Impact Hazard

Lecture 7: Impact Hazard

Introduction to Impact Hazard

  • Topics Discussed:

    • Faults

    • Earthquakes (EQ)

    • Seismic Waves

    • Seismology

    • EQ Hazards

    • EQ Preparedness

    • Asteroids and Comets

    • Meteors and Meteorites

    • Impact Hazard

    • Mass Extinction Events

    • Trajectory Predictions

    • Mitigation Strategies

  • Learning Objectives for the Module:

    • Explain why the impact hazard is unique.

    • Describe the properties of asteroids, comets, meteoroids, meteorites, meteors, fireballs, and bolides.

    • Calculate the energy associated with an impact.

    • Describe the consequences of large impacts.

    • Explain mass extinctions and extinction-level impacts.

    • Explain how asteroids are discovered and tracked.

    • Describe the Torino threat scale.

    • Explain impact avoidance strategies.

Orbits of Potentially Hazardous Asteroids

  • The Unique Nature of Impact Hazard:

    1. Can wipe out civilization.

    2. Can be prevented.

Nature of the Impact Hazard

  • Transportation accidents typically kill fewer than 100 persons.

  • Rare natural disasters can kill millions.

  • Only impacts have the potential to kill more than 100 million persons per event (Chapman and Morrison, 1994).

Impact Hazard Statistics

  • Solar system objects include:

    • Planets, moons, dwarf planets

    • Kuiper Belt containing icy objects beyond Neptune's orbit

    • Oort Cloud housing billions of icy objects ~10,000 AU away

    • Asteroids: rocky or metallic objects larger than 1 m

    • Comets: icy rock masses from the Kuiper Belt or beyond

    • Meteoroids: small (<1 m) rocky or metallic objects

  • Impact Statistics:

    • Most space objects vaporize upon entering Earth's atmosphere.

    • Impacts of small objects are frequent (~100 tons/day).

    • Some impacts have changed the course of life’s evolution.

    • A large impact today would be considered a global catastrophe.

    • Extinction-level events threaten the existence of life on Earth.

Characteristics of Asteroids

  • Definition:

    • 'Asteroid' is derived from Greek meaning "star-like".

    • Small, rocky objects that appear star-like in telescopes and move across the sky like planets.

  • Origins:

    • Some are leftover from the formation of the Solar System.

    • Others are fragments from larger celestial bodies that broke apart.

  • Asteroid Populations Based on Location/Orbit:

    • Main Belt: Between Mars and Jupiter (approximately 1,000,000 asteroids larger than 1 km).

    • Trojan Asteroids: Located at Jupiter's distance.

    • Near-Earth Asteroids: In the inner solar system (about 1,000 asteroids larger than 1 km in Earth-crossing orbits).

  • Gaps in the Main Belt:

    • The main belt is not uniformly populated.

    • Asteroids are absent in areas disturbed by Jupiter's gravitational influence, known as Kirkwood gaps.

    • Objects can escape these gaps and become Earth-crossing.

Largest Asteroids

  • Notable Asteroids:

    • 1 Ceres: Diameter (D) = 940 km, a = 2.77 AU, discovered by Giuseppe Piazzi in 1801.

    • 2 Pallas: D = 540 km, a = 2.77 AU, discovered in 1802.

    • 4 Vesta: D = 525 km, a = 2.36 AU, discovered in 1807.

  • Total of 13 asteroids have D > 250 km.

  • Initially classified as planets but were demoted in 1852.

  • Ceres and Vesta were targets of the UCLA-led Dawn mission.

Physical Properties of Asteroids

  • Determined via telescopic remote sensing:

    • Albedo: Percentage of reflected sunlight ranges from 1% (dark) to 50% (bright).

    • Spin Period: Length of an asteroid's day.

    • Shape: Determined from radar and light curve variations.

    • Mass and Density: Usually determined only for binary asteroids.

  • Asteroid Composition:

    • Revealed through spectroscopy and radar observations:

    • C type: Dark, carbon-rich (albedo ~5%).

    • S type: Brighter, stony (albedo 10–20%).

    • D type: Dark, red spectra.

    • M type: Metallic, very radar-bright.

    • C- and D-types are primitive; S-types are more evolved; M types come from differentiated parent bodies.

Comets

  • Definition:

    • 'Comet' derives from Greek "kometes" (meaning "long-haired").

    • Composed of:

    • Nucleus: Solid inner part primarily consisting of loose rock, dust, and ice (usually a few km in diameter).

    • Coma: Fuzzy bright region of gas and dust surrounding the nucleus.

  • Characteristics of Comets:

    • Produce ion and dust tails when traveling around the Sun.

    • Types of Comets by Orbital Period:

    • Short-period comets (<200 years, Halley type and Jupiter family).

    • Long-period comets (>200 years).

Origins and Behavior of Comets

  • Origins:

    • Trans-Neptunian region or Kuiper Belt (source for Jupiter-family comets).

