University of Wisconsin-Madison

Notes:

ASTEROIDS (seen by light reflected from their surfaces)

Main Belt Asteroids

Orbital region between Mars and Jupiter

951 Gaspra (from Galileo spacecraft, 16.200 km, Oct. 29, 1991)

243 Ida (from Galileo spacecraft)

Meteorite reflectance spectra indicate most come from these asteroids

Figure: Comparison of spectral reflectance (NASA 1992) (to be supplied later)

However, there are major types of meteorites for which the asteroid parent is not known and major classes of asteroids for which there are no known meteorite analogs (Nelson, et al, 1993)

Near Earth Asteroids (NEA)

Estimates are that about 1000 exist (Wetherill and Shoemaker, 1982)

Ejected from Main Belt by interactions with Jupiter.

Collisions

Chaotic dynamics increase orbital eccentricity.

Relatively short (10-100 Myr) lifetimes and thus must be replenished rapidly compared to the age of the solar system. (Greenberg and Nolan, 1993)

Amor type

Orbit outside the Earth's

Apollo type

Orbit crosses Earth's with a period of > 1 year.

Aten type

Orbit crosses Earth's with a period of < 1 year.

Reflectance spectra indicate many NEAs are similar to Main Belt Asteroids

Others appear to be extinct comet nuclei

Surface volatiles depleted

Inert crust seals remaining volatiles inside

COMETARY OBJECTS (have a visible atmosphere or tail)

Unprocessed (unheated) primordial material of the original solar nebula.

Enter the inner solar system from distant reservoirs

Oort cloud and/or Kuiper belt way outside the orbits of the planets.

Gravitational perturbations due to galactic forces cause some comets to head our way.

Interactions with Jupiter shortens orbital periods.

Shoemaker-Levy encounters Jupiter (Hubble views, 1994)

Halley's Comet (~ 10km) studied in some detail in 1986.

Nucleus of Halley's Comet (ESA image, 1986)

Additional Halley Info

Nucleus irregular and dark

Ices (50%), dust (50%), and rock - ~ 1 g/cm³ density

non volatiles consist of both silicates and hydrocarbons

ices: water (80%), CO (15%), plus formaldehyde, carbon dioxide, methane, and hydrocyanic acid.

EARTH-CROSSING ASTEROIDS (ECA)

Class of NEAs with the potential to impact our planet

Definition (Shoemaker, 1990)

"...an object moving on a trajectory that is capable of intersecting the capture cross-section of the Earth as a result of on-going long-range gravitational perturbations due to the Earth and other planets. In this case "long-range" refers to periods of tens of thousands of years."

128 ECAs are known. (Their discovery, using current systems, depends on having an absolute magnitude >13.5 and varies with reflectivity of their surfaces as well as size.)

25% are Amors

66% are Apollos

9% are Atens

General Nature

Majority are dark, C-type asteroids (carbonaceous chondrite meteorites)

Low density, volatile-rich, much opaque (carbon-bearing?) material

Current detectable minimum size is 14 km.

Many are S-type asteroids (chondrite and achondrite meteorites)

Either stony, chondrite-like objects or stony-iron objects or a combination of the two.

Current detectable minimum size is 7km.

A few metallic (Ni-Fe) and basaltic types.

Physical characteristics

Highly irregular shapes

Well developed regoliths

Some very rapid spins

Some may be contact binaries or loose aggregates.

HAZARD RELATED TO ECAs (see NASA, 1992)

The 130+ known impact craters on the Earth show that hits occur

Distribution of terrestrial impact craters (NASA, 1992)

One, the Ries event 14 million years ago produced a crater now 26km in diameter.

Quiet Crust Stage impacts on the Moon are known to be "young"

Copernicus: ~900 million years

Tyco: ~100 million years

Figure: Estimated number of ECAs (NASA, 1992) (to be supplied later)

The work of Alverez, et al (1980) strongly indicates that a large impact event wiped out the dinosaurs and many other species 65 million years ago (the K-T event)

Focused much more attention on evaluating the risks associated with this identified hazard.

This risk is in the "low probability, high consequence, we can do something about it" category.

Global danger is that stratospheric dust and general atmospheric chemical changes will depress global temperatures and increase acid rain, and ozone depletion long enough to seriously endanger civilization.

Effects of a > 1km diameter impacting on the Earth at about 20km/sec (Silver and Schultz, 1982)

Crater:

> 26km diameter with 100 times the mass of the impactor ejected

Major ejecta to one or more crater diameters

Significant ejecta to many crater diameters

Tsunami of extraordinary scale if impact is in the ocean

Atmosphere

Large quantities of NO formed in high temperatures of the bow shock wave (1-10% of the atmosphere?) and Cl₂ and SO₂ if impact is in the ocean

Ozone depletion (NO) and acid rain (NO, Cl₂, SO₂) if impact is in the ocean

Reduced sunlight transmission due to NO

Large quantities of fine dust (10% of mass of impactor?)

Complete solar blockage for 3-6 months?

collapse of photosynthesis

subfreezing temperatures on the continents

widespread snow

Large quantities of CO₂ if impact is in carbonates or in the ocean

rebound may be to a temporary greenhouse situation.

Geology

Geochemical signatures (like Ir, etc.)

Tectites

Penetration of ocean crust

Tsunami effects at ocean margins

disruption of normal land and ocean geological and biological processes

Volcanism

Seismicly induced fracturing to activate or reactivate existing magma sources (Decan flood basalts in India seem to coincide with the K-T event

Release of lithostatic pressure above materials near melting point (Sudbury crater in Canada.

Biosphere

Mass extinctions

--several others beside the K-T event now suspected

Rapid speciation of surviving forms.

