Asteroids, also known as minor planets are the mostly rocky remnants, or planetessimals that formed the terrestrial planets, and perhaps the cores of the giant gas planets. The largest asteroid, Ceres is 945 kilometers (587 miles) in diameter, and the smallest asteroids can be as tiny as 1 meter (~3 feet) across or smaller, although there is no formal size cutoff for what defines an asteroid. In addition to displaying a wide range of sizes, asteroids show incredible variation in composition, shape, density, and orbital families; and collectively these variations carry clues about the structure and formational history of the solar system, including even how life on Earth may have begun.  Most asteroids appear to be composed of rocky matter, although a much smaller population of asteroids are primarily metallic bodies, beleived to be derived from the inner cores of once larger and differentiated asteroids that were shattered by collisions with other asteroids (or comets). Some asteroids contain ices, including water ice, and also water bound within the crystal latices of hydrated minerals; still other primitive asteroids are known to contain organic molecules, and have a chemical compositon believed to represent that of the early solar nebula from which the sun and planets were born. Learn more on NASA Science Asteroids.

Above: This view of the asteroid 243 Ida is a mosaic of five image frames acquired by the Galileo spacecraft's solid-state imaging system at ranges of 3,057 to 3,821 kilometers (1900 to 2375 miles) on August 28, 1993, about 3.5 minutes before the spacecraft made its closest approach to the asteroid. Galileo flew about 2400 kilometers (1,500 miles) from Ida at a relative velocity of 12.4 kilometers per sec (28,000 miles per hour). Asteroid and spacecraft were 441 million kilometers (274 million miles) from the Sun. Ida is the second asteroid ever encountered by a spacecraft. It appears to be about 52 kilometers (32 miles) in length, more than twice as large as Gaspra, the first asteroid observed by Galileo in October 1991. Courtesy NASA.

NEOs are 'Near-Earth Objects' and may be either asteroids or comets, although the vast majority of known NEOs are asteroids. They are termed 'near-Earth' because their orbits either cross or come very close to intersecting Earth's orbit. Therefore NEOs can approach near-Earth space and as we know, occaisionally impact Earth. More formally, the orbit of an NEO has a perihelion (closest approach distance to the sun) less than 1.3 astronomical units (AUs). An asteroid NEO is termed an NEA (near-Earth asteroid), and technically a comet NEO is an NEC (near-Earth comet). Near-Earth comets must also have a perihelian less than 1.3 AUs, and have an orbital period less than 200 years. As of February 2021, there are over 25,000 known NEOs. More on NEOs available on JPL's Center for NEO Studies (CNEOS).

Animation below showing the near-Earth approaching asteroid 2017 EA. This asteroid is only about 3 meters (10 feet) wide, but grazed by the Earth soaring only 9000 miles (14,500 km) above the eastern Pacific Ocean on March 2, 2017. The asteroid was so close that its path carried it within the ring of man-made geosynchronous satellites that orbit our planet. Animation courtesy of JPL.

Near-Earth Objects (NEOs) which include mostly asteroids, and some comets have orbits that either intersect or bring them very close to Earth's orbit. There are three distinct and dominant orbital classes or groups of NEOs: the Apollos, Amors, and Atens, and a fourth lesser group, the Atiras. These classes are named after the first object to be discovered belonging to the respective group. Specifically, Apollo asteroids are in Earth crossing orbits with semi-major axes larger than Earth's. Aten asteroids are also in Earth crossing orbits with semi-major axes smaller than Earth's. The Amor class contains Earth-approaching asteroids with orbits outside that of Earth, though within the orbit of Mars. The fourth asteroid NEO class, the Atiras, follow orbits that are completely interior to Earth's orbit. Atira's orbit relatively close to the sun making them difficult to discover and observe, rendering them the rarest of the NEO groups. Finally, NEOs also include dozen of comets, also whose perihelion is less than 1.3 AUs, and whose orbital periods are less than 200 years. Learn more on NEO Basics: NEO Groups.

The figure above illustrates the four main classes of asteroid NEOs (not showing short period comet NEOs). Diagram from JPL/Caltech/Center for Near Earth Object Studies.

All bodies, such as asteroids that are in an orbital (gravitationally-bound) relationship with another solar system body have orbital elements that describe the precise nature of the orbit. Asteroids, like their larger cousins, the major planets orbit our sun, and their orbits can be described by six orbital elements, sometimes referred to as Keplerian elements, after Johannes Kepler who first developed laws of planetary motion.