    • Orbits perturbed by giant planets, primarily Jupiter.

    • Most long-period comets come from the Oort Cloud (20,000 to 100,000 AU).

  • Comet Tails:

    • Two tails develop as they approach the Sun:

    • Dust tail.

    • Ion tail.

Meteoroids and Meteorites

  • Definitions:

    • Meteoroid: Small object with diameters <1 m.

    • Meteorite: Solid fragment of a planetary body that falls on another planetary body.

  • Types of Meteorites:

    • Stony Meteorites: 93% of all meteorites, primarily silicate rock, may contain chondrules.

    • Iron Meteorites: Composed of an iron-nickel alloy.

    • Stony-Iron Meteorites: Contain both metal and rock.

  • Meteoroid Statistics:

    • Approximately 100 tons of meteoroids enter the atmosphere each day; most vaporize.

    • Earth accumulates about 90,000 kg of additional mass annually.

  • Meteor Formation:

    • Meteors are streaks of light resulting from heated air from meteoroids compressing as they enter the atmosphere.

    • Ram Compression Effects:

    • Air heats to ~1,700°C.

    • Causes glowing vapor and dust to form, resulting in meteor phenomenon.

Fireballs, Bolides, and Impact Speed

  • Fireballs: Smoke-like trails from meteors.

  • Bolides: Produce explosions, shockwaves, and sonic booms.

  • Impact Speed Determinants:

    • Mass and size significantly impact impact speed:

    • <7,000 kg (1 m diameter) impacts at ~500 km/h.

    • >1,000,000 kg (4 m diameter) impacts at ~70% cosmic speed.

    • >100,000,000 kg (15 m diameter) impacts at cosmic speed.

Historical Meteorite Events

  • Notable Meteorite Events:

    • 1954: Woman in Alabama bruised by a meteorite.

    • 1992: Meteorite damaged a car in New York.

    • 2003: Fireball debris injured villagers in India.

  • Extraterrestrial Impacts:

    • Lunar impacts observed with telescopes.

    • Shoemaker-Levy 9 comet fragments struck Jupiter in 1994, releasing >6 million megatons of energy, observable from Earth.

Significant Historical Impacts

  • Tunguska Event (1908):

    • A 50–200 m asteroid exploded 5–10 km high, unleashing 10–15 megatons of energy and flattening 80 million trees with no reported fatalities due to its remote location.

  • Chelyabinsk Event (2013):

    • A meteoroid of approximately 10 million kg and 18 m in diameter caused an explosion that shattered glass and injured ~1,500 people due to an extensive air blast.

EnergyRelated to Asteroid Impacts

  • **Energy Calculations:

    • The kinetic energy (KE) associated with impacts can be calculated using the equation:
      KE = \frac{1}{2} mv^2

    • Example from Deep Impact (1998): KE of impact at 5600 m/s is >4e12 J.

    • Convert energy measurements to kilotons of TNT for comparison; 1 kiloton of TNT = 4.2e12 J.

Crater Formation Dynamics

  • Factors Influencing Crater Characteristics:

    • Size, density, and coherence of the impactor.

    • Impact velocity and angle.

    • Composition of the planetary surface.

  • Stages of Crater Formation:

    • Excavation Stage:

    • Shattering of target rock and creation of ejecta.

    • Formation of transient crater.

    • Modification Stage:

    • Transient crater rebounds, leading to a raised rim and possible formation of impact melt.

The Chicxulub Crater Impact

  • Caused by a ~10 km diameter asteroid approximately 66 Ma ago, contributing to mass extinction.

  • **Key Effects:

    • Estimated energy release: 75 million megatons of TNT equivalent.

    • Generated massive shockwaves comparable to MW10–MW13 earthquakes.

    • Produced mega-tsunamis with waves likely reaching 1.5 km high near impact site, diminishing in height to 300 m upon reaching distant shores.

Current Threat Assessment of Near-Earth Objects (NEOs)

  • NEOs Definition:

    • Comprise asteroids, comets, and meteoroids with pericenters <1.3 AU and within 50 million km of Earth’s orbit.

  • Tasks outlined by NASA:

    • Find 90% of NEOs >1 km by 2005.

    • Identify 90% of potentially hazardous objects >140 m (40% found to date).

  • Technologies for discovery include surveys and tracking systems.

Addressing the Impact Hazard

  • Impact Mitigation Strategies:

    • Deflection Techniques:

    • Incline spacecraft trajectory to prevent Earth impact.

    • May require long warning times; various methods including kinetic impactors and gravity tractors.

    • Disruption Techniques:

    • Employing nuclear devices to fragment the asteroid.

    • Short warning times needed but infrastructure for rapid deployment required.

Conclusion

  • The impact hazard is unique as it can lead to civilization-level extinction but is also preventable.

  • Historical and current efforts to understand and mitigate threats posed by potential impacts continue to evolve.