active planetary radar telescopes are the Arecibo (=13 and 70 cm) and Goldstone (=3.5 and 16cm) instruments.

5.0 NEO OBSERVATION PROGRAMS

There are several programs which detect and track NEOs. These are both ground-based and spaceborne. Some of these programs mentioned below will be discussed in some detail.

5.1 GROUND-BASED PROGRAMS

5.1.1 SPACEWATCH

Spacewatch has been making observations since 1983, using the 0.91-meter (36 inch) f/5 Newtonian Reflector of the Steward Observatory on Kitt Peak in Southern Arizona.

Figure 7: Spacewatch Observatory, Arizona.

A Tektronix 2048×2048 pixel CCD array is used during the 18 nights each lunation centered on the new Moon to image the sky and identify Near-Earth Asteroids. Currently, 3 NEAs are discovered every month. A new 1.8-meter telescope is also being fabricated.

Detection Modes (Gehrels et al., 1996)

Spacewatch uses the charges generated on the CCD due to a star’s image when it crosses off the CCD to read off of an automatic computer program. Each such scan is repeated 3 times over the same section of the sky. As the image is being read from the CCD and displayed on the computer screen, a program called Moving Object Detection Program (MODP) developed by Rabinowitz D. L., and Scotti J. V., searches for point sources as well as steaks which may be a near-by fast moving object.

Figure 8: Time Series of Spacewatch images.

This is most efficient for fast moving objects with angular rates that are greater than 2 deg/day. For rates in the region of 0.02 – 2 deg/day, the MODP detects objects based on their consistent motion from pass to pass. Very fast moving objects within 0.2 AU of the Earth have angular rates > 2deg/day and are missed by the automated program and thus need to be detected by an observer based on faint and/or long trails. The faintest NEAs with H > 22 are detected by the observer. The detection modes are shown in the following figure.

Figure 9: Spacewatch detection modes as a function of rate of motion and distance.

The following table shows some of the more interesting objects that have been discovered by Spacewatch in recent years. The table lists the object’s designation along with its closest approach to the Sun (perihelion), furthest distance (aphelion), and inclination of the orbit to the plane of the ecliptic.

Table 1: List of objects discovered by Spacewatch in recent years.

Near-Earth Asteroid Belt

Another interesting suggestion based on Spacewatch observations (Rabinowitz, 1993, 1994) is that the NEAs belong to two distinct populations. One is the “traditional” and the second group consists of objects with diameters less than 50 m and having orbital elements like semi-major axis, eccentricity and inclination similar to the Earth’s. This suggests the presence of a Near-Earth Asteroid “Belt”. However, this is still a controversial suggestion.

5.1.2 Near Earth Asteroid Tracking (NEAT) Program

NEAT, a cooperative effort between NASA/JPL and the U.S. Air Force. is an autonomous celestial observatory located at the USAF/Ground-based Electro-Optical Deep Space Surveillance (GEODSS) site on Haleakala, Maui, Hawaii. NEAT is designed to complete a comprehensive search of the sky for near-Earth asteroids and comets. JPL designed, fabricated, and installed the NEAT camera and computer system on a 1-m GEODSS telescope

The NEAT principal investigator is Dr. Eleanor F. Helin, with co-investigators Dr. David L. Rabinowitz and Dr. Steven H. Pravdo, also the project manager. NEAT began observing in December 1995 and observes for 12 nights each month centered near the new moon.

Discoveries are reported to the Minor Planet Center, whose WWW site contains new objects which require confirmation by observers (NEAT Homepage).

Techniques

Since NEAT is an autonomous detecting system of asteroids and comets, a simple solution to finding NEAs is shown. NEAT detects moving objects – asteroids and comets – by observing the same part of the sky 3 times during an interval of about 1 hour. The automatic data analysis system searches for moving objects by comparing the 3 images and subtracting out the fixed stars.

Dr. Rabinowitz developed a representation of the 3 images, shown in Fig. 10.

Figure 10: Processed raw image of NEAT showing 3103 Eger.