The six elements include:

Eccentricity (e)

Semimajor axis (a)

Inclination (i)

Longitude of the ascending node ( or Ω)

Argument of periapsis (ω)

True anomaly (ν, θ, or f)

The first two, eccentricity and semimajor axis describe the shape of the orbit, specifically eccentricity describes how much the elliptical shape of the orbit deviates from a circle, and the semimajor axis indicates how large the ellipse is (or the mean radius of the elipse). The remaining four elements describe exactly how the asteroid's orbital elipse is oriented in space, with respect to arbitrary frames of reference. For example inclination describes the angle of the orbital plane of the elipse with respect to the plane defined by earth's orbit  (known as the ecliptic). The longitude of the ascending node indicates the angle between a fixed point of celestial longitude called the vernal point (or Vernal  Equinox; arbitrarily the sun's point on the first day of Spring, symbolized as ) to the point on the asteroid orbit where it crosses upward through the ecliptic plane. The Vernal Equinox is analagous to Earth's zero longitude being defined arbitrarily at Greenwich, UK.  The argument of periapsis specifies the angle measured between the point of the ascending node and the perihelion point of the asteroid's orbit, this essentially defining the direction of the long axis of the orbit elipse. Finally, the true anomaly indicates exactly where along the orbital elipse the asteroid is located at a specific time (defined by the angle drawn from the objects perihelion point to the current position of the object).

Left figure shows elipses with varying eccentricity values. Right figure shows orbit semimajor axis (a), and its relation to eccentricity (e); longitude of ascending node (Ω)with respect to the Vernal Equinox (♈); the agument of perihelion (ω). Diagram from University of Arizona Astronomy 250 lecture notes.

Above figure shows inclination (i), longitude of ascending node (Ω) with respect to the Vernal Equinox (♈); the agument of perihelion (ω), and the true anomaly (ν). See more about Orbital elements.

Asteroids are tiny compared to their relatives, the larger terrestrial and gas giant planets. However, their importance to our solar syatem's evolution, their potential threat to Earth as impactors, and the fact that they are the keys to our next step in space exploration and development, render them perhaps the most important objects of study. There are three principle reasons that make asteroids essential objects of study:

Planetary Defense – Asteroids (and comets too) have been striking the Earth since the earliest history of the solar system, some 4.3 billion years ago. Although the frequency impacts from the largest and most dangerous asteroids has greatly diminished, the Earth is still struck by numerous small asteroids, and there remains a possibility that the planet could be impacted by large asteroids in the future (>500 meters diameter). One great incentive to survey and study asteroids is for humans to learn how to best deflect a potentially dangerous impactor.

Space Resources -  Asteroids contain an abundance of minerals and elements required by humans to survive and evolve here on Earth, and to explore and develop space. Additionally, many asteroids are less energetically expensive and less costly (for spacecraft) to visit than even our moon. This is partly true because near-Earth asteroids, living up to their name, reside within orbits that periodically bring them to us, making them attractive targets for mineral exploration and extraction. There are already well-funded private organizations formed that have begun the technical steps to explore and ultimately extract resources from near-Earth asteroids.

Science – Asteroids, despite being smaller in size than their relatives the larger planets, comprise a significant structural component of our solar system. Their size, shape, composition and orbital families contain information about the past structure of the solar system, how it evolved through time to appear as it does today, and how it is further evolving to affect other solar system bodies. Understanding the principle of solar system formation and planetary evolution contributes towards our understanding of how life evolved and how it's been affected by impact processes.

The study of asteroids crosses several disciplines, so one does not necessarily have to become an astronomer to study asteroids. There are scientists who study asteroids with backgrounds ranging from astronomy, physics, mathematics, chemistry, geology, and engineering. A physicist or mathematician might explore the detailed motions or the orbital dynamics of asteroids and asteroid 'families' which provide key evidence of how the solar system formed and continues to evolve. A chemist (geochemist or cosmochemist) will explore what elements and minerals asteroids are made which is important to understand their evolution, and for possible resource deposits. These scientists might also study meteorites that fall to earth that provide direct geochemical information about their 'parent' bodies, the asteroids. Since asteroids are made of rocks and minerals, and the impact of asteroids upon Earth (and other planets) over deep time is a geologic process, geologist also study asteroids. Interestingly, we also live in a time when commercial organizations and governments are becoming highly interested in studying asteroids, mostly as they consider how we might exploit them as space resources for their contained mineral wealth. Asteroids and their resources will become very important for the development of long-term space exploration and our eventual colonization of space. For this purpose, engineers are needed to design instruments (such as ground and space-based telescopes) to further survey and characterize asteroids, and aerospace and instrument engineers to design probes, spacecraft, and robots to visit and begin exploiting asteroid resources. For those with curiosity and a thirst for scientific adventure, asteroids offer endless possibility for discovery!