WHAT'S THE RISK?

For a truly global catastrophe, the minimum mass required at 20km/sec is on the order of 10¹⁰ billion tons or a ground burst explosion approaching 10⁶ megatons tons TNT.

This implies a diameter of the ECA of between 1 and 2km depending on density and velocity.

Frequency of such ECA impacts (NASA. 1992)

Average interval estimated as 500,000 years.

However, paleontological and geological evidence has not confirmed this, in fact, so far it appears somewhat pessimistic.

DETECTION OF ECAs (NASA, 1992)

With current technology, we could discover and track for orbit determination nearly all asteroids and short period comets >1km in diameter.

moderate sized ground telescopes

small constellation of small, imaging satellites

The few observers now looking discover several new ECAs/mo out of estimated thousands.

A ground system using current technology would cost an estimated 50M and $10M/year for detecting, tracking and coordination.

DEFLECTION OF ECAs

Should the human species worry about this hazard?

Should a detection and tracking system be considered a high priority along with everything else?

If so, should a continuously upgraded capability be established to deflect a threatening ECA?

Deflection options

rockets and nuclear explosives

long duration low thrust propulsion attached to the ECA

other?

RESOURCES OF NEAs

General nature of meteorites (Lewis and Hutson, 1993)

* Stones: silicate dominated (96% of all falls)

* Chondrites (88%)

* Primitive, unmelted, undifferentiated materials, 4.6 eons

* Abundances of rock-forming elements close to solar proportions

* Usually contain glassy "droplets" called chondrules

* Achondrites (8%)

* Very silicate-rich igneous textured objects (99% silicates and oxides)

* Formed by density-dependent differentiation (gravity field)

* Most 4.6 eons

* Stony-Irons (1% of all falls)

* About 50% ferrous metal alloys, 50% silicates

* Irons (3% of all falls)

* About 99% metallic Fe-Ni-CO alloys

* Inclusions of FeS, phosphides, carbides, graphite, silicates

Other than deflection of a threatening ECA, what might the capability to work at an ECA be used for? (On the other hand, if we could go to an ECA to deflect it, we could go to any NEA to get resources)

Any resources we find on the Moon for use in space probably would be found on the NEAs, and in some cases, in significantly greater concentrations.

On the other hand, is there anything the Moon could not supply more economically and reliably?

One exception may be the volatiles from carbonaceous asteroids, including extinct cometary nuclei: water, CO, and CO₂ (Nichols, 1993, and Weissman and Campins, 1993)

Probably not, except in the case of Phobos and Deimos, asteroid like moon of Mars, where supplies for Mars-related shuttles might be extracted.

Phobos

Additional Phobos Info

Deimos

Additional Deimos Info

What might be of commercial interest for use on Earth? (Kargel, 1994)

Whatever it might be, like ³He from the Moon, it must provide a return on investment commensurate with the risk of the loss of that investment.

Note that success would bring a drop in the price of the commodity of interest due to increased supply.

However, Kargel suggests that some NEAs, if judged only on the chemical analyses of meteorites, have sufficient gold and platinum group metals (Pt, Ir, Os, Pd, Rh, Ru) to pay a huge return on investment even in the face of significantly deflated prices.

If a NEA 1km in diameter contains 100ppm precious metal (and some meteorites do) 400,000 tons of such metal could provide $320B at deflated market prices ($5.1T at current prices).

At the lower prices, increased use may increase returns on the investment.

Major cost factors to consider:

Earth to asteroid launch costs (might be shared with cost to commercialize ³He and to have capability to deflect ECAs)

Spacecraft and extraction hardware costs (also might be so development cost sharing possible)

Low gravity fields at NEAs probably makes operations more difficult

Recurring costs of sustained operations

Cost of capital

Text:

References:

Alverez, L.W., et al, 1980, Extraterrestrial cause for the Cretaceous-Tertiary Extinction, Science, v208, 1095-1108.

Greenberg, R. and Nolan, M.C. , 1993, Dynamical relationships of near-Earth asteroids to Main-Belt asteroids, in Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 473-492.

Kargel, J.S., 1994, Metalliferous asteroids as potential sources of precious metals, Journal of Geophysical Research, v 99, 21129-21141.

Lewis, J.S., and Hutson M.L., 1993, Asteroidal resource opportunities suggested by meteorite data, in Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 523-542.

Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 977p.

NASA, 1992, The Spaceguard Survey, D. Morrison, Chair, Report of the NASA International Near-Earth-Oject Detection Workshop, January 25, 1992.

Neal, V., et al, 1989, Extravehicular Activity in Mars Surface Exploration, Report on Advanced Extravehicular Activity Systems Requirements Definition Study, NASA-17779.

Nelson, M.L. et al, 1993, Review of Asteroid Compositions, in Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 493-522.

Nichols, C.R., 1993, Volatile products from carbonaceous asteroids, in Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 543-568.

Shoemaker, E.M., et al, 1990, Asteroid and comet flux in the neighborhood of Earth, in Geological Society of Americal Special Paper 247, 155-170.

Silver, L.T., and Schultz, P.H., 1982, Geological Implications of Impacts of Large Asteroids and Comets on the Earth, Geological Society of America Special Paper 190, 527p.

Wetherill, G.W., and Shoemaker, E.M., 1982, Collision of astronomically observable bodies with the Earth, in Silver, L.T., and Schultz, P.H., 1982, Geological Implications of Impacts of Large Asteroids and Comets on the Earth, Geological Society of America Special Paper 190, 1-14.

Weissman, P.R., and Campins, H., 1993, Short-Period Comets, in Lewis, J.S., et al, 1993, Resources of Near-Earth Space, University of Arizona Press, 569-618.