Near-Earth Objects

Abstract

The chances of a random piece of rock striking Earth still exists, but probably to a much reduced level of threat. During the early years in the creation of our Solar System, there was plenty of debris that had yet to settle into some form and order, with constant collisions of large bodies before any structure was imposed. As time passed and natural order and stability were imposed, collisions were reduced in number, and, with it, the chances of being hit subsequently fell. The remnants of this settling down phase were either ejected into deep space or conformed to some kind of symmetry, even though within that symmetry the threat of being hit remains.

The Recognized Threat

The chances of a random piece of rock striking Earth still exists, but probably to a much reduced level of threat. During the early years in the creation of our Solar System, there was plenty of debris that had yet to settle into some form and order, with constant collisions of large bodies before any structure was imposed. As time passed and natural order and stability were imposed, collisions were reduced in number, and, with it, the chances of being hit subsequently fell. The remnants of this settling down phase were either ejected into deep space or conformed to some kind of symmetry, even though within that symmetry the threat of being hit remains.

One such chaos within the order is presented to us by near-Earth objects. NEOs have been in existence since the formation of the Solar System, and, while knowledge has been gathered as to their size and orbital paths, it is unlikely that a point will ever be reached when the Solar System settles down into a steady and rhythmic accord, where everything within it behaves in a pattern that is constant, predictable, and hazard-free. There is a state of flux within stability itself, and that’s without considering influence from outside of the Solar System. We pride ourselves in the predicting and modeling of astronomical occurrences, from our Moon’s phase to the occultation of a star by a planet, but there remain many instances where the application of modeling simply can’t be applied with a healthy degree of certainly that what we are told matches that which eventually transpires.

The universe cannot function in a healthy manner on such a diet of predictability. A random element must be introduced so that the so-called existing order is allowed to grow and evolve. This applies to NEOs , for here, monitoring affords us relative peace of mind that some governance on order is maintained. However, it does not account for a random collision in the Asteroid Belt, or a comet release from the Oort Cloud . It does not account for the movement of said bodies and their eventual resting place. We can observe, but we can do little more.

‘Near-Earth objects’ is the collective term for all objects in the vicinity of Earth , with near-Earth asteroid (NEA) defining that particular object clearly and distinctly as an asteroid, rather than any other body, such as a comet. By the end of August 2016, 14,686 NEOs had been logged and their positions recorded, with 1729 of these classed as potentially hazardous asteroids . If discussing the potentially hazardous phenomenon in its entirety, the term used would be potentially hazardous objects (PHOs). However, careful study is required to establish the exact classification of the body (Fig. 6.1).

Fig. 6.1

When first observed by NASA’s Neowise Team on December 31, 2013, Comet Catalina gave the distinct appearance by its movement of being that of an asteroid rather than a comet (courtesy of NASA)

With regard to the NEAs in particular, much research has been undertaken in an attempt to understand what makes them ‘tick,’ not only in terms of their chemical composition, about which spacecraft have enlightened us enormously, but, most importantly, their movements over a period of time. One such research project has been investigating spin states of NEAs, analyzing the fundamental core properties of designated asteroids. Understanding the pole directions and spin rates of this sample could be key to our overall knowledge as to how the population works as a whole. Spin rates, with regard to shape and size, can be helpful in predicting the long-term trajectories of such objects, not only providing crucial data as to where they are expected to end up but also passages close to Earth in the near future, which will offer us an opportunity to send probes to investigate and analyze such bodies close up. Also, when the technology exists, we could possibly redirect or steer them clear.

The ability to influence the direction of a body is both a comfort and a worry. Knowing that in the future there would be some possibility of halting or deflecting an incoming rock is a great reassurance, although a body of a certain size would be a challenge. The worry element is the disruption, albeit for Earth’s benefit, to the order of the Solar System. Altering the path or direction of one rock could create an imbalance, as if the order of the Solar System had been tampered with.

The NEAR Mission

Whereas lunar and planetary exploration were the primary targets for exploration over many decades and by many countries, the focus has now switched into understanding other components of the Solar System, with particular attention paid to the study of NEOs . Launched on February 17, 1996, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft made successful flybys of asteroid 253 Mathilde on June 27, 1997, and asteroid 433 Eros on December 23, 1998 (Fig. 6.2).