The short answer is that there are millions of asteroids within the solar system. As of early 2021 there are 546,846 numbered minor planets, and 498,491 un-numbered. Most of the un-numbered asteroids are known to exist but require further observation before they receive official numbered status. The vast majority of the numbered and un-numbered asteroids reside within the main belt of asteroids, between the orbits of Mars and Jupiter; far fewer of the these asteroids are Near-Earth asteroids, some of which have orbits that intersect Earth's orbit and also dip into the main belt.

It is estimated that there are between 700,000 and 1,700,000 asteroids equal or larger than 1 kilometer across. If we include all possible asteroids down to only a few meters across (e.g. 8 to 20 feet), there are likely 100s of millions out there! The smaller the asteroid, the larger the population.

A subset of all asteroids are the Near-Earth asteroids (NEAs). As of February 2021 there are > 25,000 known NEAs, and asteroid surveys are adding > 2000 new NEAs each year.

Most asteroids have been discovered through optical observations using ground-based telescopes. The first asteroid discoveries were rather serendiptous, with early 19th century astronomers occaisionally finding them while visually looking for other phenomena. With precise measurements of their postitions recorded against the background of (the apparently) fixed stars, astronomers were able to determine asteroid orbits and recognized them as 'planet-like' bodies, similar too though smaller than the larger planets.

From the late 19th century through the mid-20th century, many asteroids were discovered on photographic plates, with their precise positions and orbits being determined again with reference to the background stars on the glass plates. Specially designed measuring engines were built to aid in the precision measurement of newly discovered asteroids and comets.

The early 1970s saw the establishment of the first dedicated astronomical survey for near-Earth asteroids, the Palomar Planet-Crossing Asteroid Survey (PCAS), founded by Eugene Shoemaker and Eleanor Helin. This survey utilized photographic plates and a blink comparator to identify new asteroids and comets. In the 1980's Eugene Shoemaker, along with his wife Carolyn founded the Palomar Asteroid and Comet Survey (PACS). PACS utilized hypersensitized Kodak Tech-Pan 4415 photographic film and a stereoscopic microscope to detect near-Earth asteroids and comets. Precise astrometric positions were measured with the use of a photo-microdensitometer instrument located at Lowell Observatory. The PACS program was very succesful and in 1993 included the astonishing discovery of comet Shoemaker-Levy 9, a comet captured and torn apart by Jupiter's gravity field. In July 1994, the world's ground and space-based telescopes were all pointed towards Jupiter, and the world watched in awe as the numerous broken cometary fragments of SL-9 plumeted one by one into Jupiter, releasing incredible explosions. More on SL-9 available on Comet Shoemaker-Levy Collision with Jupiter.

Despite the succeses of the photographic surveys, by the mid-late 1980's, a revolutionary new technology was being developed for astronomical imaging: the use of digtially-based 'charged-coupled devices' (CCDs). The University of Arizona's Spacewatch program founded by Tom Gehrels and Robert McMillan was the first to begin formal survey's of near-Earth asteroids using CCDs, and throughout the 1980s and early 1990s Spacewatch developed the instrumentation and software techniques that transformed the way near-Earth asteroids are discoved and tracked, essentailly marking the end of the era for photographic surveys. The Spacewatch project enjoyed many 'firsts' including the first NEA dicovered with a CCD, and the first NEA and comet discoved with computer software. Later in the 1990s, projects such as MIT's Lincoln Near-Earth Asteroid Research program (LINEAR) rapidly accelerated the application of CCDs and moving object detection algorithms and increased asteroid discovery rates by 2-3 orders of magnitude over photographic techniques.

