IAL 16: Small Bodies of the Inner Solar System and Target Earth

Don't Panic


  1. Introduction
  2. Asteroids
  3. Target Earth
  4. Watching the Skies

  1. Introduction: Reading Mostly: Don't Lecture All

  2. There are non-moon small Solar System bodies (SSSBs)

    In IAL 10: Solar System Formation, we recognized them as LEFTOVERS: material that was NOT combined into the large bodies during solar system formation.

    The LEFTOVERS are planetesimals or protoplanets or fragments thereof.

    Gas and dust in
    Solar System is probably all of relatively recent formation (Wikipedia: Cosmic dust: Dust grain formation). The gas is just the solar wind. It is continually blowing off the Sun and making its way to interstellar space. Dust is probably mainly from the explosive evaporation of comets ??? and perhaps from collisional events ???. Some primordial dust is found embedded in primordial meteorites??? and is, of course, a valuable clue in understanding Solar System formation (4.6 Gyr BP).

    Here we will NOT consider dust and gas, except in passing, and concentrate on SSSBs bigger than dust.

    These bodies are somewhat evolved by impact geology, space weathering, and, in some cases, primordial-radiogenic heat geology (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat).

    The bodies are classified into various categories.

    But there's some confusion because:

    1. Some older categories are now officially disfavored (but might end up being favored again).
    2. There are newer official categories (which may NOT last).
    3. The categories sometimes overlap in that some bodies fall into more than one category.
    4. Some bodies straddle the line between the categories.
    5. There is some hierarchy of categories.
    6. It's hard to keep up with all the nomenclature.

    The upshot is NO short description of the classification can be perfect or complete.

    1. Who Sets Up the Categories?

      The main setter-upper is the International Astronomical Union (IAU)---an international organization of astronomers (founded in 1919) who have taken on themselves the job of running the universe. These are the folks who decided that there were only 88 constellations (which are patches of sky, NOT stars) and who decided to degrade Pluto from planet to dwarf planet.

      It's NOT the IAU in the figure below (local link / general link: solvay_1927.html),---but very like them.

      Besides the IAU, some organizations and individuals set up or stick to categories they like and then there's the court of public opinion which sometimes just doesn't given in to the man.

    2. A Brief List of the Categories:

      The categories are summarized in the Euler diagram in the figure below (local link / general link: solar_system_objects.html).

      Now here is a brief list of categories with brief descriptions. It's NOT exhaustive or finicky---but includes
      planets and moons because its finicky NOT to.

    3. Mass and Number of Leftovers:

      In TOTAL MASS there is NOT a lot of leftover material in dwarf planet, small Solar System bodies, meteoroids, and dust and gas:

      1. The estimated total asteroidal mass is 0.03 % of the Earth's mass (Cox-293; La-154)

      2. The total rocky-icy body mass is harder to estimate ???, but it is probably ???? a lot smaller than the Earth's mass.

      But in NUMBER there is no natural limit---as one goes down in size, the bodies tend to become more numerous: at the bottom of the size scale you have dust and gas.

    4. Reservoirs of Leftovers:

      The small bodies (except maybe dust and gas) can't exist everywhere in the Solar System for long periods of time.

      In most parts of the Solar System, gravitational perturbations by the planets will eventually scattering them away or cause them to impact on a planet (HI-281; ???) or the Sun (Se-560).

      There are certain reservoirs where small solar system bodies (SSSBs) have continued to exist since formation or at least a very long time.

      The main minor planet reservoirs are shown in the cartoon in the figure below and listed below that.

      The Main Reservoirs:

      1. Asteroid belt: Located at about 2.1--3.4 AU. Out to about 2.7 AU the asteroids tend to be rocky (or stony) and metallic (mainly iron) and comparatively highly reflective.

        From 2.7 out carbonaceous asteroids dominate. They are black in color and reflect only about 4 % of the light that strikes on them.

        But as we mentioned above some and perhaps many asteroids have subsurface ices.

        Although there may be of order 25 million asteroids bigger than about 100 m in size scale in the Asteroid belt (Wikipedia: Asteroid: Size distribution), they are far apart in space and if you stood on one you wouldn't see a swarm of others: other asteroids would usually just be faint stars at best (HI-257).

        The asteroids mostly orbit close to the ecliptic plane and all (or almost all) orbit eastward (FMW-250; La-146).

        For the asteroid belt and the Jupiter Trojan asteroids, see the figure below (local link / general link: solar_system_inner.html).

      2. Trojan Asteroid Zone: Trojan asteroids occupy gravitationally stable points called Lagrangian L4 and L5 points that lead and trail Jupiter by 60 degrees on Jupiter's orbital path at about 5.2 AU.

        The Trojans tend to be carbonaceous and black.

      3. Kuiper belt: It is located at about 30--100 AU and was proposed as circa 1950 as a reservoir for rocky-icy bodies that could become short-period comets. See IAL 17: Pluto, Rocky-Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

      4. Scattered disk: It is located at about 30 to beyond 100 AU. The orbital inclinations relative to the ecliptic plane go as high as 40 degrees---hence the name Scattered disk. The bodies are all rocky-icy bodies.