Fig. 6.2

Artist’s rendering of the NEAR (Near Earth Asteroid Rendezvous) spacecraft encounter with the asteroid Eros (courtesy of NASA)

Discovered by Austrian astronomer Johann Palisa on November 12, 1885, this main-belt asteroid, measuring around 50 km across, travels in an elliptical orbit and takes around four years to orbit the Sun. A portion of 253 Mathilde’s orbit sees the asteroid reach the limb of the belt, but still well within the parameters of both Mars and Jupiter , and not intersecting either planet’s orbit.

The rate of rotation for 253 Mathilde exceeds 17 days, making the body a relatively slow rotator, with other asteroids normally completing a rotation between 2 and 24 h. Only two other asteroids, 1220 Clocus and 288 Glauke, are known to have longer rotational periods. This slow rotation was at first a puzzle, but a credible explanation would stem from the fact that the asteroid has a companion, its own satellite.

NEAR encountered 253 Mathilde at a distance of 1200 km but, despite taking over 500 images, was only able to capture one hemisphere. The very dark appearance of the asteroid is comparable to fresh asphalt, hence the low albedo, reflecting just 4% of its surface. This C-type asteroid’s composition, essentially dominated by phyllosilicate minerals, is shared by carbonaceous chondrite meteorites. The NEAR spacecraft was also able to establish that 353 Mathilde consisted of essentially a collection of very loosely packed debris, meaning that up to 50% of the asteroid’s inner volume is space.

NEAR’s photographs revealed 254 Mathilde to be a crater-strewn world, with two large craters, Ishikari, 30 km across, and Karoo, 31 km across. The appearance of the craters would seem to imply that some of the asteroid’s mass had been lost in collisions, resulting in these defined gouges. The albedo deep within all of the photographed craters mirrored that of the surface, with no discernible shift in brightness levels even at these lower levels. At a distance of 17,895 km, NEAR was able to reveal one particular impact crater that was nearly 10 km wide.

The primary mission of NEAR was to encounter Eros late in 1998. The rendezvous with 253 Mathilde was an afterthought from scientists on Earth, only discovering a year before NEAR’s launch that it was possible not only to visit Eros but to rendezvous with 253 Mathilde. The deliberations over targeting another asteroid and not just Eros could have jeopardized the whole mission. Should they take a risk in using precious fuel to divert NEAR to 253 Mathilde then go on to Eros? The great uncertainty as to whether NEAR could survive the first asteroid encounter would have to be considered, given the close contact with all manner of debris associated with close flybys. However, the gamble was taken, and it duly paid off, with this rather low-cost mission of NASA’s delivering invaluable scientific and photographic data of not just one but two asteroids.

The slow rotation of an asteroid merits further investigation. With 253 Mathilde taking 17 days to rotate, perhaps examination of other slow-rotating asteroids would also show a companion? If so, would the presence of a satellite or indeed a possible binary partner account for all asteroids that rotate slowly? Could any combination have a crucial bearing on orbital deviations over a long period of time, perhaps steering them on a collision course with a larger body, such as a planet?

The Eos Family

The first of the aforementioned asteroids, 1220 Clocus , is a member of a group of asteroids known as the Eos family, with photoelectric studies conclusively showing two noticeable periods where there is variation in reflective light output. Could 1220 Clocus have a satellite, or could it be a binary asteroid? That would explain the noticeable spikes in light. Or is it just that, at certain times, the side presenting itself towards Earth has significantly less reflective material than the rest of the body? This proved to be an unlikely theory, as no such matter difference on an asteroid’s surface could account for that.

The Eos family, or Eaon family, is a prominent group of asteroids that orbits within the Asteroid Belt between Mars and Jupiter . Named after one of its members, 221 Eos , the bulk of the family hold a steady and consistent orbit around the Sun, although a number of bodies within the family have rebelled and left the group. These more radical elements are noticeably younger within what is essentially an old collection of fragments, the result of an ancient collision. It is possible that these younger members were perhaps captured or realigned into the group by planetary resonance, with their orbit less stable and less cohesive than more established family debris, possibly orbiting in a path wider and more susceptible to influence. In turn, their wider orbital path would leave them exposed to outside influence, allowing a number of the fragments to break away. As a further development, those that haven’t broken away from the fold appear to have informally clustered in their own groups.