Finally, into the early 2000s, the advent of ever-faster computer processors, improved CCD sensors, practically limitless data storage, and continually improving motion detection algorithms saw the rise of surveys such as the University of Arizona's Catalina Sky Survey (CSS), Lowell Observatory Near-Earth Object Search (LONEOS) , JPLs Near-Earth Asteroid Tracking (NEAT) program, and the University of Hawaii's Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) project. Today the CSS and Pan-STARRS projects dominate near-Earth asteroid (NEA) discoveries and are approaching discovery rates of ~2000 new NEAs per year.

The frequency of asteroid impacts is generally related to the size of the asteroid: smaller objects strike more frequently than larger objects. Interestingly, the Earth gets showered in almost 100 tons of meteoric dust each day. Also each day, the planet is bombarded by numerous objects the size sand grains and pebbles. Objects the size of the smallest asteroids, ~1-meter across (3 feet) strike a few times per year. The asteroid that impacted Earth over the Russian city of Chelyabinsk in 2013 was approximately 19-meters across (~62 feet); events of this size occur on average every 60-80 years. In 1908, a 50-meter (~165 feet) asteroid impacted the remote Tunguska region of Siberain Russia; an object of this size impacts on average every 200-300 years. So you get the idea, the bigger the space rock, the less common the threat of impact. Really big asteroids, larger than 1-kilometer across (~3300 feet) are considered the most dangerous, as their impact energy is large enough to be extremely disruptive, or catastrophic for life on Earth. A 1-kilometer asteroid or comet impacts Earth, on average every 500,000 years. Asteroids or comets larger than 10-kilometers (6-miles) are potentially 'planet killing' asteroids, like the asteroid that struck ~65 million years ago killing off the dinosaurs and much of the other life forms on Earth. More info on small impacts is available on JPL's "New Map Shows Frequency of Small Asteroid Impacts, Provides Clues on Larger Asteroid Population" article.

Figure above shows observed and modeled estimates of Near-Earth asteroids as a function of size (lower axis scale). The right-side scale indicates the estimated interval between impacts in years for a givewn asteroid size (as read off the curved trace of the circles above the selected diameter). Impact energy, given in units of megatons TNT.  from Asteroids IV, 2015, Michel, DeMeo, and Bottke (eds.), University of Arizona Press

Asteroids are potentially dangerous, particualrly near-Earth asteroids: those asteroids with orbits that can bring them into close approach and possible impact with the Earth. We know fairly well the impact effects of asteroids the size of large boulders, up to asteroids the size of big city busses. This is because humans have either witnessed these effects firsthand, or studied effects after impact (see FAQ info on the Tunguska and Chelyabinsk asteroids impacts, and frequency of impacts). TThe 3013 Chelyabinsk impact in Russia, despite the asteroid displacing much of its impact energy in the upper atmosphere, still sent over 1500 to seek medical attention for cuts, bruises, burst ear drums, etc. Scientists have also studied the effects of very large size asteroid impacts (asteroids >1km across) by close geologic, geophysical and geochemical examination and mapping of the global rock record, including the fossil record to determine how life was affected.  For example the impact of a 10-km wide (~6-miles) 'Chicxulub asteroid that impacted earth ~66 million years ago, that killed off numerous life forms including the dinosaurs has been well documented (although significant questions remain). The impact energy that asteroids impart into a target (such as Earth) is largely dependent upon it's mass and velocity. The larger the asteroid, and the faster it is travelling, the larger the energy of impact. Asteroid velocities range between 12-40 km per sec (7-25 miles per sec), and larger asteroids tend to retain this cosmic velocity, meaning that they are not slowed much by Earth's atmosphere.  Other factors include the asteroid's composition (rock or metal; or ice if it's a comet); it's bulk density, angle of entry, target material (e.g. ocean patform or continental crust), etc. To calculate the effects of various types of your own modeled impacts check out this cool Earth impactor tool.

Meteor Crater in northern Arizona. The 1.2-km (0.75 mile) crater was produced about 50,000 years ago by the impact of a ~50-meter (165-ft) metallic iron-nickle asteroid.

To be clear, the probability that any person living today will be injured or killed by an asteroid impact is about as close to 0% as you can get. However, into the future, if we do  nothing to discover, track, and ultimately evade asteroid impacts, the probability that life on Earth will be seriously disrupted or destroyed is about 100%.  Fortunatley we are doing something: In the USA we fund surveys, like the Catalina Sky Survey, that are wholly dedicated to survey and discover potentially hazardous asteroids; and more survey projects are in development to increase our understanding of the asteroid threat. Also, there is ongoing funded research into how we might deflect a potentially dangerous asteroid. It is a matter of time before the nations of te world commit to funding a series of at-the-ready deflection projects.