      5. Oort cloud: The Oort cloud extends perhaps from 2000 AU to 200,000 AU (which is about 3 light-year) (see Wikipedia: Oort cloud: Structure and composition). It is envisioned as a spherical shell and NOT a planar (or semi-planar) structure unlike the other reservoirs. The Oort cloud is entirely theoretical, but it must exist to resupply long-period comets. See IAL 17: Pluto, Rocky-Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

      In addition to the main reservoirs there are small bodies in other locations temporarily.

      Near-Earth asteroids (NEAs) are asteroids that come into the inner Solar System at least as far as crossing the orbit of Mars. Jupiter causes their orbits to apsidally precess (see section Watching the Skies below), and so they must eventually impact on an inner planets or the SunSolar System by a close encounter with a large body (Se-560).

      NEAs can only survive 1 to 10 million years (HI-256).

      Centaurs are objects that are mostly between Jupiter and Neptune. They were first taken to be asteroids, but since they can show cometary behavior, they must be icy (HI-249). See IAL 17: Pluto, Rocky-Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

      Centaurs probably must eventually impact on some planets within a few million years or less.

      Both NEAs and Centaurs must be resupplied from the main reservoirs.

      Collisions or gravitational perturbations put them into their short-lived NEA or Centaur orbits.

  3. Asteroids

  4. We define asteroids as being rocky bodies smaller than planets in the inner Solar System or in the neigborhood of Jupiter's orbit.

    Some certainly have signficant ices below their surface, but they are NOT conspicuously rocky-icy bodies.

    Our definition corresponds to what historically has been thought of as an asteroid.

    A very well studied asteroid is Vesta which we preview in the film in the figure just above/below (local link / general link: 004_vesta_rotating.html).

    Now for some
    asteroid topics.

    1. Discovery of Asteroids:

      The first asteroid was discovered on 1801 Jan01 by Guiseppe Piazzi (1746--1826) at the Palermo Observatory in Sicily and named Ceres for the Roman goddess Ceres, the goddess of the harvest---from her name, we get cereal.

      Ceres was expected to be the missing planet between Mars and Jupiter according to the Titius-Bode law.

      The Dawn probe (2007--2018) gave us a detailed close-up of Ceres during its active life in orbit 2015--2018 plus (see Wikipedia: Dawn Spacecraft: Mission conclusion).

    2. Origin and Survival:

      It was once speculated that the asteroids were mostly a broken-up planet that exited between Mars and Jupiter.

      This theory is now considered untenable.

      The tenable theory is that the asteroids are leftover planetesimals and protoplanets or fragments thereof.

      Fragmentation occurred though collisions among the planetesimals, protoplanets, and asteroids. The rate of collisions was high in the early Solar System and has decreased continuously since then, but will never turn off.

      Why did the asteroids between Mars and Jupiter not coalesce into a planet?

      The theory is that the strong gravitational perturbations of Jupiter prevents this. Somehow they keep the asteroids spread out and also shepherds then from leaving the asteroid belt. For further explanation of how this happens, we just handwave.

      So asteroids in the asteroid belt have survived since Solar System formation because of Jupiter.

      The same argument for existence and survival, mutatis mutandis, applies to the Trojan asteroids in the Jupiter Lagrangian L4 and L5 points.

      The orbits of the asteroids are always evolving due to gravitational perturbations.

      This means that occasionally there will be strong gravitational encounters or body-on-body collisions that can kick asteroids out of their safe reservoirs in the asteroid belt and the Jupiter Lagrangian L4 and L5 points.

      Such ejected asteroids CANNOT survive outside of their safe resevoirs for more than a few million years???.

      Gravitational perturbations due to the planets cause the ejected asteroid orbits to evolve until the Trojan asteroids collide with something (e.g., a planet, a moons, or the Sun) or, perhaps, only gravitationally collide and get kicked out of the Solar System.

    3. Number of Asteroids:

      How many asteroids are known and how many are estimated to exist?

      1. How many known is slightly tricky because it depends on the exact definition one uses and its also a number that is constantly changing because of updates.

        But simple answer is just to count the minor planets and dwarf planets excluding "distant objects": 1091257 - 4056 = 1,087,201 ≅ 10**6. the asteroid belt (IAU Minor Planet Center: Latest Published Data).

        The very large number of minor planets is due to automated searches.

        In 1995, there were less than 28,000 minor planets with known orbits????.

        So the number of minor planets is rapidly increasing number and one must check the IAU Minor Planet Center: Latest Published Data frequently to keep up.

      2. How many asteroids are estimated to exist?

        This is tricky number to find too.

        One estimate is ≅ 25*10**6 greater than 100 m in size scale. (see Wikipedia: Asteroid: Size distribution).

        There could be millions more if one extends one's range down to the lower limit of ∼ 1 m in size scale below which one calls objects meteoroids.

      Instead of looking at actual counts that are always changing, it's probably better to look at Table: Approximate Number of Asteroids N Larger than Mean Diameter D which gives the size distribution of asteroids based on estimates and exact counts for the largest asteroids.

        Table:  Approximate Number of Asteroids N Larger than Mean Diameter D
          D           N           D            N 
        (km)                    (km)
        900           1        10.0        10,000   
        500           3         5.0        90,000   
        300           6         3.0       200,000  
        200          28         1.0       750,000  
        100         200         0.5       2*10**6 
         50         600         0.3       4*10**6 
         30        1100         0.1      25*10**6
        NOTE.---The numbers for diameters larger than 500 km are exact counts. The numbers for diameters larger than 300 km and 200 km are nearly exact counts. The only source of uncertainty is that the mean diameters of asteroids are a bit uncertain and asteroids near the diameter bin boundaries lines may be marginally in the wrong diameter bin. It is likely that all asteroids larger than about 100 km have been discovered, but the table only gives approximate numbers for the bins from 100 km down in mean diameter. As the mean diameter get smaller, the number of asteroids become more and more uncertain.

