Lecture 16: Minor Planets, Asteroids, Icy Bodies, Meteoroids, and Target Earth

Don't Panic

Sections

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

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Introduction

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There are small non-moon bodies in the solar system.

In IAWL 10: Solar System Formation we recognized them as leftovers: material that was not combined into the large bodies during the solar system formation.

The bodies are themselves planetesimals or protoplanets or fragments thereof

They are somewhat evolved by impacts or in some cases RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

The fragments go down in scale to dust.

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MINOR PLANETS AND METEOROIDS

Collectively, the largest leftovers can be called MINOR PLANETS.

Physically the range from basically metallic/rocky/carbonaceous to metallic/rocky/carbonaceous/icy bodies: or rocky and icy bodies for short.

There is no hard distinction between rocky and icy: there are a continuum of specimens.

The rocky ones tend to be inward of the vicinity of Jupiter's orbit and the icy ones beyond where ices (water, CO_2, NH_3, and others) condensed easily in the early solar system.

Although there doesn't seem to be any hard rule, I will just call the rocky bodies inward of the vicinity of Jupiter's orbit ASTEROIDS.

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Any body smaller than about 10 m in size scale is given the generic name METEOROID (Cox-333).

METEORS are meteoroids that burn up penetrating the atmosphere or an alternative definition is the streak of light created by any infalling body (i.e., a shooting star).

To produce a visible meteor in Earth's atmosphere, a meteoroid must be about 1 centimeter or larger.

Meteoroids smaller than 0.01 centimeter don't burn up, but instead settle to the Earth's surface (Cox-333). They must reach a lowish terminal velocity????.

METEORITES are the fragments of meteoroid that hit the ground (HI-243; Se-416).

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In TOTAL MASS there is NOT a lot of leftover material:

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

2. The total 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 practical limit: as one goes down in size, the bodies tend to become more numerous: at the bottom of the size scale you have dust.

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The small bodies (except maybe dust) 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 bodies have continued to exist since formation or at least a very long time.

A cartoon of main minor planet reservoirs (HI-248--249, 256--257; Se-569).

The main reservoirs (HI-248--249, 256--257; Se-569):

1. MAIN 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.

   Although there may be of order a million asteroids bigger than about 100 m in size scale in the belt (La-151), 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).

   A cartoon of the Asteroid Belt (HI-257).

2. TROJAN ASTEROID ZONE: Trojan asteroids occupy gravitationally stable points called Lagrangian 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: Located at about 30--100 AU. was proposed as circa 1950 as a reservoir for icy bodies that could become short-period comets. See IAWL 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

4. OORT CLOUD: Located at of order 20--150 kAU. The Oort Cloud is envisioned as a spherical shell [and not a planar (or semi-planar) structure unlike the other reservoirs] is entirely theoretical. See IAWL 17: Pluto, 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 precess, and so they must eventually impact on an inner planet (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 IAWL 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

CENTAURS probably must eventually impact on some planet within a few million years or less.

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

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

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Asteroids

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The first ASTEROID was discovered on 1801jan01 by Guiseppe Piazzi (1746--1826) at the Palermo Observatory in Sicily and named CERES for the Roman goddess of the harvest: from her name we get cereal.

CERES was expected to be the missing planet between Mars and Jupiter, but soon other asteroids were discovered and they were all found to be small bodies.

[It was decided that they were too small to be called planets.

Because early observers couldn't resolve them into disks as planets could be, they were called ASTEROIDS which means star-like in the sense of being unresolvable.

William Herschel (1738-1822), the most famous observer of his day, introduced the name ASTEROID, but only provisionally.

For a discussion of early asteroid nomenclature, see When Did the Asteroids Become Minor Planets? by J. Hilton.]

It was once speculated that the ASTEROIDS were a broken-up planet, but now it is considered much more likely that they are leftover planetesimals and protoplanets or fragments thereof.

As of 2004may04, there are about 277,090 MINOR PLANETS (not counting comets) with known orbits ( IAU Minor Planet Center statistics page)---although poorly known in many cases.