PHA's are Potentially Hazardous Asteroids. PHAs are a subclass of near-Earth asteroids (asteroids that approach Earth's orbit) that are defined by two criteria: 1) PHAs are at least 140-meters across (~460 feet), and 2) their orbits bring them within 0.05 AU of Earth's orbit (= 4.6 million miles or 7.5 milliom km).  This distance is roughly 20x the Earth-moon distance. Currently there are about 2200 known PHAs, including 157 larger than 1-km. The largest PHA is 7-km across. More information is available on JPL's NEO Basics: NEO Groups page.

Image credit: CSS Orbit View, D. Rankin

On February 15, 2013 a small asteroid (also known as a superbolide) about 20-meters wide (~65-feet) impacted over the city of Chelyabinsk, Russia. The asteroid entered the Earth's atmosphere at a speed of about 19 kilometers per second (= 12 miles per second or ~41,000 mph) and it's brightness - as it exploded into the atmosphere - exceeded that of the sun. Numerous early-morning commuters captured live video of the streaking asteroid; video that later played a key role in determining the pre-impact orbital path of the asteroid. The asteroid came in at a fairly shallow angle and therefore its impact energy was largely dissipated into the atmosphere about 30-kilometers (18 miles) above the earth's surface. The pre-impact kinetic energy of the asteroid exceeded 400 kilotons TNT, or about 30x the blast yield of the atomic bomb released over Hiroshima in WWII. Numerous small fragments were strewn about the snowy landsacpe and into frozen lakes, and many fragments were recovered by locals and researchers.

The Chelyabinsk asteroid shown above during its explosive entry into Earth's upper atmosphere went completely undetected since it came from the direction of the sun, a 'blind spot' for existing asteroid surveys. Photo M. Ahmetvaleev.

On June 30, 1908 deep in the remote recesses of central Siberia, Russia, and 50-100 meter (150-330 feet) asteroid exploded into the atmosphere over the Tunguska region. The ensuing blast yield- estimated to be 3-5 megatons TNT -  was the largest ever recorded by humans. Some estimates place the blast yield up to 30 megatons TNT. Despite the fact that the asteroid was almost completely destroyed as it disintegrated into the atmosphere, about 5-10 kilometers (3-6 miles) above the Earth's surface, the blast wave was powerful enough to decimate over 2000 km2 (~800 square miles) of taiga forest. Fortunatley, because of the remote, sparsely populated region, no one is beleived to have bee killed by the blast. The first research expedition to the site in 1927 failed to find any meteorite fragment at the site leading to speculation that the blast was perhaps the result of a comet impact (a comet whose ice might have rapidly dissipated into the atmosphere but still produce the destructive blast wave). However, later expedtions did identify tiny spherules with high nickel to iron concentrations, consistent with metals originating from cosmic, not terrestrial origins. An impact event on the scale of the Tunguska impact occurs on average every 200-300 years. More info on NASA's The Tunguska Impact--100 Years Later.

Above: Flattened trees of the Siberian taiga in the Tunguska region. Photo Leonid Kulik, 1927.

OSIRIS-REx is a NASA-funded asteroid sample return mission run by the University of Arizona. OSIRIS REx is an acronym that stands for the 'Origins Spectral-Interpretation Resource-Identification Security Regolith-EXplorer". In the words of the project investigators:

"OSIRIS-REx seeks answers to the questions that are central to the human experience: Where did we come from? What is our destiny? Asteroids, the leftover debris from the solar system formation process, can answer these questions and teach us about the history of the sun and planets.

The OSIRIS-REx spacecraft is traveling to Bennu, a carbonaceous asteroid whose regolith may record the earliest history of our solar system. Bennu may contain the molecular precursors to the origin of life and the Earth’s oceans. Bennu is also one of the most potentially hazardous asteroids, as it has a relatively high probability of impacting the Earth late in the 22nd century. OSIRIS-REx will determine Bennu’s physical and chemical properties, which will be critical to know in the event of an impact mitigation mission. Finally, asteroids like Bennu contain natural resources such as water, organics, and precious metals. In the future, these asteroids may one day fuel the exploration of the solar system by robotic and manned spacecraft."