        The asteroids are the small rocky bodies inward of about Jupiter's orbit that are NOT moons. Some of them probably have significant ices below the surface.

        Below about 10 meters in size scale, small bodies are considered meteoroids rather than asteroids, but there seems to be no exact lower cut-off for asteroid mean diameter.

        Note we are NOT including trans-Neptunian objects, centaurs, and scattered disk objects.

        References: Wikipedia: Asteroid: Size distribution, Wikipedia: List of notable asteroids: Largest by diameter.


      An out-of-date cartoon that conveys similar information to Table: Approximate Number of Asteroids N Larger than Mean Diameter D is in the figure below.

      Clearly, as size goes down, asteroids become much more numerous. The lower cut off for asteroids is about 10 m in mean diameter, below which a body is a meteoroid. But no hard line between asteroids and meteoroid seems to have been defined.

    4. Names of Asteroids:

      Originally, asteroids for people or beings from Greek mythology and Roman mythology. This just followed ancient tradition.

      The discoverer just chose the name.

      International Astronomical Union (IAU) nowadays regulates the names, but I think the disoverer still gets a say on a name.

      IAU or someone earlier decided to give the asteroids a prefix number that records their order of discovery.

      So Ceres is formally 1 Ceres.

      The number of known asteroids grew so great in the 20th century that mythological names were exhausted and other names were allowed such as the names of spouses (e.g., 253 Mathilde for the wife of astronomer Maurice Loewy (1833--1907)) or celebrities.

      Many asteroids have only temporary designations that indicate the date of their discovery???.

      For more information on how the asteroids are named, see IAU Minor Planet Center: How Are Minor Planets Named?.

    5. Largest Asteroids:

      All the bigger asteroids (i.e., larger than 150 km) were discovered long ago: the last one, 1437 Diomedes (mean diameter about 170 km), in 1937 (Cox-317--319).

      Nowadays the
      asteroids we find are getting smaller and smaller on average. Many of the recent discoveries are just big boulders.

        Question: Asteroids are mostly NOT RESOLVED. So how is their size determined?

        1. Size usually has to be estimated from reflected light and its spectrum and assumptions about reflectivity and shape of the asteroid.
        2. Size can be estimated knowing mass which can be determined from their orbital speed.
        3. Sheer guesswork.

        Answer 1 is right.

        For small bodies orbiting large, the mass of the small body is almost negligible in determining the orbital parameters: the large body's mass is very important of course.

        Size is frequently uncertain to about a factor of 3 or more (La-150).

        For asteroids of scale size less than about 300 km, self-gravity is NOT enough to force them to be roughly spherical (La-150).

        Remember only the pressure force and the centrifugal force (to some degree) can withstand gravity if the body gets too massive.

        At smaller size scales the electromagnetic solid forces and the centrifugal force can allow them to have odd shapes.

      The Table: Largest Asteroids below (local link / general link: asteroid_largest_table.html) shows the characteristics of the largest asteroids.

      You can see that asteroids go down rapidly in size from the Ceres.

    6. Images of Asteroids:

      Relatively few asteroids have been imaged: i.e., seen as more than an unresolved or barely resolved light sources.

      Good images usually require a spacecraft to do a flyby or go into orbit around the asteroid.

      The Hubble Space Telescope (HST) can do poor quality images of the largest asteroids and crude radar mapping can be done from the Earth.

      As of 2021, 13 asteroids have been imaged from relatively nearby by spacecraft (see Wikipedia: List of minor planets that have been visited by spacecraft plus a 14th Dactyl which the Wikipedia doesn't list separately). This number is likely to grow slowly over the coming decades.

      List of asteroids with close-up images:

      1. 1 Ceres in 2015 by the Dawn probe.
      2. 4 Vesta 2011--2012 by the Dawn probe.
      3. 21 Lutetia in 2010 by Rosetta.
      4. 243 Ida in 1993 by Galileo on its way to Jupiter.
      5. a) 243 Ida I Dactyl (AKA Dactyl) in 1993 by Galileo on its way to Jupiter. The Galileo spacecraft (1989--1995-arrival--2003) discovered Dactyl which is a moon of Ida---it was the first known moon of an asteroid.
      6. 253 Mathilde in 1997 by NEAR Shoemaker.
      7. 433 Eros in 1998--2011 by NEAR Shoemaker. NEAR Shoemaker orbited and landed on Eros at the end its mission.
      8. 951 Gaspra in 1991 by Galileo on its way to Jupiter.
      9. 2867 Steins in 2008 by Rosetta.
      10. 4179 Toutatis in 2012 by Chang'e 2.
      11. 5535 Annefrank in 2002 by Stardust. It's named for Anne Frank (1929--1945).
      12. 9969 Braille in 1999 by Deep Space 1.
      13. 25143 Itokawa in 2005 by Hayabusa.
      14. 162173 Ryugu in 2018--2019 by Hayabusa2 (2014--2020).

      Let's look at some close-up images of asteroids.

