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Astro 202

Spring 2008

COMETS and ASTEROIDS

Small bodies in the solar system

Impacts on Earth and other planets

The NEO threat to Earth

Lecture 1

Don Campbell

ARMAGEDDON

DEEP IMPACT

http://www.youtube.com/watch?v=fUwRyJzJSOELast Days On Earth Part 1 – History Channel/ABC

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The Cretaceous-Tertiary (K-T) impact and extinction of the dinosaurs (plus a lot of other species)

Painting by Don Davis

EXTINCTION OF THE DINOSAURS

Chicxulub crater ~ 180 km diameter- seismic data showing buried crater- dated to 65 million years ago

(about a 10 km diameter impactor)

Articles on NEO Threat in Science, March 7 Issue 2008

http://www.sciencemag.org/cgi/reprint/319/5868/1326.pdf

http://www.sciencemag.org/cgi/reprint/319/5868/1329.pdf

ASTRO 202 SPRING 2008

REFERENCE MATERIAL FOR NEAR-EARTH OBJECT DISCUSSION

Near-Earth Object Survey and Definition Analysis of AlternativesReport to CongressMarch 2007 Summary document (790k)http://www.nasa.gov/pdf/171331main_NEO_report_march07.pdf

Background material and analysis (15 MB)http://www.hq.nasa.gov/office/pao/FOIA/NEO_Analysis_Doc.pdf

Near –Earth Object Issues:Should we be worried?

Finding all or most of them – search techniques

Determining precise orbits so know if there is a possibility of impacting Earth

Characterizing general population and specific potentially hazardous objects (PHOs)

Mitigation techniques – i.e. doing something about a high probability PHO

Political/policy issues – Who is responsible?

Cost – Who is going to pay?

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COMETS:

First known small bodies;

Atmospheric phenomenon or further away from Earth?

Ptolemaic (Greek) view of the universe - Sun and planets form the first 7 crystal spheres and the stars the 8th. Outside that is the sphere of the heavens.

An Earth centered universe

until the 16th century

Thought to be incorruptible and immutable – nothing ever changes

Historically:

Comets thought to be harbingers of bad things.

“Like the red star that from his flaming hair

Shakes down disease, pestilence and war”

Homer, 900 BCE

Comets were thought of as stars with “flaming” hair.

This was a persistent theme until almost modern times because comets broke the regularity of the “heavens” which were supposed to be fixed and immutable.

Tycho Brahe (Danish astronomer) challenged principle of celestial incorruptibility

- he showed that the comet of 1577 was at least 4 times the distance of the Moon so could not be an atmospheric phenomenon. He used parallax –several other astronomers measured positions from other observatories.

Destroyed the idea of immutable crystalline spheres.

Tycho Brahe (1546 – 1601)

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The idea that the “heavens” change and that the Earth and other bodies in the solar system can be “impacted” by small bodies in the solar system was only slowly accepted.

Affected the interpretation of craters on the Moon as late as 50 years ago – volcanic or impact origin – big fight.

The New Solar System:

Read; Chapter 24, Comets by John Brandt

Chapter 25, Asteroids by Clark Chapman

SMALL BODIES – ASTEROIDS AND COMETS

Asteroids -

Most in Main Belt between Mars and Jupiter – source region for NEAs

Near Earth Asteroids (NEAs) - Inside orbit of Mars

Potentially Hazardous Asteroids (PHAs) - NEAs that cross Earth’s orbitCould hit Earth if orbits intersect

Comets -

Periodic comets – There are about 200 comets in short period – < 200 years –orbits about the Sun. Another ~200 have been detected buthave either been lost, destroyed or not completed at least two orbits

“New” comets - Ones that have never been into the inner solar system beforeor have periods so long that this appears to be the case.

N.B. Not always easy to distinguish between asteroids and comets – old comets thathave exhausted most of the volatiles near their surfaces – i.e. no longer have aa coma or tail can look like an asteroid.

COMETS:

What is a comet?

A small body orbiting the Sun with a composition that includes ices, primarily water ice.

As the comet comes near the Sun in its orbit, ices on and below the surface sublimate into gases such as water vapor. The gas escapes through localized “breaks” in the surface carrying off large amounts of dust and small fragments forming the coma and tails that we see.

Comets have two tails, a dust tail and a ionized gas tail (blue in the image).

Some asteroids are thought to be “gassed out” comets.

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Dirty “snow ball” model of nucleus by Fred Whipple (1950)

Ion tail is blue because ions of carbon monoxide fluoresce under sunlight

Halley’s comet in 1910 apparition

Halley’s comet nucleus from ESO Giotto mission – about 15 km in size.

Comets: The nucleus!

Halley’s comet returned in 1986 – its orbital period is 76 years.

First spacecraft flyby of a comet –European and Russian (Soviet Union) missions.