The candidate object appears to move from the upper left to the lower right. Each small panel is a 25 x 25 pixel sub-image extracted from the 4096 x 4096 pixel raw images. The moving object is found at 3 different positions during the 3 observation times. The top 3 panels are centered at position 1 during times 1-3. The moving object is centered in panel 1 only. The middle 3 panels are centered at position 2 during times 1-3. The moving object is centered in panel 2 only. The bottom 3 panels are centered at position 3 during times 1-3. The moving object is centered in panel 3 only. Notice that fixed stars, if any, remain in the same relative position in all 3 horizontal panels. The moving objects can only appear in the center of the panels in the upper left to lower right diagonal. All the other panels are “anti-coincidence” tests. The object can not be centered in any of them.

Results

NEAT has detected 8162 asteroids since 1995, of which 4527 are new objects. There have been 698 new designations.

5.2 SPACEBORNE MISSIONS

Ground-based observations, inspite of their large network, can cover only a small portion of the sky and can operate only at night subject to proper weather conditions. In lieu of these problems, satellite based sensors have become popular. High altitude satellites ( 20,000 Km) can provide a very good coverage of the Earth’s surface without being hampered by day/night and weather. Also, since most of the impacting objects deposit their energies at high altitudes, it is easier to detect their emissions by a spacecraft, rather than by ground-based sensors which do not see the emissions below the cloud top (Taglaiferri, 1994).

5.2.1 Infrared Astronomical Satellite(IRAS)

The Infrared Astronomical Satellite (IRAS) was designed to perform an unbiased, sensitive all sky survey at 12, 25, 60 and 100 microns of infrared point sources. IRAS did an asteroid and comet survey that derived data on 1811 known asteroids and 25 comets.

IRAS was a spacecraft with a telescope mounted in a liquid helium cooled cryostat with a f/9.6 Ritchey-Chretien design having a 5.5 m focal length and a 0.57 m aperture. The mirrors were made of beryllium and cooled to around 4 K.

IRAS was launched in January of 1983 into a sun-synchronous near-polar (99 degree) orbit, which precessed by about a degree each day. The celestial sphere was divided into “lunes” bounded by ecliptic meridians 30 degrees apart. The survey covered the sky by “painting” each lune in overlapping strips as the satellite scanned through the lune during each orbit. Most (96%) of the sky was covered by at least two hours-confirming scans (HCONs) and 2/3 of the sky was covered by a third (which used a slightly different observing strategy). Some sources, near the ecliptic poles, had more than three hours-confirmed coverages.

One HCON was separated from the next by up to several months. Thus the data combined into a single image from the survey may have been taken over a period of many months, and are subject to variations in the actual observed foreground due to the changing geometry with which the zodiacal light was observed and to variations in detector response.

The focal plane assembly contained the survey detectors, visible star sensors for position reconstruction, a Low Resolution Spectrometer (LRS) and a Chopped Photometric Channel (CPC). The focal plane assembly was located at the Cassegrain focus of the telescope and was cooled to about 3 K.

IRAS was decommissioned after nine months of service due to lack of liquid helium to cool the focal plane arrays. Data was collected by IRAS and can be ordered by contacting IPAC.

5.2.2 Infrared Space Observatory (ISO)

The infrared region of the spectrum is of great scientific interest, not only because it is here that cool objects (10-1000 K) radiate the bulk of their energy, but also because of its rich variety of diagnostic atomic, ionic, molecular and solid-state spectral features. Measurements at these wavelengths permit determination of many physical parameters of astronomical sources, such as energy balance, temperatures, abundances, densities and velocities.

ISO provides astronomers with a unique facility of unprecedented sensitivity for a detailed exploration of the universe ranging from objects in the solar system right out to the most distant extragalactic sources for eighteen months.

Satellite Configuration

The satellite essentially consists of a large liquid-helium cryostat, a telescope with a 60-cm diameter primary mirror, four scientific instruments, and the service module, as seen in Fig. 11.

Figure 11: ISO satellite drawing.

The instrument complement consists of an imaging photo-polarimeter, a camera, a short wavelength spectrometer, and a long wavelength spectrometer. These instruments are cooled to between 2K and 8K. The following Table shows the characteristics of the instruments.

Table 2: Main Characteristics of ISO Instruments.

Orbit

The ISO is in a 24 hour period highly-elliptical orbit with perigee at 1000 km and apogee at 70.500 km. The lowest parts of the orbit lie inside the Earth’s van Allen belts of trapped electrons and protons. Inside these regions, the majority of ISO’s detectors are scientifically unusable due to effects caused by radiation impacts.