      1. 1 Ceres: The Largest Asteroid:

        The Dawn spacecraft deliver many up-close images of Ceres starting in 2015.

        See a good example of an image of Ceres in the figure below (local link / general link: 001_ceres.html).

      2. 4 Vesta: The Second Largest Asteroid:

        Vesta the Dawn spacecraft. is shown in the figure below (local link / general link: 004_vesta_dawn.html) and in the film in the second figure below (local link / general link: 004_vesta_rotating.html), both from the Dawn spacecraft (2007--2018).

        to-scale collage of Vesta and other smaller asteroids is shown in the figure below (local link / general link: asteroid_collage.html).

      3. 243 Ida and 243 Ida I Dactyl (AKA Dactyl):

        Ida and Dactyl are shown in the figure below (local link / general link: 243_ida.html).

      4. 433 Eros: The Love Asteroid:

        An image of Eros---the love asteroid---is shown in the figure below (local link / general link: 433_eros.html).

        Eros' mean orbital radius (AKA semi-major axis) is 1.458 AU and it has eccentricity 0.223 (Cox-319).

          Question: Does Eros ever come closer to the Sun than the Earth's mean orbital radius?

          1. Yes. Its perihelion is about 0.9 AU.
          2. No. Its perihelion is about 1.1 AU.
          3. The question can't be answer with the information given.

          Answer 2 is right.

          perihelion = (1-eccentricity) x a  = about 0.8 x 1.5 = 1.2 AU
                     = 1.13 AU more exactly

      5. 162173 Ryugu: The First Asteroid with a Sample Returned From:

        See the film of 162173 Ryugu in the figure below (local link / general link: 162173_ryugu_rotating.html).

    7. Videos of Asteroids:

      See Asteroid videos below (local link / general link: asteroid_videos.html):


    8. Properties of Asteroids:

      Asteroids can have quite heterogeneous properties:

      1. The smaller ones have all kinds of shapes.

        Complex and somewhat random formation and impact fragmentation history will causes this if gravity and the pressure force do NOT dominate the shape.

      2. Their composition is various: stony, iron, carbonaceous, or some mix these probably with ices sometimes.

        For the image of an iron meteorite, see the figure below (local link / general link: meteorite_iron.html).

      3. Densities can vary from about 8 g/cm**3 (the probable density of metallic asteroids) down to 1 g/cm**3.

        The low-density asteroids must have voids and/or large abundances of ices.

        They may be piles of rubble held together by gravity (Ze2002-229).

      Asteroids are probably all pretty similar in one respect.

      Probably most asteroids are CRATERED and covered in regolith.

      We've only seen a few up close, but we suppose the others look similar.

        Recall regolith is rock and pebbles broken up by meteoritic impacts.

        In many cases micrometeoritic impacts have pulverized the material to fine, glassy, slippery dust (Ze2002-177; HI-142).

        We know most about regolith on the Moon where we've actually touched it, but probably it covers many old, airless surfaces in the Solar System.

        The composition of the regolith probably varies a lot.

      The surface similarity of asteroids is because their geological activity is similar.

      There is no water/weather erosion and internal heat geology is probably negligible these days.

      There is almost only space weathering (mainly micrometeritic weathering and diurnal temperature cycle weathering.

    9. The Future of Asteroid Research:

      Asteroid research is likely to go on and on for at least 3 reasons:
      1. Asteroids contain important information about Solar System formation and evolution.
      2. They are interesting targets for human spaceflight technology.
      3. They can be a threat to civilization. See section Target Earth below.

  5. Target Earth

  6. What did a Founding Father have to say on Target Earth? See the figure below (local link / general link: thomas_jefferson.html).

    Although shooting stars (
    meteors) and meteorites have been known since prehistory, only in the 19th century did scientists universally accept meteorites as facts (see Wikipedia: Meteorite: Meteorites in history).

    Now we know that infall of small meteoroids and dust (i.e., particles smaller than about 0.0001 g) is continuous and sums to about 40*10**6 kg per year (Cox-335) which is roughly equivalent to the mass a spherical mass of rock of diameter 30 meters.

    Larger impactors occur frequently, but typically fragment and partially evaporate in the Earth's atmosphere and often over the oceans where no one notices---except military satellites. They amount to ???? per year.

    About 25 fresh meteorites usually of order a kilogram mass??? each are recovered each year (FMW-273, HI-260).

    Older impactors are often recovered from some regions in Antarctica where often ice erosion has laid bare impactors accumulated and concentrated over thousands of years (FMW-273; Wikipedia: meteorite: Antarctica).

    Do impactors present any danger and how much?

    Let's investigate.

    1. Do Impactors Present a Danger?

      Let's look at some cases of impactors impacting the humanity:

      1. In 1982nov, Robert and Wanda Donahue of Wetherfield, Connecticut while watching M*A*S*H on television had a 3 kg meteorite come through their roof making a hole in the living room ceiling, bounce back up into the attic, and finally come to rest under the dinner table (FMW-275).

      2. In 1992oct, Michelle Knapp (then 18) of Peekskill, New York woke up one morning to discover the 1980 Chevy Malibu she'd just bought from her grandmother had been impacted by a 1.5 kg meteorite that also cratered the driveway (FMW-275).

        This was the Peekskill meteorite (1992oct09). See the video Peekskill Meteorite Fall | 0:34 in the linked videos below (local link / general link: impactor_videos.html).