COMETARY NUCLEI WERE THOUGHT TO BE COMPOSED MAINLY OF ICES AND SO VERY GOOD REFLECTORS OF SUNLIGHT

WRONG – Missions to Halley showed that they are very dark – about the darkest bodies in the solar system.

Therefore much larger than we thought

See gas and dust jets on Halley

Comet Borelly from Deep Space 1 spacecraft

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DEEP IMPACT – July 4, 2005

Comet Tempel 1

Impact speed 37,000 km/hr

Only ~1% of surface covered in ice

STARDUST Mission to Comet Wild 2. Flyby Jan 2, 2004

Returned to Earth, Jan 15, 2006

Not primitive material!!

Comets: Where do they come from?Two reservoirs, Kuiper belt and Oort Cloud. Orbits disturbed by passing star, etc causing comets to come into the inner solar system.

COMET ORBITS

http://janus.astro.umd.edu/orbits/outerSS.html

Halley – close approach to Sun = 0.587 AU; eccentricity = 0.967

Semi-major axis = 17.8 AU; Farthest distance from Sun = 35 AU

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Comet velocities:Very high when close to the Sun – i.e. at perihelion

Very low for high eccentricity orbits far from the Sun - i.e. at aphelion

At Perihelion – how high?

Halley’s comet: ~58 km/sec = 200,000 km /hr = ~130,000 miles/hr

What is Earth’s velocity in its orbit = 2π R/P = 2π 1.5 x 108 / 3 x 107

= 30 km/sec

Relative velocity between 90 km/sec and 30 km/sec

Assume typical comet impact velocities to be ~40 km/sec

IMPACT ENERGIES:

Kinetic energy = ½ M V2 M = mass of the object, V = velocity

Assume V = 40 km/sec impact velocity

M = 4/3 π R3 x density R = radius of (spherical) object

Assume R = 2 km and density = 1 g/cm3 - i.e. density of water

Kinetic energy = ½ x 1.7 1010 x 1.6 109 Joules

= 1.4 1019 Joules

How much is that?

One ton of TNT (a common explosive) = 4 109 Joules

THEREFORE COMET KINETIC ENERGY = 3 109 tons of TNT equivalent

= 3,000 megatons of TNT

Do we know of any comet impacts on Earth?

Hard to distinguish between comet and asteroid impacts but:

Tunguska – a giant explosion occurred on June 30, 1908 in Russian Siberia

Energy estimated at 5 to 10 megatons

No expedition reached scene until about 20 years later

Devastated area up to 40 km from the center.

Originally thought to be a comet impact but now thought to be a stony asteroid about 50m in diameter traveling at about 20 km/sec.

½ M V2 = 10 megatons TNT

The air explosion from 60 km away

A few minutes after the explosion

Paintings by William Hartmann –artist and cratering expert

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Comet Shoemaker-Levy

Breakup and Impact into Jupiter

July 1994

HST image just before last impacts showing impact scars

Crater diameter approx. = 0.1 times the cube root of the energy E in kilotons

Glenn Beck: The $53 trillion asteroid

NEW YORK (CNN) -- Let's say a giant asteroid was headed toward Earth right now and experts say it has a good chance of ending civilization as we know it. Let's also say that we've known about this asteroid for years but even as it gets closer and closer our leaders do nothing.

"Don't worry," they tell us, "The next administration will figure something out."

Stardust

STARDUST Mission to Comet Wild 2. Flyby Jan 2, 2004Returned to Earth, Jan 15, 2006

Comet is 4.5 km in size.

Originated in the KuiperBelt

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Stardust

Stardust Stardust

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Stardust landed gently

Genesis did not

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Particle is ~10-5 of a meter in size, 3 components, ion sulphide, Magnesium silicate (Enstatite) and a fine grained material with a composition close to that of chronditesin meteors. The first two are the result of high temperature “processing” close to the forming Sun so particle a mix a materials that originated near and far from the Sun.

Brownlee et al, Science, 314, 1711, 2006

Further analysis found no primitive material – i.e. material (interstellar dust) from which the Sun and solar system was made that was not heated by the forming Sun.

Ishii et al, Science, 319, 447, Jan 2008

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Deep Impact Deep Impact

Impactor weighs 370 kg (about 800 lbs) and the impact velocity was ~10 km/sec.

Kinetic energy (1/2 MV2) was about 19 109 Joules = ~5 kilotons of TNT

Deep Impact - January 12, 2005

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Final crater estimated to be about 100 m across and 30 m deep -implies very porous material with density about half that of water ice. Very fine grained particles reflected sunlight making it difficult to see the crater.

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Mix of primitive material and material that underwent high temperature processing near the early sun as it formed.

* Comet Churyumov-Gerasimenko

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Positions of all known asteroids projected onto the plane of the Earth’s orbit on Jan 20, 2008

Red ones (Apollo and Alten) cross the orbit of Earth so are potentially hazardous. Yellow ones (Amors) don’t cross Earth’s orbit now but could in the future if their orbits are disturbed.

NEAR-EARTH ASTEROIDS