About 16 hours of each day are spent outside the radiation belts and thus allowing all detectors to be operated. Since there is no on-board storage, ISO needs to be in continuous contact with a ground station while doing scientific studies.

Figure 12: ISO Orbit showing the Earth’s Radiation Belts.

Science

Comets retain the original content of the primordial solar nebula in the form of ice and trapped dust from which our solar system condensed. Studying comets allows a unique probe into the history of the solar system and its relation to the interstellar medium. ISO is able to detect comets at large heliocentric distances (5 AU) and study the onset of activity (emission of gas and dust, the evolution and composition of the coma) when a comet approaches the Sun. Cometary dust and nucleus have a low temperature and albedo, and are thus best detectable in the infrared. The spectral, spatial and sensitivity capabilities of ISO allows for a thorough comparison of the general interplanetary dust with the properties of dust close to its probable sources, comets (cometary trails) and asteroids (asteroidal bands). Appendix D contains the list of asteroids and comets found in the database of ISO.

5.2.3 Near Earth Asteroid Rendezvous (NEAR):

Mission summary and mission profile

The Near Earth Asteroid Rendezvous (NEAR) mission was launched on February 17, 1996 from a Delta II launch vehicle and is the first launch in the Discovery Program.

Figure 13: NEAR Spacecraft.

It has the shape of an octagonal prism and will be the first spacecraft to orbit an asteroid with its primary mission being a rendezvous with asteroid 433 Eros on February 6, 1999. The following figures show the mission trajectory and rendezvous geometry.

Figure 14: NEAR Mission Trajectory.

Figure 15: NEAR Rendezvous Geometry.

The primary scientific goals are to measure:

Bulk properties: Size, shape, volume, mass, gravity field, and spin state

Surface properties: Elemental and mineral composition, geology, morphology, and texture

Internal properties: Mass distribution and magnetic field.

The various scientific instruments which form a part of its payload are:

Multispectral Imager System (MIS)

X-ray/Gamma-Ray Spectrometer

Near-IR Spectrograph (NIS)

Magnetometer

Laser Rangefinder

Radio Science

The figure below is a schematic representation of the wavelength regions in which the above instruments operate.

Figure 16: Regions of instruments operations.

After a brief section on the comets 433 Eros and 253 Mathilde, which the NEAR will be approaching, the MIS and NIS instruments will be discussed in detail.

433 Eros

Eros is the first NEA (belongs to Amors group) to be discovered and is the second largest. It is an S-type (silicaceous), potato-shaped asteroid with an estimated dimension of 40.5 by 14.5 by 14.1 kilometers and a geometric albedo of 0.16.

Figure 17: 433 Eros.

As mentioned earlier, its orbit crosses Mars’ path but doesn’t intersect that of Earth. It has an orbital period of 1.76 years and an inclination of 10.8 degrees to the ecliptic. Its closest approach to Earth so far has been 0.15 AU.

The interest in Eros has stemmed from the fact that it has approached the Earth several times, and although it will never hit the Earth, it has helped in refining the value of the Astronomical Unit. It has a varied composition with one side appearing to have a higher pyroxene content and a facet-like surface, while the opposite side displays higher olivine content and a convex- shaped surface. It has shown no evidence of air or water on its surface and its temperature varies from 100 C to -150 C between day and night, respectively.

The MSI and NIS instruments along with other instruments will provide detailed information on its morphology and composition.

253 Mathilde

Mathilde is a C-type asteroid with moderate absorption features in the ultraviolet region but an almost flat spectra in the visible region (Richard P. Benzel et al., 1996). It has an orbital period of 4.30 years and an inclination of 6.7 degrees. Its geometric albedo is 0.036 and has a diameter of 61 kilometers

Figure 18: 253 Mathilde.

The NEAR mission will give an opportunity to study the close-up images of a C-type asteroid, for the first time, when it is closest to Mathilde on June 27, 1997. The MSI will be used to make observations. The relative reflectances of Mathilde (with respect to the clear filter MSI wavelength of 7000A) at the central wavelength region for the MSI, is given in the following table (Binzel et al., 1996).

Table 3: Relative reflectances of Mathilde with respect to the clear filter MSI wavelength.