Of the 4,400 members of the Eos family, the majority fall into the S-type category of asteroid, although examination of the group members using infrared showed differences within the category itself. As a result, and in order to keep some reign on the classification, the family was given its very own category: K-type asteroids. C-types are the most popular category of asteroid composition, S-types are second, and approximately 17% of all asteroids fit into the K-type category.

Under the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) classification, K-type asteroids are further grouped together with other similar strand members, the entire range being placed under the S-type. The SMASS was a project initiated at the Massachusetts Institute of Technology (MIT) in 1990 with the purpose of studying small asteroids within the main Asteroid Belt, in order of their spectral shape and color. This, together with their albedo rating, defines what types of asteroid they are.

Before SMASS , the first taxonomy used on asteroids was proposed by American astronomer David J. Tholen in 1984. Named the Tholen Spectral Classification , it used a combination of data collected during the Eight-Color Asteroid Survey (ECAS) in the 1980s, plus albedo measurements. Tholen based his classification on 978 asteroids, with 14 different asteroid types established. In turn, SMASS analyzed 1447 asteroids, classifying the findings into 26 different asteroid types.

In 1918, Japanese astronomer Kiyotsugu Hirayama (1874–1943), while studying at Yale University, discovered that more asteroid orbits were similar to one another than chance would allow. Hirayama concluded that therefore families of asteroids existed, with 19 members of the Eos family. By 1993, this figure had risen to with 289.

Hirayama’s studies were fundamental in establishing that certain groupings of this kind exist, with the orbits of 790 asteroids studied. By 1919, five Hirayama families, as they were collectively known, were established. Apart from the Eos, which at this point had climbed from 19 to 38, there was the Themis family with 31; the Koronis family with 23; the Maria family with eight; and the Flora family with 81.

Kiyotsugu also hypothesized that these families, of which many more have subsequently been discovered, were the result of catastrophic collisions with a parent body, an interpretation still widely accepted in the astronomical community today. In the early formation of the Solar System, significantly larger bodies most likely existed where only fragments are present today, with many years of further collisions taking place before an historical chain presented itself. The chain consisted of larger bodies that survived the encounters intact, right down to the tiniest fragment that perhaps still travels in the orbit of the Asteroid Belt. The collision that resulted in the formation of the Eos family places its members’ collective age at 1.1 billion years old.

Discovered by Karl Theodor Robert Luther (1822–1900) in 1890, and named after Glauke, a daughter of Creon, who was king of Corinth in Greek mythology, 288 Glauke has a rotational period of 50 days. 288 Glauke is an S-type asteroid, and although comparisons with other S-type asteroids place it in a class of its own for rotation periods, there is a similarly slow but not as slow same-class asteroid, 4179 Toutatis , with a rotation period of just over seven days.

4179 Toutatis

First sighted on February 10, 1934, and named as object 1934 CT, 4179 Toutatis was considered lost as an asteroid for many years. It was presumed that the asteroid was destroyed in a collision with another body or, if not destroyed, had its orbit radically altered so that it left the Solar System, never to return. A fair proportion of asteroids and other such debris are only ever recorded once then lost forever. However, on January 4, 1989, French astronomer Christian Pollas rediscovered it, giving it its current name Toutatis, after the Celtic god of tribal protection.

Pollas, who also has a main-belt asteroid named in his honor, has many a find to his name, including 4179 Toutatis, which he co-discovered with Belgian astronomer Eric Walter Elst. Elst’s find count is staggering, literally thousands, including 4486 Mithra, which, like 4179 Toutatis, is a potentially hazardous object. Furthermore, 4179 Toutatis is both an Apollo and an Alinda, with its orbit intersecting that of Mars . Earth and Jupiter have affected the asteroid’s orbit, resulting in frequent approaches to Earth.

The asteroids of the Alinda family are held in their orbit by a 1:3 resonance with Jupiter , which results in their being close to a 4:1 resonance with Earth. Any asteroid, or any object for that matter, in this resonance has its orbital eccentricity steadily increased over time by the gravitational interactions with Jupiter, until it eventually has a close encounter with an inner planet, such as Venus or Earth, subsequently disturbing this pattern and altering its path.

The first of the Apollo family to be discovered was 1862 Apollo, found by German astronomer Karl Reinmuth (1892–1979) on April 24, 1932. Apollo 1862 is a PHA itself and, most notably, the Chelyabinsk meteor was a member of this family.