If one subtracts 149??? Centaurs and scattered-disk objects ( IAU Minor Planet Center centaur page) and 787??? trans-Neptunian objects ( IAU Minor Planet Center trans-Neptunian object page), the probable number of ASTEROIDS (as we define them here) is 277,054 which must be about right.

The very large number is due to recent automated searches.

In 1995, there were less than 30,000 minor planets with known orbits (according to IAU Minor Planet Statistics site on 2004apr27).

So the number of MINOR PLANETS is rapidly increasing number and one must check the IAU Minor Planet Center statistics page frequently to keep up.

Some more ASTEROID statistics (La-151).
1. There are 10 with diameters or size scales greater than 250 km.

2. There are about 100 with diameters or size scales greater than about 100 km.

3. There are estimated about 40,000 with size scales greater than about 1 km.

   Of the about 240,000 we know, many are smaller than 1 km, and so we certainly do NOT know all the 1 km or greater ones yet.

4. There are estimated about 10**6 with size scales greater than about 100 m.

5. As one goes smaller, they get more and more numerous it seems, and so there may be quasi-infinity of 10 m ones.

All the bigger ASTEROIDS (i.e., larger than 150 km) were discovered long ago: the last one, 1437 DIOMEDES (diameter about 171 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 assumptions about reflectivity and shape.

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.

ASTEROIDS are given a name and a number: the number is the order of discovery: e.g., Ceres is formally 1 Ceres. See IAU Minor Planet Center: How Are Minor Planets Named?.
Elvis Presley and other rock stars have been honored with minor planets (IAU Minor Planet Center: Rock & Roll Minor Planets).

Many have only temporary designations that indicate the date of their discovery ( IAU Minor Planet Center: Provisional Designations).

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Largest Asteroids

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 No.  Name          Year of        Diameter    Semi-Major   Eccentricity
                    Discovery      (km)        Axis

  1   Ceres         1801           913         2.77         0.078 
  2   Pallas        1802           523         2.77         0.234
  3   Vesta         1807           501         2.36         0.091
 10   Hygiea        1849           429         3.14         0.120
511   Davida        1903           337         3.18         0.178
704   Interamnia    1910           333         3.06         0.148
 52   Europa        1858           312         3.10         0.100
 15   Eunomia       1851           272         2.64         0.185
 87   Sylvia        1866           271         3.49         0.083
 16   Psyche        1852           264         2.92         0.134 

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Reference: Cox-317.

Notes

1. CERES is actually oblate because of the centrifugal force caused by its relatively fast rotation with period of about 9 hours (La-151).

   From precise occultation measurements, CERES has equatorial diameter 959+/-5 and polar diameter 907+/-9 (La-148, 150-151).

   CERES rotates rapidly with about a 9 hr rotation period. The centrifugal force causes it to be oblate.

2. VESTA is approximately triaxial with dimensions: 566 x 531 x 437 km (La-148).

   Thus even bodies larger than 300 km in size do not have to be exactly spherical or oblate spherical.

   Rigid body forces can partially withstand gravity for Vesta-size objects.

3. 52 EUROPA is not to be confused with Jupiter's Galilean moon Europa which is a much bigger object.

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

Let us look at some images of ASTEROIDS.

First a large one: 2 VESTA.

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2 Vesta: direct image, elevation map, model. HST, 1997sep04.

Vesta, the 2nd largest asteroid, has average diameter 523 km. But despite its large size, it isn't all that spherical as one can see.

A large crater with a central peak near the south is clear on the elevation map. The crater has a diameter of about 400 km.

Vesta has basaltic rock, and so is thought to have undergone volcanism and chemical differentiation in the early solar system.

Perhaps many asteroids larger than 100 km underwent some chemical differentiation and volcanism in the early solar system.

But their residual heat of formation and past radioactive heat were lost quickly and they became internal-heat geologically inactive early on.