The spectral characteristics of Mathilde is shown in the following plots.

Figure 19: Spectral Characteristics of 253 Mathilde.

Multispectral Imager System (MSI)

The Multi-Spectral Imager System(MSI) is one of the primary instruments on the NEAR spacecraft and its measurements will be critical in understanding the surface processes, composition and compositional variation, and geologic history of Eros.

The MSI consists of a five-element refractive telescope with passively cooled Si CCD and electronics, a filter wheel, and a computer (digital processing unit, or DPU).

Figure 20: MSI Instrument.

The telescope is f/3.4 with a 168-mm focal length. The imager provides a field of view of 2.25° x 2.9°, divided into an array of 244 x 537 pixels on the CCD. Pixel angular resolution is 95 x 161 microradians, corresponding to 9.5 x 16.1 m from a distance of 100 km.

Brightnesses are encoded to 12 bits. The Si CCD is sensitive to the wavelength range of 400-1100 nm (visible and short-wavelength near-infrared light). A filter wheel has seven spectral filters designed primarily to discriminate iron-containing silicate minerals, and one broadband filter for low-light imaging and optical navigation.

The DPU contains software that controls the instrument and supports automatic exposure time control, acquisition of image sequences, and a three-tiered compression system offering several modes of lossless compression and seven lookup tables for converting the data from 12 to 8 bits.

MSI will image the surface of EROS at several spatial resolutions, as high as 3-5 meters. These high resolution images will reveal the distribution and thickness of the asteroid’s fragmental surface layer or “regolith,” the history of impacts by fragments of other asteroids and comets that is recorded in craters, the character and locations of fractures of the asteroid’s body, and the processes that affect the surface layer. MSI along with radio tracking and laser altimeter will aid in determining the exact shape and density of the asteroid, which is unknown so far. The shape will help in determining the origin of the asteroid. MSI’s seven spectral filters are capable of distinguishing between the spectra of sunlight reflected by the major iron-containing mineral constituents of Eros’s surface. Due to its greater spatial resolution as compared to the near-infrared spectrograph, color imagery from MSI can be used to obtain the compositional information down to a spatial scale of meters. Correlation of the compositional variations with specific surface features such as fractures or craters will allow discrete rock units to be mapped and can thus be a beacon to the internal structure.

Figure 21: Hyakutake NEAR MSI Image..

Figure 21 shows the MSI photograph of the comet Hyakutake also designated as Comet C/1996 B2. This comet approached within 0.1 AU of the Earth (about 15 million km) on March 25, 1996. This gives an idea of the imaging capabilities of the MSI instrument.

Near Infrared Spectrograph

The Near-Infrared Spectrograph (NIS) is one of the five instruments on the NEAR spacecraft. It is designed to map the mineralogic composition of 433 Eros using the spectrum of reflected sunlight. Spectra measured during approach, flyby, and orbit of Eros will cover surface regions as small as 300 meters. Each spectrum has 64 spectral channels covering the near-infrared wavelength region from 800-2700 nm, which allows mapping of the mineral composition of Eros especially the rocks.

The NIS is a scanning spectrometer that measures near-infrared light in the wavelength range 800-2700 nm.

Figure 22: NIS Instrument.

A gold scan mirror that rotates over an angle of 140 degrees controls the direction of viewing. Light reflected from the scan mirror enters through either of two shutters that provide a 0.38 degrees x 0.76 degrees or a 0.76 degrees x 0.76 degrees field of view, to accommodate different illumination conditions. These provide spot sizes of 0.65×1.3 km or 1.3×1.3 km from a 100-km distance. This light is dispersed off a diffraction grating onto two detectors. A 32-element germanium (Ge) detector measures the wavelengths 804-1506 nm in 21.6-nm increments; a 32-element indium-gallium arsenide (InGaAs) detector measures the wavelengths 1348-2732 nm in 43.1-nm increments. The gain of the Ge detector can be set at 1x or 10x, to accommodate various illumination conditions. A movable shutter can block the aperture to provide calibration measurements of the background dark level. A solar-illuminated gold calibration target is viewable for radiometric calibration. The computer (digital processing unit, or DPU) is shared with the NEAR magnetometer, and provides the software to control instrument function.