      3. Circa the 1990s in Las Vegas, a taxi driver had a stone fly in through his open cab window and suspected it had to have come from outer space.

        He reported this to yours truly as he was driving me home from the airport after the 1998/9 Christmas vacation. (I don't really put much faith in this one, but you never know---it was in Las Vegas after all.)

      4. 61 meteoritic impacts on human artifacts were reported in the period 1790--1990 (FMW-275).

      5. The 2007 Carancas impact event (2007sep15) (see also Meteoritic Society: Carancas meteorite). apparently caused illness in locals who approached the impact site. The illness may have been caused by inhaling arsenic fumes that were released when the hot impactor plowed into the ground and ground water (see Wikipedia: 2007 Carancas impact event: Illness complaints). The Carancas meteorite is an ordinary chondrite. For images of the Carancas impact event crater, google Carancas crater.

      6. On 2013 Feb15, the Chelyabinsk impactor (2013feb15) exploded over Chelyabinsk Oblast (near the city of Chelyabinsk) in Russia.

        The Chelyabinsk impactor (2013feb15) was of order 20 meters in size scale, entered the Earth's atmosphere at an estimated 19.16(15) km/s, exploded in an air burst with enegy of about 0.5 megatons TNT at a height of about 29.7 km, created an intense flash and a shock wave.

        The shock wave caused considerable damage in the Chelyabinsk area mainly through shattering glass window.

        About 1500 people were injured, some seriously. The injuries were mainly from moving glass.

        The Chelyabinsk impactor (2013feb15) broke up in the air burst, but fragments have been found. The largest one probably impacted in Lake Chebarkul (see Wikipedia: Chelyabinsk impactor: Strewn field).

        See the videos of the Chelyabinsk impactor (2013feb15) below (local link / general link: impactor_videos.html).

      7. On 1908 Jun30, in Tunguska, Siberia a fireball was seen to explode in the sky with a blinding flash and intense heat pulse (Se-575, HI-266).

        The blast was heard up to 1000 km away.

        This was the Tunguska event.

        Due to the remoteness of the location and distracting episodes such as the WWI and the Russian Revolution, no scientific study was done until 1927.

        It was found that trees were flattened over an irregular region extending up to 30 km from ground zero. No crater was found, but carbonaceous dust was scattered around.

        There was lots of ecosystem damage. See the figure below.

        The impactor is still a bit uncertain, but one modern theory is a 30 m size-scale carbonaceous asteroid hitting at 15 km/s and exploding in the atmosphere (HI-266). Alternatively, it may have been a stony asteroid of 30 m size scale (Se-576).

      Yes. Impactors do present some danger to human society.

      But the cases looked at above have had much, much less impact on humanity than war, plague, and famine. See the figure below (local link / general link: death_pale_horse.html).

    2. Do Impactors Present a Big Danger?

      Let's look at the evidence.

      1. In 1980, Luis Alvarez (1911--1988) (1968 physics Nobel laureate) and his son Walter Alvarez (1940--) proposed that the mass-extinction 66 million years ago, the Cretaceous-Tertiary (K-T) extinction event , that wiped out the dinosaurs was due to an impactor of order 10 km in size scale.

        For dinosaurs, see the figure below (local link / general link: dinosaur_collage.html).

        impact crater has been identified as being probably the Chicxulub crater in Mexico. See the Mexico map in the figure below (local link / general link: map_mexico_cia.html).

        Chicxulub crater straddles the northern coast of the Yucatan Peninsula with center near Progreso. It is centered near the village of Chicxulub (pronounced chick-shoe-lube I believe.)

        The Chicxulub crater is 170 km in diameter and is the 3rd largest crater known on Earth. But it is entirely covered by sediments.

        It was discovered by finding shock-exposed rock and subsequent geological investigation.

        The Chicxulub impactor hit about 66 million years ago and probably the caused K-T extinction event that included the extinction of the dinosaurs---at least the dinosaurs that weren't birds.

        The impactor probably threw ejecta up in plume that may have fallen back all over the Earth.

        The impactor and ejecta may have touched off worldwide fire storms and caused dust in that atmosphere that a multi-year winter (Se-574).

        The initial evidence was a worldwide layer rich in iridium at the stratigraphic K-T boundary: iridium is an element rare on Earth, but common in some meteorites (Se-573--574).

          ``Worldwide'' doesn't mean the layer is everywhere. Only where the rock strata for the that epoch survive is the layer found. But those strata exist at many places around the globe.

        See images of the K-T boundary at K-T extinction event.

        Let's look at the Chicxulub impactor in artist's conception in the figure below.

        K-T extinction event was of, course, good for us.

        It led to the Age of the Mammals (AKA the Cenozoic). See a typical adorable mammal in the figure below (local link / general link: guinea_pig.html).

      2. In 1994, Comet Shoemaker-Levy 9 impacted on Jupiter with catastrophic results.

        See the figures below.


          Caption: Comet Shoemaker-Levy 9 after it had fragmented. HST, 1993 Jul01.

          This HST image shows approximately 20 fragments.

          Comet Shoemaker-Levy 9 was gravitationally captured by Jupiter sometime in the past (Se-501--503).

          In 1992, it went too close to Jupiter and was broken up into at least 21 fragments by Jupiter's tidal force.

          The size scale of the fragments is about 5 km.

          It was discovered at about this time by Eugene and Carolyn Shoemaker and David Levy.