Vesta and smaller asteroids might have owed their chemical differentiation and internal heat to aluminum-26 (half-life about 0.7 Myr) and other relatively short-lived radioactive nuclides.

The short-lived species would have to have been produced by a supernova that went off shortly before the solar system formation and that seeded the solar system with its debris.

See Se-565.

Credit: NASA/HST; download site Views of the Solar System by Calvin J. Hamilton.

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Only five ASTEROIDS have been imaged from up close (La-151):

1. 951 Gaspra in 1991 by Galileo on its way to Jupiter.
2. 243 Ida in 1993 by Galileo.
3. Dactyl, a moon of Ida, in 1993 by Galileo.
4. 253 Mathilde in 1997 by NEAR (Near Earth Asteroid Rendezvous).
5. 433 Eros in 2000 by NEAR. NEAR orbited and landed on Eros at the end its mission.

These are all smaller asteroids and quite aspherical.

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Asteroid 243 Ida and its moon Dactyl.

This is a false color image of 243 Ida taken on a fly-by by the Galileo probe on its way to Jupiter in 1993.

North is at 1:00.

The bright blue areas may indicated enrichment in iron-bearing minerals.

Ida is 58 km along its long axis. Dactyl is about 1.5 km in size scale (Se-563; Ze2002-229).

Ida like mostly smaller asteroids is not round. Its self-gravity is insufficient to pull it into a spherical shape against its electromagnetic force structure and its centrifugal force due to its rotation.

Credit: NASA: Galileo probe.

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433 Eros from the NEAR probe, 2000feb12.

Eros is approximately 33x13x13 km and we are viewing it from about 1800 km away.

The sequence of images covers 5.5 hours and shows Eros in approximately the color seen by the unaided eye.

NEAR would go into orbit around Eros and land on it as the final act of its mission.

You can see that the surface is cratered, but smoothish.

The surface has probably been pulverized to regolith: slippery, glassy dust with pebbles and rocks of various sizes????.

Eros, discovered in 1898, is a NEA: i.e., a Near-Earth asteroid.

It's semi-major axis is 1.458 AU and it has eccentricity 0.223 (Cox-319).

Question: Does Eros every cross Earth's orbit?
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

      

Because Eros is named for the love god, many of the features on Eros have all been named for famous/infamous lovers: Don Quixote & Dulcinea, Orpheus & Eurydice, Catherine & Heathcliff, Don Juan, Casanova, Genji, and Lolita (Views of the Solar System: Cylindrical Projection Map of Eros by Grant L. Hutchison).

Credit: NASA/NEAR probe; download site Views of the Solar System by Calvin J. Hamilton.

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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, metallic, or carbonaceous.
   Question: How can you have an almost all metal asteroid?
   1. You can't. The name metallic only indicates a metal-rich asteroid.

   2. A large asteroid could have chemically differentiated when it was young due to heating by some radioactive isotope (e.g., aluminum-26, half-life about 0.7 Myr) and then the core of the asteroid might almost all metal (mainly iron). Say then that the asteroid was fragmented by an impactor and fragments of the core were flung off as new asteroids. The core fragments would almost all metal: i.e., a metallic asteroid.

   
   
   Answer 2 is right.

   Iron meteorites are one of the commonest kinds of meteorites found on Earth and probably mainly come from fragments of asteroidal iron cores (Se-554).

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   An iron meteorite from Derrick Peak, Antarctica.

   Iron meteorites (IRONS) are primarily iron and nickel.

   IRONS are probably remnants of the broken-up core of a chemically differentiated asteroids.

   IRONS constitute about 66 % of FINDS: i.e., meteorites just found on the ground without being seen to fall.

   But they are only about 6 % of FALLS: i.e., meteorites that are seen to fall and then are located. Most FALLS are STONES or STONY-IRONS.

   Question: Why are IRONS so much more abundant as FINDS than as FALLS?
   1. Just by chance.
   2. STONES and STONY-IRONS look pretty much like stones, and so don't catch the eye. IRONS look weird.

