Small Impacts

Impact Physics

Meteorite Classification

• Composition:
  • Stony: 94.6% (Solid Rock, Density ≈ 3 g/cm³)
  • Stony Iron: 1.0%
  • Iron: 4.4%

Impact Velocity

• Minimum impact velocity for an object falling to Earth from a large distance: $11.2 \text{ km/s}$ (25,000 mph).

Fireballs

• Fireballs are tracked by US Government Sensors.
• Data available from 1988-Apr-15 to 2023-Feb-19.
• Data includes Impact Energy, logged in kilotons (kt).
• Source: https://cneos.jpl.nasa.gov/fireballs/

Impact Frequency vs Size

• A chart illustrates the average time between Earth impacts relative to object size.
• Examples:
  • 10 km object: impacts every 100 million years.
  • 100 m object: impacts every 10,000 years.
  • 1 m object: impacts yearly.
  • 1 cm object: impacts hourly

Projectile Size and Effects

1 cm Projectile

• Rate: impacts once per hour.
• It burns up during atmospheric traverse.
• No sound or energy experienced by observer.
• Often unobserved during daylight.

10 cm Projectile

• Rate: impacts approximately every 5 days.
• Breaks up at an altitude of 151,000 ft.
• Bursts into fragments at 150,000 ft.
• Residual velocity after the burst: $8.2 \text{ km/s}$.
• No crater is formed, but fragments may strike the surface.
• Air blast arrives 2.36 minutes after impact.
• Maximum wind velocity: $0.00679 \text{ mph}$.
• Sound Intensity: 2 dB (Barely Audible).

1 m Projectile

• Rate: Impacts every few months.
• Breaks up at an altitude of 155,000 ft.
• Bursts into fragments at 144,000 ft.
• Residual velocity after the burst: $9.6 \text{ km/s}$.
• No crater is formed, although large fragments may strike the surface.
• Air blast arrives 2.28 minutes after impact.
• Maximum wind velocity: $0.00772 \text{ mph}$.
• Sound Intensity: 23 dB (Easily Heard).

10 m Projectile

• Rate: Impacts approximately every 10 years.
• Breaks up at an altitude of 155,000 ft.
• Bursts into fragments at 92,900 ft.
• Residual velocity after the burst: $8.1 \text{ km/s}$.
• No crater is formed, but large fragments may strike the surface.
• Air blast arrives 1.52 minutes after impact.
• Maximum wind velocity: $2.07 \text{ mph}$.
• Sound Intensity: 52 dB (Traffic).

25 m Projectile

• Rate: Impacts approximately every 127 years.
• Breaks up at an altitude of 155,000 ft.
• Bursts into fragments at 55,400 ft.
• Residual velocity after the burst: $6.1 \text{ km/s}$.
• Energy of burst: $0.26 \text{ MT TNT}$.
• No crater is formed, although large fragments may strike the surface.
• Air blast arrives 59.4 seconds after impact.
• Maximum wind velocity: $12.2 \text{ mph}$.
• Sound Intensity: 67 dB (Heavy Traffic).
• Glass windows shatter.

75 m Projectile

• Rate: Impacts approximately every 2,700 years.
• Breaks up at an altitude of 155,000 ft.
• Bursts into fragments at 3,180 ft.
• Residual velocity after the burst: $2.24 \text{ km/s}$.
• Energy of burst: $10 \text{ MT TNT}$.
• No crater is formed, although large fragments may strike the surface.
• Air blast arrives 30.4 seconds after impact.
• Maximum wind velocity: $165 \text{ mph}$.
• Sound Intensity: 91 dB (Very Loud).
• Glass windows shatter.
• Wood-frame buildings collapse.
• 90% of trees are blown down; remaining trees are stripped.

100 m Projectile

• Rate: Impacts approximately every 6,000 years.
• Breaks up at an altitude of 155,000 ft.
• Impacts ground at $4.58 \text{ km/s}$.
• Energy of impact: $4 \text{ MT TNT}$.
• Crater is formed.
• Air blast arrives 30.3 seconds after impact.
• Maximum wind velocity: $88 \text{ mph}$.
• Sound Intensity: 85 dB (Heavy Traffic).
• Impact Crater Diameter = $1.23 \text{ km}$
• Depth = $262 \text{ m}$
• Breccia Thickness = $122 \text{ m}$
• The major seismic shaking will arrive 2 seconds after impact. Richter Scale Magnitude: 5.0

200 m Projectile

• Rate: Impacts approximately every 41,000 years.
• Breaks up at an altitude of 155,000 ft.
• Impacts ground at $9.81 \text{ km/s}$.
• Energy of impact: $188 \text{ MT TNT}$.
• Crater is formed.
• Air blast arrives 30.3 seconds after impact.
• Maximum wind velocity: $586 \text{ mph}$.
• Sound Intensity: 105 dB
• Impact Crater Diameter = $2.96 \text{ km}$
• Depth = $630 \text{ m}$
• Breccia Thickness = $292 \text{ m}$
• The major seismic shaking will arrive 2 seconds after impact. Richter Scale Magnitude: 6.0

Other Effects

• Ejected Material:
  • Arrives 45.2 seconds after impact.
  • Average Ejecta Thickness: 28 cm (11 inches).
  • Mean Fragment Diameter: 8.04 meters (26.4 feet).

Regional vs. Local Effects

• For impactor sizes greater than ~200 meters, impact effects become regional rather than local.

Asteroid 2024 YR4

• Estimated to be about 40 to 90 meters wide.
• Initial analysis indicated a more than 1% chance of impacting Earth on Dec. 22, 2032.
• Later calculations significantly lowered the risk; impact probability to 0.004%.
• No significant potential for impact for the next century, as of Feb 24, 2025.

Asteroid Size and Cumulative Volume

• A graph illustrates the number of asteroids vs. diameter.

Important Caveat

• The presented data is based on a specific setup: rock, 45° impact angle, $11.2 \text{ km/s}$ velocity.
• Different materials, angles, and velocities will produce different results.
• Small changes can lead to very different outcomes; the relationship is not linear.

Meteor Crater

• Impactor Diameter: 50 meters.
• Composition: Iron.
• Mass: 500,000 tons (1 billion lbs).

Earth Impact Effects Program

• An interactive web site for estimating regional environmental consequences of an impact on Earth.
• Estimates ejecta distribution, ground shaking, atmospheric blast wave, and thermal effects.
• Calculates the size of the crater produced.
• Website: https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/

The Color of a Meteor

• The color depends on its chemical composition.
  • Nitrogen/Oxygen
  • Iron
  • Sodium
  • Calcium
  • Magnesium

Terminal Velocity

• The highest velocity attainable by an object as it falls through a fluid (air).
• Terminal velocity of a 10 cm diameter rock is ~ 100 - 200 mph

Historic Meteor Events

• Wold Cottage Meteorite (Dec 13, 1795): A stone fell from the atmosphere in Breadth 28 inches, Length 36 inches and Weight was 56 pounds.
• L'Aigle (April 26, 1803): Between 2000 and 3000 stones rained down on the village of L'Aigle on a clear day at 1:00 pm in the afternoon by Jean-Baptiste Biot.
• Tunguska event (June 30, 1908): An explosion over Eastern Siberian Taiga flattened 2,000 square kilometers of forest; largest impact event on Earth in recorded history.
• Chelyabinsk Meteor (15 Feb 2013)