The spectrum of reflected sunlight can be used to study the minerology of Eros, since different mineral have a characteristic reflectance spectrum as can be seen in the following plot.

Figure 23: Spectral Reflectance of Eros.

Compositional variations, which have been recorded by ground-based observations will be studied in greater detail by the NIS instrument. This detailed study will aid in understanding the geologic evolution of Eros and thus identify its relation to meteorites.

5.2.4 Small Missions to Asteroids and Comets (SMACS)

SMACS (Small Missions to Asteroids and Comets) is a series of four low cost Pegasus XL to be launched in November 1998, for flyby encounters to the following specially selected targets in near-Earth space:

P/Honda-Mrkos-Pajdusakova, an active comet nucleus.

Phaethon, an F-type asteroid likely to be a dormant or extinct comet nucleus.

1986 DA (M-type), possibly a fragment of the core of a collisionally destroyed planet.

Ra-Shalom (C-type), primitive material from which the planets were constructed.

The mission will characterize the physical, compositional, and geological state of extreme types of asteroids and cometary nuclei and conduct in situ studies of NEOs. It will consist of a pushbroom infrared/visible imaging spectrometer (PIRVIS) with a 20-cm telescope, l-deg FOV, panchromatic, visible color, and short-wave infrared imaging in the range of 0.85-4.8 micron with / ~300. The visible range multicolor camera is designed to provide ~4700 spectral and geological images and over 5000 spectra (0.85-4.8m) of resolved locations at each of the targets (~1.2 Gbit/encounter) with surface resolutions up to 4 m/pixel for some encounters.

Figure 24: PIRVIS.

6.0 Spectral Properties of NEOs

Relatively few spectroscopic observations of NEAs have been obtained due to their small sizes, faint apparent magnitudes, and limited intervals of visibility. Studies of the visible spectra of majority of NEAs indicate that they are similar to the ordinary chondrite meteorites (Binzel et al., 1996). Spectroscopic measurements, in the visible wavelength range from 0.45 – 0.95 micrometers, of 35 NEAs were conducted using a low-resolution spectrograph and solid-state charge coupled device detectors attached to the 2.4 m Hiltner telescope at Kitt Peak, AZ. Most of the NEA spectra have a moderate absorption band in the 1 micron region due to the presence of olivine and pyroxene. However some of the NEOs display a strong absorption band at this wavelength and their spectra resemble those obtained from OC meteorites as can be seen in the following figure.

Figure 25: Comparisons of 6 near earth asteroids having deep 1 micron absorption bands. (Please refer to Appendix C for additional spectra.)

The reason for the NEOs to have a continuous range of spectral properties as seen in this figure could possibly be due to the fact that they are composed of distinct lithologic units derived from larger asteroids, thus rendering them with a variety of mineral and spectral properties. This continuum could also be due to the variation in opaque mineral particle size, which could lead to a range of absorption band depths. The varying surface age of these small objects (since they have a tendency to experience a lot of collisions) would lead to a continuous mineral spectra and it is seen that the asteroids with the youngest surfaces resemble the OC meteorites most closely.

Thus the study of the spectral characteristics of NEOs can shed light on the role played by these objects in the form of a link between the previously separated domains of OC meteorites and the most common S-type asteroids.

7.0 Conclusions: FUTURE HAZARD MITIGATION

As mentioned at the beginning of the report, impacts due to comets and asteroids have had hazardous effects on the Earth. With visible, IR and radar technology it is now possible to detect and track these NEOs both from the ground as well as in space. However, just being aware of these objects will not mitigate their impact probabilities. Several methods have been suggested to defend the Earth from these NEOs. Amongst these are ones where we try to deflect or fragment the objects.

7.1 Orbit Deflection and Fragmentation

Just as perturbations bring objects into the Earth’s trajectory, it is possible to perturb the orbit of these NEOs to deflect them away from the Earth, if they are found to have Earth-impacting trajectories. Typically, NEOs having a size range from 0.1 to 10 Km have a collision flux in the range from 10^-3 to 10^-8 per year. Deflections can be achieved either on a short or long time scale as compared to the orbital period. A small NEO can be deflected by direct impact or by using gravity controlled impact and explosions. A neutron rich device can also be used to transfer the momentum impact from a nuclear explosion to the asteroid. Another method is to use a surface charge to crater the NEO. This however needs care, since the ejecta from the crater can be big enough to threaten the Earth. However, charge burial in a NEO requires in situ drilling of objects under low gravity conditions. Thus it is more feasible to mitigate hazards due to NEOs by just deflecting their orbit by the application of a change in velocity delta v, along the line of motion. This leads to a maximum deflection.(Ahrens, 1994).