The spacecraft was succesfully launched Sepetmber 8, 2016 from Cape Canaveral aboard an Atlas V 411 rocket and will arrive at asteroid Bennu in 2018. Upon arrival, the spacecraft will spend many months orbiting Bennu, mapping and characterizing the asteroid in never-before achieved detail in prepation for selecting a site from which the spacecraft will 'tag' the surface to obtain a pristine sample of Bennu for transport back to Earth. The spacecraft, containing the asteroid sample will take about 2 years to return to Earth where it will be collected in the Utah desert in Septmber 2023. The material from Bennu will provide decades of research and labwork for teams of eager cosmo-chemists, planetary scientists, meteoriticists, organic chemists, metallurgists, and other scientists; and it will likley change our understanding of solar system and planetary formation and possibly provide valuable clues to how life itself began on Earth. See the OSIRIS-REx website for more information.

Comets are primitive solar systems objects composed primarily of ice, and also contain components of rock and dust. As comets approach the inner solar system the sunlight begins to activate the frozen molecules within the comet's nucleus and it begins spewing dust, gas, and ions forming a large gaseous atmosphere around the nucleus called the 'coma', and a long 'tail' of gas and dust. Cometary coma can be a large as planets, and comet tails can extend for millions of miles into space. Large comets that venture into the inner solar system with close approaches to the sun, and are well positioned for viewing from Earth can put on spectacular displays. Some comets have been bright enough for daytime viewing. The source regions for comets include three regions within the solar system: 1) the Oort cloud, a spherically-shaped halo of icy bodies at the very edge of our solar system (5,000 to 200,000 AU); 2) the Kuiper Belt, another distant region of icy worlds, shaped more like a disk and located outwards from Neptune up to 50 AU; and 3) within the Main Belt of asteroids, some apparent 'asteroids' have been observed to have comet-like activity; the source of these Main Belt active asteroids or comets remains un-resolved.

Discovered in 2015 by CSS observer Jess Johnson, Comet Johnson c/2015 V2 bacame a backyard observable object, visible with binoculars in May 2017. This comet displayed two tails: a wide and bright dust tail, and a more narrow ion or gas tail at nearly a right angle to the dust tail. Image by Rolando-Ligustri.

Centaurs are minor planets that are generally found in orbits between Jupiter and Neptune. That is, their perihelion distance is beyond the orbit of Jupiter, and their semimajor axes are less than that of Neptune. Centaurs are named for the half-horse / half-man mythological beasts, for the reason that these strange objects sometimes appear as icy comet-like bodies, and sometimes like typical rocky asteroids. The largest centaur discovered to date is 10199 Chariklo which is 260-km across (160-miles). The orgins of centaurs are not fully understood. Some researchers beleive they are derived from the Kuiper Belt, while others beleive they are part of the family of scattered disc objects.

A trans-Neptunian object (TNO) is a minor planet whose orbit is out beyond that of Neptune (~30AU). That is, their semimajor axes are > 30 AU.  Pluto is in fact a TNO, although it was not recogzied as such when it was discovered in 1930. Not until the discovery of  (15760) 1992 QB1 did astronomers recognize this new class of solar system object. Now there are over 2700 known TNOs, which typically fall into one of two dynamical subgroups: Kuiper belt objects (KBOs) and scattered-disc objects (SDOs). The KBOs reside relatively closer in to Neptune than do the more distant SDOs (see below).

Figure above showing locations of Kuiper belt objects and Scattered disc objects, relative to major planets in te outer solar system. Several KBO-Neptune resonant relationships are also shown in the red scale.


The 'YORP effect' is short for the 'Yarkovsky-O'Keefe-Radzievskii-Paddack effect', named for the four researchers who proposed various aspects of it. First, it's helpful to define the Yarkovsky effect which describes how the orbits of small asteroids (about less than 30-40m diameter) can be altered by the tiny amount of thrust they exhibit as sunlight is re-radiated from the spinning body (Fig a). More specifically, an asteroid will absorb some of the light striking it from the sun, and re-emit that energy as thermal radiation producing a very tiny amount of thrust to the asteroid. Thrust vectors may arise from both diurnal and seasonal effects as a function of the orientation of the asteroids spin axis (Figs a and b). In any case, the thrust is very, very small and although some orbital effects can be measured in only decades of time, it may take millions to billions of years before the asteroids orbit can be changed substatially. The long time-scale alteration of asteroid orbits from the Yarkovsky effect in part explains the continuously 'refreshed' population of near-Earth asteroids that we observe, since it is estimated that they should have all vanished by now (in the ~4.3 billion years of our solar system) by either impacting the sun, a planet, or being ejected from the solar system. Interestingly this effect was postulated by the Russian engineer I.O. Yarkovsky...in 1901; and it's taken almost a century before it had gained real attention. The Yarkovsky effect is today a major focus of study for solar systems dynamicists and aerospace engineers. The Yarkovsky-O'Keefe-Radzievskii-Paddack effect is an extension to the Yarkovsky effect that includes possible changes to the asteroids rotation rate and obliquity (Figure c).