          The fragments formed a long chain on an elliptical orbit that looped away from Jupiter and than returned so close to Jupiter that the fragments impacted over a period of 6 days in 1994jul.

          Unfortunately, all the impacts happened on the far side from the Earth and so were unobserved.

          But the impact sites were rotated into view quickly and were impressively obvious. Recall Jupiter's rotational period is 9 hr, 50 min, 30 sec.

          At the time---as yours truly recalled---people wondered if the impactors would splash down without a trace.

          Credit/Permission: NASA, STScI, H.A. Weaver, T.E. Smith, 1993 / Public domain.
          Download site: Views of the Solar System: Comets by Calvin J. Hamilton.
          Image link: Itself.


          Caption: Comet Shoemaker-Levy 9 impacts on Jupiter in UV. HST, 1994jul21.

          This is an false-color ultraviolet image of Jupiter: wavelength 255 nm.

          Impact site H is just rising on the dawn limb of Jupiter. The impact happened only about 15 minutes earlier.

          Impact site R is about 2.5 hours after impact.

          The impact sites look dark because dust deposited by the breaking up fragments. The dust is NOT very reflective.

          Remember that the Jupiter diameter is about 11.2 Earth diameters. Some sites are bigger than the Earth in size scale.

          Thus the impact-affected regions are huge and the impactors were only about 5 km in size scale.

          Each impactor's kinetic energy was equivalent to a few MILLION megatons TNT---so of megamegatons or teratons.

          This energy was transformed into explosion energy.

          Fireballs rose to about 3000 km and in the infrared there were glowing hot scars.

          The dark dot north of the Jovian equator must be a Galilean moon. Probably Io since it most likely to be transiting Jupiter at any given time because it is the closest Galilean moon to Jupiter.

          See the discussion of Se-501--503

          Credit/Permission: NASA, Hubble Space Telescope (HST) Comet Team, 1994 / Public domain.
          Download site: Views of the Solar System: Comets by Calvin J. Hamilton.
          Image link: Itself.

        See also the thrilling Comet Shoemaker-Levy 9 video Comet Shoemaker Levy colliding with Jupiter | 0:35 in Impactor videos below (local link / general link: impactor_videos.html).

        Each impactor from
        Comet Shoemaker-Levy 9 had kinetic energy equivalent to a few MILLION megatons TNT---so of megamegatons or teratons.

        We can compare this with the nuclear weapons.

        The record yield for a nuclear bomb is believed to be 100 megatons TNT in design and 50 megatons TNT in actual detonation (The Nuclear Weapon Archive). This was the Soviet Tsar Bomba.

        Somewhat lesser nuclear bomb explosions are shown in the two figures below (local link / general link: explosion_1954_bikini.html; local link / general link: nuclear_explosion_las_vegas.html).

      Given the teraton nature of the impacts of the Comet Shoemaker-Levy 9 fragments, one must believe that similar impactors on striking Earth would lead to global devastation.

      Worldwide firestorms and cloud cover (Se-574).

      Personally, I find the Comet Shoemaker-Levy 9 impacts pretty convincing evidence that impactors on Earth can cause devastation up to and including mass extinction.

      Because of its large gravity and large size, Jupiter probably gets impacted much more often than Earth. But since its surface is a churning fluid the traces get erased probably rather quickly.

      See the Comet Shoemaker-Levy 9 videos in Impactor videos below (local link / general link: impactor_videos.html).

    3. The Impactor Threat Is Real:

      So the impactor THREAT is real, but what is the likelihood of devastating impact at any time on Earth?

      Recall that humanity has never been seriously impacted by impactors in all of recorded human history.

      Well people have done estimates of AVERAGE IMPACT RATES. Note the word AVERAGE: the impactors come pretty much randomly.

      See the cartoon of an impactor threat diagram in the figure below.

      From the threat impactor diagram in the figure just above, one can see that the effect of the impact goes up rapidly with diameter or size scale if the impactor is NOT round.

      The diagram in the figure below shows why this is so.

        Question: If impact events like the Tunguska event occur on average about once a century (which is NOT certain), why has no one ever noticed these impact events, except for the Tunguska event itself?

        1. They have been noticed. You just havn't read the right history books.

        2. They probably mostly happened over the open ocean (which covers about 70 % of the Earth) and they left no trace. Or they happened over remote land regions and left no crater (like the Tunguska event itself) and no permanent trace. In either case, no one capable of making a historical record noticed any major impacts.

        3. We've been lucky.

        4. They just didn't happen and our estimates of frequency are all wet.

        Well 2, 3, and 4 may all be part of the truth.

      In fact a re-evaluation of impactor has found that Tunguska-like impact events should occur NOT every century, but about every 2,000 to 3,000 years (J. Scott Stuart et al., 2003sep05). So the small object threat seems to have gone way down.

      But 1-kilometer impact event is still estimated to occur about every million years: every 600,000 years was the exact estimate.

      However, the newer estimates are NOT necessarily right either.

  7. Watching the Skies

  8. In the 1980s there was a movement to search for potentially hazardous NEOs (near Earth objects) stimulated by the dinosauricidal impactor theory (as the cause of the Cretaceous-Tertiary (K-T) extinction event) yours truly believes.

    But UFOs not so much. See the figure below (local link / general link: ufo_new_jersey.html).