With the advances in Remote Sensing instruments, detection and tracking of Near Earth Objects has become better studied and productive field. Although there is no immediate danger of any impacts by these objects on the Earth, it is always nice to be ahead in the “Hide and Seek Game” of Near Earth Objects.

8.0 References

Ahrens, T. J., and Harris, A. W., 1994. Deflection and Fragmentation of Near-Earth Asteroids: in Hazards Due to Comets and Asteroids, T. Gehrels, ed., Univ. of Arizona Press, p. 897 – 927.

Binzel, R. P., Burbine, T. H., and Bus, S. J., 1996, Ground-based reconnaissance of asteroid 253 Mathilde: Visible wavelength spectrum and meteorite comparison, Icarus 119, 447-449

Binzel, R. P., Bus, S. J., Burbine, T. H., and Sunshine, J. M., 1996, Science Vol 273, p. 946- 948

Carusi, A., T. Gehrels, E. F. Helin, B. G. Marsden, K. S. Russell, C. S. Shoemaker, E. M. Shoemaker, and D. I. Steel, 1994, Near-Earth objects: present search programs: in Hazards Due to Comets and Asteroids, T. Gehrels, ed., Univ. of Arizona Press, pp. 127-147.

Chapman, C. R., A. W. Harris, and R. Binzel, 1994, Physical properties of near-Earth asteroids: Implications for the hazard issue: in Hazards Due to Comets and Asteroids, T. Gehrels, ed., Univ. of Arizona Press, p. 537-549.

Douglas, G., Starting Out Right in CCD Imaging, http://www.skypub.com/ccda/ccdalib/startout.html.

Helin, E. F., and E. M. Shoemaker, 1979. Palomar Planet-Crossing Asteroid Survey 1973-1978. Icarus 40, p. 321-328.

Hill, David K. ÒGathering Airs Schemes for Averting Asteroid Doom.Ó Science 268:16 June 1995, pp 1562-3.

Tagliaferri, E., Spalding, R., Jacobs, C., Worden, S. P., and Erlich, A., 1994, Detection of meteoroid impacts by optical sensors in Earth orbit: in Hazards Due to Comets and Asteroids, T. Gehrels, ed., Univ. of Arizona Press, pp. 199 – 220

Verschuur, Gerrit L ÒThe End of CivilizationÓ. Astronomy, Setpember 1991, pp 51-54.

Weissman , P. R., 1990, The cometary impactor flux at the earth. In Global Catastrophes in Earth History, eds. V. L. Sharpton and P. D. Ward, Geological Society of America Special Paper 247 (Boulder: Geological Soc. Of America), pp. 171-180.

Xu, S., Binzel, R. P., Burbine, T. H., and Bus, S.J., 1995, Small Main-Belt Asteroid Spectroscopic Survey: Initial Results, Icarus vol. 115, pp 1-35.

http://cfa-www.harvard.edu/cfa/ps/NEO/TheNEOPage.html

http://ccf.arc.nasa.gov/sst/report/neo_rpt.html

http://www.ipac.caltech.edu/ipac/iras/iras.html

http://isowww.estec.esa.nl/

http://huey.jpl.nasa.gov/~spravdo/neat.html

http://sd-www.jhuapl.edu/NEAR/

Appendix A – List of NEOs

To obtain a complete listing of Known Near Earth Objects, please refer to the NEO homepage: http://cfa-www.harvard.edu/cfa/ps/NEO/TheNEOPage.html.