Figures a and b showing diurnally and seasonally induced thrust, respectively, from the Yarkovsky effect (after Bottke et al. 2006 (Annu. Rev. Earth Planet. Sci. 2006. 34:157–91)

'Planet X' is the name given for a possible, as yet undiscovered ninth planet within our solar system that was recently theorized to exist. The postulation for the occurence of such a new planet does not imply that it actually exists at all, however, researchers have uncovered evidence that it may indeed exist, and several groups of astronomers have begun searching for it. The evidence for a Planet X is indirect and comes from detailed observations of the orbtits of several planet-like bodies orbiting on the distant edges of the solar system. These are the so-called Kuiper Belt Objects (KBOs) which include dwarf planets and other icy-like bodies. The orbits for a number of these KBOs appear to cluster together in a way that can be explained by the gravitational influence of another larger body, possibly a Planet X. The theory implies that Planet X, should it exist would be up to 10 times the mass of Earth and orbit about 56 billion miles outwards from the sun (20 times further out than Neptune's average distance from the sun). At such distances the object would take somewhere between 15,000 to 20,000 years to orbit the sun. Check out this informational video with the two astronomers who theorized the existence of Planet X.

The most recent argument for 'Planet x' rests on the apparent orbital (perihelion) alignment of six icy bodies residing at the furthest distances in the solar system. The idea is that their 'aligned' orbits were perhaps gravitationally sheparded through the action of the proposed Planet X.  Figure from Science, Jan 20, 2016.

CSS does not routinely provide tours of it's facilities, typically because we're occupied most nights with surveying and telescope operational tasks; we surely don't want any near-Earth asteroids slipping through our survey net! Fortunately, there is night-time access to the Mt Lemmon site through the Mt Lemmon Sky Center, operated by Steward Obsertvatory. The Sky Center operates several programs available to the public including the opportunity to observe through their summit telescopes which are adjacent to a couple of our telescopes. Please contact the Sky Center directly.

There are many paths to becomming involved with researching asteroids. Many asteroid researchers have begun as amatuer astronomers, learning the principles of astronomy and astrophysics from self-study and observation with their own telescopes. In this regard it helps to develop skills in physics and mathematics as early as possible (for example in middle school and high school); and there are numerous colleges and universities that provide excellent physics and astronomy curricula and instruction at the Bachelor of Science through PhD levels. Fortunately, the study of asteroids spans several disciplines, so one does not necessarily have to become an astronomer to study asteroids. There are scientists who study asteroids with backgrounds ranging from astronomy, physics, mathematics, chemistry, geology, and engineering. A physicist or mathematician might explore the detailed motions or the orbital dynamics of asteroids and asteroid 'families' which provide key evidence of how the solar system formed and continues to evolve. A chemist (geochemist or cosmochemist) will explore what elements and minerals asteroids are made which is important to understand their evolution, and for possible resource deposits. These scientists might also study meteorites that fall to earth that provide direct geochemical information about their 'parent' bodies, the asteroids. Since asteroids are made of rocks and minerals, and the impact of asteroids upon Earth (and other planets) over deep time is a geologic process, geologist also study asteroids. Interestingly, we also live in a time when commercial organizations and governments are becoming highly interested in studying asteroids, mostly as they consider how we might exploit them as space resources for their contained mineral wealth. Asteroids and their resources will become very important for the development of long-term space exploration and our eventual colonization of space. For this purpose, engineers are needed to design instruments (such as ground and space-based telescopes) to further survey and characterize asteroids, and aerospace and instrument engineers to design probes, spacecraft, and robots to visit and begin exploiting asteroid resources. For those with curiosity and a thirst for scientific adventure, asteroids offer endless possibility for discovery!