   
   
   Answer 2 is right.

   Given the 66 % to 6 % ratio, most FINDS are IRONS by more than chance alone.

   IRONS are pretty resistant to burning up in the atmosphere, and so they the large abundance that we find on Earth is probably a biased sample of meteoroids.

   Iron meteoroids are probably a small fraction of meteoroids.

   See Se-554.

   Credit: NASA/JPL; download site Views of the Solar System: Meteorites.

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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.

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

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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.

Only impacts, including micrometeoritic impacts, drive asteroid geology.

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Target Earth

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``It is easier to believe that Yankee professors would lie than stones would fall from heaven.''---attributed to Thomas Jefferson (HI-260).

Although shooting stars (meteors) and meteorites have been known since prehistory, only in the 19th century did scientists universally accept meteorites as facts (HI-260).

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 burn up in the atmosphere and often over the oceans where no notices---except military satellites. They amount to ???? per year.

About 25 fresh meteorites usually of order 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).

Do impactors present a THREAT?

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).

   A common problem for aliens.

3. Circa 1990s in Las Vegas, a cab driver had a stone fly in through his open cab window and suspected it had to have come from 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 Vegas after all.)

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

5. In 1908jun30, 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.

   Due to the remoteness of the location and 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.

   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).

6. In 1980, it was proposed that the mass-extinction 65 million years ago that wiped out the dinosaurs was due to an impactor of order 10 km in size scale.

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   A small political map of Mexico

   The Chicxulub crater straddles the northern coast of the Yucatan Peninsula with center just east?? of 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 65 million years ago and probably the caused a mass extinction at the end of the Cretaceous period that included the extinction of the dinosaurs.

   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).

   Credit: Central Intelligence Agency (CIA); download site Perry-Casta~neda Library Map Collection University of Texas Austin. Most of the maps are in the public domain and can be downloaded. There are historical maps.

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   An artist's conception of the Chicxulub impactor at the moment of impact.

   In 1980, it was proposed that the mass-extinction 65 million years at the end of the Mesozoic era/Cretaceous period and beginning of the Cenozoic era/Paleogene period was caused by an impactor of order 10 km in size scale.

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

   [``World-wide'' 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.]

   The mass extinction ended the age of the dinosaurs and began age when mammals were the dominant large animals. See Se-573--574; and Sm-92--93.

   The crater of the impactor is the Chicxulub crater centered near Chicxulub village, Yucatan, Mexico. It is 170 km in diameter, but is entirely covered by sediments.

   It was discovered in the early 1990s by drilling??? and other geological means.

   It is thought that the impactor threw up a huge plume of hot debris that spread around the planet and ignited massive forest fires.

   Subsequently, soot from the fires and dust from the debris may have caused a multi-year global winter.

   Both the fires and the multi-year global winter would have extinguished considerable life.

   The world-wide debris included impactor iridium which gave the iridium-rich layer at the Cretaceous/Paleogene boundary.

   That an impactor hit near the end of the Cretaceous is solidly established.

   There are, however, still a few doubters that it was the cause of the mass extinction. They rebut the leading theory. See Keller et al., 2003sep25 and Sm-93.

   Credit NASA/artist Don Davis.

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7. In 1994, comet Shoemaker-Levy 9 impacted on Jupiter with catastrophic results.

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   Comet Shoemaker-Levy 9 after it had fragmented. HST, 1993jul01.

   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: STScI/NASA/H. A. Weaver/T. E. Smith; download site Views of the Solar System: Comets by Calvin J. Hamilton.

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

   This is an ultraviolet image of Jupiter: wavelength 255 nm. The color, of course, is false.

   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.

   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.

   This energy was transformed into explosion energy.

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

   See the discussion of Se-501--503

   Credit: NASA/Hubble Space Telescope Comet Team; download site Views of the Solar System: Comets by Calvin J. Hamilton.

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

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   A 15 megaton bomb on Bikini Atoll, 1954feb28.