Appendix B – Facts of 433 Eros and 253 Mathilde

B.1 Eros Facts

Size: 40.5 km x 14.5 km x 14.1 km

Approximate mass: 5 x 10^15 kg

Rotation Period: 5.270 hrs

Orbital Period: 1.76 yrs

Spectral Class: S

Semimajor Axis: 1.458 AU

Perihelion Distance: 1.13 AU

Aphelion Distance: 1.78 AU

Orbital Eccentricity: 0.223

Orbital Inclination: 10.8 deg

Geometric Albedo: 0.16

B.2 Mathilde Facts

Diameter: 61 km

Approximate mass: 200. x 10^15 kg

Rotation Period: 418. hrs

Orbital Period: 4.31 yrs

Spectral Class: C

Semimajor Axis: 2.645 AU

Perihelion Distance: 1.94 AU

Aphelion Distance: 3.35 AU

Orbital Eccentricity: 0.2663

Orbital Inclination: 6.7 deg

Geometric Albedo: 0.036

Appendix C – Spectral Reflectances of some NEOs

Appendix D – List of Asteroids and Comets Tracked by ISO

ASTEROIDS

1984 BC 219001

1984 KB 216063

1992 QB1 219002

1993 FW 219003

1993 HA2 219004

1993 RO 219005

1993 RP 219006

1993 SB 219007

1993 SC 219008

1994 ES2 219009

1994 EV3 219010

1994 GV9 219011

1994 JQ1 219012

1994 JS 219013

1994 JV 219014

Adonis 212101

Agamemnon 210911

Amphitrite 210029

Aneas 211172

Asterope 210233

Aten 212062

Bamberga 210324

Ceres 210001

Chiron 212060

Chloris 210410

Cybele 210065

Davida 210511

Deiphobus 211867

Desiderata 210344

Diomedes 211437

Dione 210106

Dionysus 213671

Dora 210668

Egeria 210013

Euphrosyne 210031

Europa 210052

Eurydike 210075

Feronia 210072

Fortuna 210019

Frigga 210077

Gaspra 210951

Haidea 210368

Hektor 210624

Hephaistos 212212

Herculina 210532

Hermione 210121

Hestia 210046

Hidalgo 210944

Hispania 210804

Hygiea 210010

Ida 210243

Ino 210173

Interamnia 210704

Irmintraud 210773

Juno 210003

Kassandra 210114

Kleopatra 210216

Kythera 210570

Lacadiera 210336

Lyapunov 215324

Massalia 210020

Melete 210056

Mimistrobel 213840

Nemausa 210051

Nuwa 210150

Nysa 210044

Oljato 212201

Palisana 210914

Pallas 210002

Pandora 210055

Papagena 210471

Patientia 210451

Phaethon 213200

Pholus 215145

Polyxo 210308

Rollandia 211269

Sappho 210080

Shipka 212530

Simeisa 210748

Sylvia 210087

Tabora 210721

Themis 210024

Tokio 210498

Tombaugh 211604

Toutatis 214179

Vesta 210004

Wilson-Harrington 214015

COMETS

P/Arend-Rigaux 220037

P/Ashbrook-Jackson 220002

P/Boethin 220019

P/Borrelly 220007

P/Churyumov-Gerasimenko 220013

P/Clark 220008

P/Comas Sola 220014

P/Encke 220004

P/Gehrels 2 220021

P/Giacobini-Zinner 220031

P/Grigg-Skjellerup 220023

P/Gunn 220016

P/Hale-Bopp 220048

P/Haneda-Campos 220022

P/Hartley 1 220042

P/Hartley 2 220028

P/Helin-Roman-Alu 1 220033

P/Helin-Roman-Crockett 220038

P/Honda-Mrkos-Pajdusakov 220012

P/IRAS 220017

P/Jackson-Neujmin 220010

P/Johnson 220025

P/Klemola 220039

P/Kopff 220015

P/Longmore 220011

P/Machholz 1 220044

P/Metcalf-Brewington 220030

P/Mueller 1 220041

P/Pons-Winnecke 220049

P/Reinmuth 1 220035

P/Russel 3 220026

P/Schwassmann-W. 1 220001

P/Schwassmann-W. 2 220003

P/Schwassmann-W. 3 220032

P/Shoemaker-Holt 1 220034

P/Smirnova-Chernykh 220040

P/Spacewatch 220036

P/Taylor 220027

P/Tempel 1 220006

P/Tempel 2 220005

P/Tempel-Tuttle 220029

P/Tritton 220043

P/Wild 2 220020

P/Wirtanen 220018

P/Wolf-Harrington 220024

P/d’Arrest 220009