    The first dedicated effort was
    Spacewatch headquartered at the Steward Observatory of the University of Arizona and initially proposed in 1980. There was a long ramp-up phase and the Spacewatch effort continues.

    In the 1980s and early 1990s, the searches were small-scale and often crewed by volunteers and amateurs.

    Nowadays there are several NEO search programs.

    The leading discoverers of NEOs with AUTOMATED SEARCHES are LINEAR (MIT/Lincoln Laboratory and the US Air Force) and NEAT (NASA/JPL and the US Air Force).

    Collectively, NEO search programs have come to be called Spaceguard.

    See the Alien as a Spaceguard in the figure below (local link / general link: alien_prototype_spaceguard.html).

    Let's go into the details of

    1. Spaceguard Acronyms:

      Spaceguard has lots of acronyms:

      1. NEO:= near Earth object.

        NEOs are asteroids or comets (both short-period comets and long-period comets) that have perihelion distances of ≤ 1.3 AU (NASA NEO Program: NEO groups).

        This means that at some time a NEO will be within about 0.3 AU of the Earth, unless its aphelion distance is ≤ 0.7 AU.

        A NEO-KM is a NEO that is of order 1 kilometer or larger in size scale.

      2. NEA:= near Earth asteroid. A NEO asteroid.

        A NEA-KM is a NEA that is of order 1 kilometer or larger in size scale.

      3. NEC:= near Earth comet. A NEO comet. Here we are using comet to mean only of the solid nucleus of the whole comet phenomenon: i.e., we are excluding large comet coma and tail (see FK-366).

        A NEC-KM is a NEC that is of order 1 kilometer or larger in size scale. Most bright comets are probably kilometer size or larger???.

      4. PHA:= potentially hazardous asteroid. An asteroid of order 150 m or larger in size scale and that at some time approaches Earth by 0.05 AU or less (NASA NEO Program: NEO groups).

        A PHA-KM is a PHA that is of order 1 kilometer or larger in size scale.

          Note 0.05 AU=7.5*10**6 km is 1/20 of the mean Earth-Sun distance. It is also about 20 times the Earth-Moon distance.

      5. PHC:= potentially hazardous comet.

        I've just made this acronym up myself since it was begging to exist.

    2. NEAs Are the Main Targets:

      Most of the effort has gone into looking for NEAs, because comets are considered a lesser risk.

      In any case, we can't find long-period comets anyway if they are out beyond Pluto for tens of thousand years or more where they are mostly too dim to find---but when it comes in it will be so pretty---see the figure below (local link / general link: comet_lovejoy.html).

      NEAs, on the other hand, are in orbit in the inner Solar System.

      They are mostly small and faint, but dedicated searches can find them.

    3. Finding NEOs and Determining their Hazard:

      In a sense, it is easy to find NEOs. Just take images at different times and anything that moves or appears/disappears relative to the background fixed stars may be a NEO. This process used to be done by human eye using blink comparison.

      By human eye blink comparison is illustrated in the figure below (local link / general link: blink_insert.html).

      Automated telescopes and computer scanning makes the search for moving objects relatively easy.

      Still it takes a long time and after one finds a NEO, one must observe it long enough to understand its orbit and this may take years.

      And, of course, you must understand the orbit or else you don't know if it will some day be a threat.

      Even with a good orbit determination predicting the long-range future is tricky since small bodies are subject to perturbations that make exact predictions centuries in the future uncertain. For example,

      1. All the planets are perturbers and so are small Solar System bodies and dwarf planets.

      2. Light pressure from the Sun can perturb a small body. And how much depends on its rotation and how its reflectivity varies over its surface. This is a real tricky problem when the NEO is just an unresolved point of light.

      The upshot is that all cases so far, we can only calculate a probability that potential impactor will hit the Earth.

    4. Whence NEAs?

      Where do NEAs come from?

      Astronomical perturbations (mostly gravitational perturbations) create NEAs and then can turn them into PHAs. The process is explicated in the figure below (local link / general link: nea_apsidal_precession.html).

      NEAs must mostly originate in the asteroid belt or are extinct comets.

      A Jupiter perturbation or a collision kicks an asteroid into a sufficiently near-Earth orbit and it becomes a NEA.

    5. Whereto NEAs?

      NEAs lifetimes as NEAs are probably only of order 1 to 10 million years (HI-256).

      Jupiter's gravity makes their orbits slowly apsidally precess, and so makes a strong interaction with an inner solar system body probable (Se-560).

      Maybe 1/3 will deflected into the Sun and few will be ejected from the inner Solar System or the Solar System altogether by a close encounter with something.

      The rest will impact on Mars, Earth, the Moon, Venus, or Mercury.

      See the animation of apsidal precession in the figure below (local link / general link: apsidal_precession.html).

    6. NASA harges In---Where Angels Fear to Tread:

      The interest in the impactor threat eventually brought in NASA which founded the NASA NEO Program (NASA Near-Earth Object Program) as well as NEAT.

      The NASA NEO Program collects data on all NEOs from all NEO search programs.

      The NASA NEO Program's initial primary goal was to discover and/or assess the threat of 90 % of all PHA-KMs and PHCs by circa 2010 (NASA NEO Program purpose).

      It is estimated that there are about 1000 PHA-KMs and PHCs to find.