   This was XX-92 BRAVO - Operation Castle. We've long forgotten where an innocent word for beach-wear came from and why.

   Credit: U.S. Department of Energy photograph.

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   It is easy to believe that impactors like the Shoemaker-Levy 9 fragments striking Earth would lead to global devastation.

   World-wide fire storms and cloud cover (Se-574).

   Personally, I find the Shoemaker-Levy 9 impacts pretty convincing evidence that IMPACTORS on Earth can cause MASS EXTINCTIONS.

   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.

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

Recall that HUMAN SOCIETY 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.

A cartoon of HI-264's impactor threat diagram.

From the threat diagram just above, one can see that the effect of the impact goes up rapidly with diameter or size scale if its not round.

Impactor explosion energy.

Question: If a Tunguska-like impactor hits on average about once a century (which is not certain), why has no one ever noticed these impactors, except for the Tunguska impactor itself?
1. They have been noticed. You just havn't read the right history books.

2. They probably mostly happened over the open ocean (about 70 % of the Earth) and they left no trace. Or they happened over remote land regions and left no crater (like Tunguska) 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 recent re-evaluation has found that Tunguska-like impactors 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 impactor is still estimated to be about every million years: every 600,000 years was the exact estimate.

But new estimates arn't necessarily right either.

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Watching the Skys

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In the 1980s there was a movement to search for potentially dangerous NEOs (near Earth objects) stimulated by the dinosaurocidal impactor theory I believe.

The first dedicated effort was Spacewatch headquartered at the Stewart 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 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).

We need some acronyms.
1. NEO:= near Earth object.

   NEOs are asteroids or short-period comets that have perihelion distances of 1.3 AU or less.

   This means that at some time a NEO will be within 0.3 AU of the Earth ( NASA NEO Program: NEO groups).

   A NEO+18 is a NEO that is of order 1 kilometer or larger in size scale.

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

   A NEA+18 is a NEA that is of order 1 kilometer or larger in size scale.

3. NEC:= near Earth comet. An 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+18 is a NEC that is of order 1 kilometer or larger in size scale.

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+18 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 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.

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.

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.

Asteroid brightness depends on size and reflectivity. Reflectivity can vary tremendously. It very small for black, carbonaceous objects.

The brightness scales with surface area or the 2nd power of diameter or size scale.

Thus, a 2nd object 10 times smaller than a 1st object, is 10**2=100 times fainter.

In a sense, it is easy to find NEOs. Just take images a few minutes apart and any thing that moves relative to the background fixed stars may be a NEO. This process used to be done by human eye using a blink comparator.

Blink comparison.

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 other asteroids.

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.

Perturbations create NEAs and then can turn them into PHAs.
NEAs must mostly originate in the Asteroid Belt or are burnt-out comets.

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

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

Jupiter's gravity makes their orbits slowly 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.

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

A cartoon of a NEA on a precessing orbit.

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The interest in the impactor threat eventually brought in NASA which founded NASA's Near-Earth Object Program Office as well as NEAT.

NASA's Near-Earth Object Program collects data on all NEOs from all search programs.

NASA's initial primary goal was to discover and/or assess the threat of 90 % of all PHA+18s and PHCs by circa 2010 ( NASA NEO Program purpose).

It is estimated that there are about 1000 PHA+18s and PHCs to find.

However, I expect the NASA NEO Program to continue indefinitely finding and tracking NEOs both to assess threats and for scientific studies.

Currently, as you'd expect, the NASA NEO Program is probably more than halfway to the goal.

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1. Growth of known NEAs and NEA+18s (i.e., large NEAs) 1980jan01--2005mar31.

   As of 2005apr30, there are:

   1. 3378 NEOs: i.e., near Earth asteroids and comets.

   2. 3322 NEAs: i.e., near Earth asteroids.

   3. 772 NEA+18s: i.e., near Earth asteroids of order or greater than 1 km in size scale.

   4. 695 PHAs: i.e., potentially hazardous asteroids: i.e., those approaching Earth by =< 0.05 AU.