In recent times, 4179 Toutatis has passed close to Earth in 2012 and 2016, with 2069 marked as the next closest approach, based on there being no future alterations to its current orbit. In 2012, the asteroid passed Earth at a distance of 6.9 million km, close enough to be spotted on Earth with high powered binoculars. Its next scheduled closest approach is November 5, 2069, when it will pass 2.9 million km away.

The rather odd shape of 4179 Toutatis would seem to indicate the merging of two distinct bodies, possibly a former binary, or the long-term fusing between two bodies of similar mass over time. The two lobes the asteroid presents (perhaps akin to a lopsided peanut), coupled with a ‘tumbling’ motion similar to that of 288 Glauke, would give a close-up observer a unique sight during its obscure rotational phase. As its rotation combines two separate periodic motions into a non-periodic result, the observer would see the Sun rise and set at random locations. There could be no such thing as a proper ‘day’ on 4179 Toutatis, its rotation providing the observer with periods of between 5.4 and 7.3 Earth days.

This tumbling motion of 4179 Toutatis is a result of the Yarkovsky or Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, which describes a force that acts upon the orbital motion of asteroids and other smaller bodies whose diameter tends to be less than 64 km. It is caused by light from the Sun, which heats up the asteroid to a point where the body is capable of radiating the energy away, which in turn creates a thrust-like motion. Over a long period of time this can have an effect on the asteroid’s orbit. Apart from making its presence known to Earth, 4179 Toutatis, because of its low inclination of orbit, makes frequent orbital transits, where the inner planets of Mercury , Venus , Earth, and Mars can appear to cross the Sun from the perspective of an observer on the asteroid.

Most PHAs herald from the Apollo family and, to a lesser extent, Aten asteroids. As with Apollo, the first to be discovered in this group was 2062 Aten—discovered by American astronomer Eleanor F. Helin (1932–2009) on January 7, 1976. As former principal investigator of the Near Earth Asteroid Tracking (NEAT) program, Elin recognized 2062 Aten to be a part of a group just like the other families and, although collectively smaller in number than the Apollos, still harboring a threat.

Bennu

The PHA 101955 Bennu is another member of the Apollo family, and attracts great interest because of its future Earth encounters. Discovered on September 11, 1999, by the MIT Lincoln Laboratory-funded Lincoln Near-Earth Asteroid Research (LINEAR) program, this particular asteroid is due to pass between Earth and the Moon in 2135. In doing so, calculations show that this will alter the path of Bennu , potentially putting the asteroid on a collision course with Earth during a later encounter. Measuring just under 500 m and traveling at around 63,000 mph (101,000 km/h), a collision of this magnitude is likely to cause considerable damage—not on the level of an extinction event but the equivalent of a nuclear warhead exploding, or that of a magnitude 7 earthquake (Fig. 6.3).

Fig. 6.3

The results of past impacts present us with the pitted, scarred, and cratered lunar surface of our closest companion. Picture taken by Apollo 8, December 21–28, 1968 (courtesy of NASA)

Passing every six years, Bennu is rated number three on the Palermo Technical Hazard Impact Scale , a scale defined by using the probability of impact and the kinetic energy released should it strike. Bennu and others like it are also under constant scrutiny from an automatic Near-Earth Asteroid Collision Monitoring System (SENTRY) , continually watching their movements for the possibility of orbital changes and subsequent future impact.

Earth’s Eyes on the Skies

Understanding how the field of space debris works can provide insight into dealing with the escalating problem that these fragments pose. However, even with this greater understanding, the task of monitoring and tracking debris is a substantial undertaking, particularly so given the rate of new finds and the concern over the proportion of objects that are not yet cataloged. One of the tools at our disposal is CoLiTec (CLT), a software package dedicated for the automatic discovery of any moving objects such as asteroids, comets, and other cosmic debris. Data processing for Comet ISON was carried out by CoLiTec following its discovery on 21 September, 2012 by Vitaly Nevsky and Artyom Novichonok (Fig. 6.4).

Fig. 6.4

Comet ISON shines brightly in this image taken on the morning of November 19, 2013. This is a 10-second exposure taken with the Marshall Space Flight Center 20” telescope in New Mexico. Image credit: NASA/MSFC/MEO/Cameron McCarty (courtesy of NASA)

Orbital models have been developed, the most intricate of which is NASA’s EVOLVE . Monitoring the predicted growth of low-orbit space debris, their mass, trajectory, and velocity, EVOLVE and similar programs project a future risk assessment. Several NASA-run surveys sweep 6000 square degrees of sky every night looking for NEOs. Another NASA project, using the Wide-field Infrared Explorer (WISE), has been scouring the skies since 2013.