      On 2011 Sep26, the NASA NEO Program announced that they had reached the target for PHA-KMs having discovered 911 out of an estimated 981 (Chang, A. 2011, NASA IDs 90 pct. of largest near-Earth asteroids). The search target for PHCs was NOT mentioned in the announcement. They are harder to find.

      But despite reaching this target, I expect the NASA NEO Program to continue indefinitely finding and tracking NEOs both to assess threats and for scientific studies.

      Let's look at the current statistics for the NEO search programs given in the figure below (local link / general link: nea_statistics.html) .

    7. Now I Know What You Are Thinking:

      Are we likely to find a certain Earth impactor or have we even found one?

      There are a lot of PHAs in one sense---see the figure below.

      Well judging from the threat diagram discussed above, I assume that sooner or later we will find a Tunguska-size Earth impactor (i.e., a 30 m size scale object) that will impact with a century or so.

      But we havn't found a certain Earth-impactor yet.

      And remember, the searchers arn't looking specifically for objects as faint a 30 m size-scale object: they find lots of them, but they arn't looking exhaustively for them yet.

      The NASA NEO Program maintains an automated Sentry Risk Table for objects that threaten for the next 100 years.

        Recall that all cases so far, we can only calculate a probability that that potential impactor will hit the Earth.

      The most likely object we know threatening to impact us within about a 100 years is 2010 RF12 (2010 RF12: Earth Impact Risk Summary) and it has only 5.0 % ??? chance of hitting us on 2095 Sep05 and cumulative chance of impacting us of 5.0 %. It's just a 7 m size-scale object which will probably annihilate in Earth's atmosphere without any ground effect at all.

      It would probably be an impressive fireball (a brighter than usual meteor) or bolide (a much brighter than usual meteor).

      Such events happen pretty frequently, but we've never predicted one. See Wikipedia: NEO: Impact rate and Wikipedia: Impact event: Air bursts.

      On a longer time scale what is the most threatening object known?

      It's 1950 DA.

      The orbit and radar image of 1950 DA are shown in the figure below (local link / general link: 1950_da_orbit.html).

      Another significant
      potentially hazardous asteroid (PHA) is Bennu. See the figure below (local link / general link: bennu_orbit.html).

      The reality is that we are NOT likely to find any major threats.

      Certainly, over the course of the next billion years, there will be devastating impacts, but those are probably all beyond our societal time horizon.

    8. Is a NEO Search Worthwhile?

      Is a NEO search worthwhile?

      1. It is something humanity probably should do sooner or later. The risk of a devastating impact is real if very small.

      2. And there are volunteers eager to do it probably because Doomsday is always exciting.

      3. In the 1980s and 1990s when the projects started we seemed to have more leisure to look for new risks.

      4. Also the search is good prospecting.

        Asteroids and comets are scientifically interesting as objects that have a bearing on the Formation and evolution of the Solar System. A catalog of ones that are easy to get to will eventually be useful.

        Long down the road, there might be asteroid mining.

        Some asteroids are very rich in metals that may be useful in building SPACE INDUSTRY or doing space colonization.

        In fact, crewed missions to NEAs makes some sense.

        Some small NEAs come very close to Earth and so those ones are easy to reach.

          Question: Why is landing/taking off easy on a small asteroid?

          1. Because it is essentially docking/undocking because of the very low gravity.
          2. Because the color scheme of small asteroids is right.
          3. Because the asteroids have asymmetric shapes.

          Answer 1 is right.

        It would be a easier to send a crewed mission to NEA than on a multi-year mission as Mars.

        But easier is NOT easy (see NASA Weighs Asteroids: Cheaper Than Moon, But Still Not Easy). The spacecraft would have to catch up or slow down to the NEA. So the mission could be multi-month though NOT multi-year. The NEA couldn't have too fast of an axial rotation. Too fast and the spacecraft could be walloped.

        Probably, the NEA would have to be at least 100 meters in size scale to make the trip worthwhile. Despite all the NEAs we found since 1980, there is none known that is in the right place in the time frame currently of interest to NASA: 2025---2030. Probably, a good candidate NEA will turn up if we look hard enough. Still a crewed mission to NEA will be tough.

        Of course, there would be benefits in understanding asteroids, in developing human spaceflight capabilities, and in a very-long range in possibly asteroid mining---something which has been thought of since the early days of 20th century science fiction. For asteroid mining, see the figure below.


          Caption: An artist's conception of asteroid mining.

          As long ago as 1977 when this illustration was made, NASA has been considering asteroid mining.

          The image caption isn't terribly clear. The lander miner is is obvious.

          But is the large solar array the orbiting construction platform or just an inset illustration of a solar array?

          Credit/Permission: NASA, 1977 / Public domain.
          Download site: NASA: artist conception: Denise Watt: s78_27139.html.
          Image link: Itself.

    9. What About Comets?

      Asteroids are considered the likely potential impactors, but comets could impact as well.

      No known comet poses a threat.

      But long-period comets, as mentioned above, linger dimly out in the outer Solar System for tens of thousands of years or more.

      We are incapable of finding them in the outer reaches as of now.

      There are probably many unknown comets are out there waiting for a return visit to the inner solar system.

      It is very, very unlikely that there is any comet threat for millions of years---but we don't know.

      There may be a BAD COMET---a longhaired star with an attitude---out there now.

      We'd have a few months or maybe a year or so after we discovered it as a glowing ball coming in toward the Sun.

      That's when call for Bruce Willis (1955--).