   5. 151 PHA+18s: i.e., potentially hazardous asteroids of order or greater than 1 km in size scale.

   6. 56 NECs: i.e., near Earth comets. The NEC number doesn't grow very rapidly. There were 40 as of 2000jan01.

   The curves in the figure are both still growing.

   The NEA curve approximately linearly.

   The NEA+18 curve shows a probably significant decline in slope.

   Eventually, the NEA+18 curve must plateau when almost all NEA+18s have been found.

   Probably finding the very last NEA+18s will take a long time since it takes an exhaustive search to find the very last ones.

   There is no end to the number of NEAs since one can keep looking for smaller and smaller ones. But those that are less than 10 m or so (which are are better called meteoroids) probably pose no threat at all.

   Credit: NASA/NASA NEO Program/Alan B. Chamberlin.

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   Now I know what you are thinking.

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

   Well judging from the threat diagram 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.

   [Or maybe a millennium or two if the new threat frequency estimates are right ( J. Scott Stuart et al., 2003sep05).

   Of observed asteroids, as of 2005may04 only 24 have come closer than the Moon's semimajor axis distance of 0.0025696 AU.

   These were all small Tunguska-size or sub-Tunguska-size asteroids (i.e., of order 100 m in size scale or less).

   They have all been since 1991, and so we obviously missed all the ones before and probably some since.

   Some of them may have been discovered retroactively.

   See IAU Minor Planet Closest Approach site.]

   But we havn't found a certain 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 Impactor Risk Table for objects that threaten for the next 100 years.

   The most likely object we know threatening to impact us within about a 100 years has only 1/500 chance of hitting us---and its just a 40 m size-scale object which is comparable to the Tunguska impactor.

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

   ---

   The orbit of 1950 DA (asteroid 29075) discovered 1950feb23 by C. A. Wirtanen.

   Semi-major axis: 1.69884271 AU.

   Eccentricity: 0.507424557.

   Inclination to Ecliptic Plane: 12.1843194 degrees

   Rotational period about 2.1 hr.

   The most threatening object yet found is 1950 DA which was first discovered 1950feb23 and lost, but was recovered on 2000dec31 (last evening of the 20th century by the precise definition). See NASA NEO Program: 1950 DA and Minor Planet Center: Apollo asteroids.

   1950 DA is a temporary name indicating the year of discovery. It is asteroid 29075.

   1950 DA is about 1.1 km in size scale. So according to the threat diagram above, it could cause continental devastation.

   Under one assumption about its axial tilt 1950 DA has a 1/300 chance of impacting Earth.

   Under another assumption, the chance is practically zero.

   So one can average and say a 1/600 chance of hitting.

   Why can't we be more certain? All those minute perturbations that can't be accurately included in a calculation that extends for centuries.

   The date of possible impact is 2880mar16---just before St. Patrick's Day, 2880.

   So if 1950 DA is going to impact, we have a long time to do something about it.

   What can we do if its going to hit? A small enough perturbation early on would deflect it. Just a small push perhaps or just changing its reflectivity by covering it with charcoal soot.

   The risk from 1950 DA given its chance of hitting and its size is thought to be greater than the combined risk by all other known asteroids through to 2880.

   So we can all breathe easier so far.

   Credit NASA/JPL/J. Giorgini.

   ---

   A radar image of 1950 DA (asteroid 29075). Arecibo, 2001mar04.

   I'm not actually quite sure what we are seeing.

   Presumably the horizontal and vertical scales are related to distance on the sky.

   The caption called it a radar image and gave no further explanation.

   Credit NASA/JPL/S. Ostro.

   ---

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

   Certainly, over the course of the next 100,000 million years there will be devastating impacts, but those are probably all beyond our societal time horizon.

   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 their 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 history 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 asteroid mining.

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

      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 lot easier to go to an asteroid than on a multi-year mission as MARS.

      ---

      

      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 NASA/artist concept: Denise Watt.

      ---

   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 comet 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.