The Inter-Agency Space Debris Coordination Committee (IADC) has also been set up to evaluate future risk, with many other countries now taking steps to develop policies with regard to space debris as, after all, it is a global issue. With the use of hindsight, the problem of space debris is also not just confined to NEOs , with human-generated debris an issue that should have been considered since the launch of the first satellite into space. This apparent neglect has seen a staggering amount of debris left for future generations to have to address and, as the decades pass, the litter that orbits our Earth continues to grow at an alarming rate.

Despite the show of concern about debris, the bombardment continues on a daily basis, not just from oddments of our own debris but from rocks large enough to make it through the atmosphere, many of which fall unseen on Earth’s vast oceans or on wasteland and Arctic tundra. The Minor Planet Center in Cambridge, Massachusetts, conducts one of the oldest ongoing surveys to date, having recorded and cataloged the orbits of many bodies since 1947. The attention to NEOs and acknowledgement of those bodies that cause a threat closer to home spawned NASA’s Near Earth Object Program, part of the Spaceguard program. Spaceguard is a collection of affiliated programs working in unison with regards to NEOs, their whereabouts and subsequent tracking. The UK has its own Spaceguard, the National Near-Earth Objects Information Centre, based in Powys, Wales.

Another NASA project, in conjunction with the United States Air Force, is LINEAR. The project, based at the White Sands Missile Range in Socorro, New Mexico, is made up of two 1 m telescopes and one 5 m telescope, and collectively they have been responsible for discovering thousands of objects a year. LINEAR uses very specialized equipment known as Ground-based Electro-Optical Deep Space Surveillance (GEODSS) telescopes.

In 1984, the Spacewatch project became the first to detect and discover asteroids and comets with CCDs, as opposed to photographic plates or film. A 0.9 m telescope and a 1.8 m telescope were used, the latter on fainter objects that become fainter after their discovery. Based at the Kitt Peak Observatory in Arizona, Spacewatch scours the skies for NEOs on a constant basis.

Drawing on international expertise is the German space agency’s (DLR) Institute of Planetary Research in Berlin. The DLR spearheaded NEOShield , a project set up to analyze realistic options for the prevention of collisions between Earth and NEOs. The main objective for consideration is whether to push or pull an approaching object, with such ideas including a “gravity tractor” to pull the approaching object away from Earth. Another idea on the table, which is far from new, is “blast deflection”—the detonation of a possible nuclear device near the object to literally deflect its course.

Various craft have visited NEOs , including a visit to asteroid (25,143) Itokawa. Itokawa is a 600 m potato-shaped asteroid named after Hideo Itokawa (1912–1999), a Japanese rocket pioneer. Launched on May 3, 2004, from the Kagoshima Space Center, the Japanese probe Hayabusa rendezvoused with the asteroid on September 12, 2005. Having landed on the asteroid after spending time matching Itokawa’s orbit, a first attempt to take samples of the body failed, but a subsequent effort proved successful, with a capsule containing the findings returning to Earth on June 13, 2010. Itokawa, an S-type asteroid, revealed itself to have probably been made up from two components, with the particles collected from its surface—only the third in history to be collected from an extra-terrestrial body, following NASA’s Apollo and the Soviet’s Lunar expeditions.

However, despite all the efforts to detect, track, analyze and visit all these objects, in essence, should one come our way, there is nothing we can do, especially if like the Chelyabinsk meteor it is spotted incredibly late. Technology simply has not advanced far enough to deal with such a catastrophe were a body of destructive size to head our way. We are ultimately at the mercy of NEOs and, for that matter, any comet or wandering asteroid that happens to visit our region of space.

With all the investment in the world, and even with a monumental global effort, the percentages do not favor success. Much of what is written about deflecting or dealing with any celestial body has no substance, and with the threat as real today as ever we still do little, because little is all we can do. If setting up committees, projects and programs to look into the threat were as prolific as the actual planning and subsequent construction of a ‘vehicle’ to defend Earth, then we might have some assurance that something is being done, but it isn’t, and therefore, as with the dinosaurs, we might ultimately endure the same fate.