Near Earth Objects | SciByte 12

chris

11 years ago

This week on SciByte we take a look at Near Earth Object, what qualifies as a NEO, what dangers they actually pose and some of the impacts that had already occurred on the Earth. Plus we take a quick look at at the DAWN spacecraft that is currently orbiting the asteroid Vesta and has plans to visit the asteroid Ceres as well.

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Show Notes:

What is a Near Earth Object? NEO

A Solar System object, like comets or asteroids, whose orbit brings them into close proximity, less than 1.3 AU, with the Earth.
That’s 15,245.6 times the diameter of the Earth, or 508.2 times the distance from the center of the Earth to the center of the Moon.
- 1.3 AU = 120,842,549.5 mi / 194,477,231.9 km
- Astronomical unit (AU) : 92,955,807.3 mi / 149,597,870.7 km
- Earth’s Diameter :7,926.4 mi / 12,756.3 km
- Moon Orbit ~= 238,858.2 mi / 384,405 km
- Moon Orbit ~= 30 Earth Diameters

Asteroids and Meteoroids and Meteorites … oh my!

Asteroid : A relatively small, inactive, rocky body orbiting the Sun.
Comet : A relatively small, at times active, object whose ices can vaporize in sunlight forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust and/or gas.
Meteoroid : A small particle, conventionally below 32ft / 10m, from a comet or asteroid orbiting the Sun.
Meteor : The light phenomena which results when a meteoroid enters the Earth’s atmosphere and vaporizes; a shooting star.
Meteorite : A meteoroid that survives its passage through the Earth’s atmosphere and lands upon the Earth’s surface.

What was that thing I saw in the sky?

It depends on when and what you saw …
Meteors, often called shooting stars or fireballs, streak across the sky in a matter of seconds, can leave a faint ionization trail visible for minutes, and can be as bright as the Moon sometimes.
Satellites in orbit around the Earth, are much slower moving and relatively constant in brightness. Just after sunset and before sunrise, are likely times to see them as this is the time when they are reflecting sunlight but it is still dark on the surface of the Earth. (Some satellites can flare up for a few seconds and become very bright when their solar panels reflect the sunlight.)

Asteroids / NEO’s Facts

The mass of all the objects of the Main asteroid belt, lying between the orbits of Mars and Jupiter, is about 4 percent of the mass of the Moon.
Objects spend on average a few million years as NEOs before hitting the Sun, being ejected from the Solar System, or (for a small number of them) hitting a planet.

How Many Near-Earth Objects Have Been Discovered So Far?

August 8, 2011 : 8,168 Near-Earth objects have been discovered.
- 828 have a diameter of approximately 0.6mi / 1 km or larger
- 1,243 have been classified as Potentially Hazardous Asteroids (PHAs).
- NASA – How many NEO’s have been discovered?
  *Generally the hype from an object is more due to the ‘late discovery’ of an object. With some being discovered mere days before an encounter.

Potentially hazardous object

An asteroid or comet with an orbit such that it has the potential to make close approaches, within 0.05 AU, to the Earth and a size large enough to cause significant regional damage in the event of impact.
That’s 586.4 times the diameter of the Earth, or 19.5 times the distance from the center of the Earth to the center of the Moon.
- Astronomical unit (AU) : 92,955,807.3 mi / 149,597,870.7 km
- Earth’s Diameter : 7,926.4 mi / 12,756.3 km
- Moon Orbit ~= 238,858.2 mi / 384,405 km</li> <li>Moon Orbit ~= 30 Earth Diameters
Diameter is at least 492ft / 150 m.
- Would cause regional devastation to human settlements. No impact of this size has occurred during human history.
- Such impacts would occur on average around once per 10,000 years.

How often does the Earth get a close encounter?

Objects with diameters of 16-30 ft / 5-10 m impact the Earth’s atmosphere approximately once per year. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized
- These can produce as much energy as the atomic bomb dropped on Hiroshima
- ~15,000 tonnes of TNT
- Every 2000–3000 years NEAs produce explosions comparable to the one observed at Tunguska in 1908
Objects with a diameter of one kilometer hit the Earth an average of twice every million year interval
Large collisions with five kilometer objects happen approximately once every ten million years.

Impact Craters on Earth

It was around the turn of the century that the idea that craters were due to impacts rather than volcanism.
Grove Karl Gilbert : In 1892 Gilbert would be among the first to propose that the moon’s craters were caused by impact rather than volcanism
Daniel Barringer : In 1903, mining engineer and businessman Daniel M. Barringer suggested that the crater had been produced by the impact of a large iron-metallic meteorite.
It wasn’t until 1960 that we had definitive proof that there were actual impact craters on Earth.
This was proved by Eugene Shoemaker, the same guy who co-discovered the comet Shoemaker-Levy 9 that hit Jupiter in 1994, studying Meteor Crater in Arizona.
The key discovery was the presence in the crater of the mineral stishovite, a rare form of silica found only where quartz-bearing rocks have been severely shocked by an instantaneous overpressure.
Where are all the Earth impact we know about?

How do we categorize the danger level?

Palermo Technical Impact Hazard Scale : a logarithmic scale used by astronomers to rate the potential hazard of impact of a near-earth object (NEO) and combines two types of data; probability of impact, and estimated kinetic yield, into a single “hazard” value.
- A rating of 0 indicates a low hazard level
- A rating of +2 would indicate the hazard is 100 times more likely
Torino Scale : a method for categorizing the impact hazard associated with near-Earth objects (NEOs) such as asteroids and comets. It is intended as a tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known kinetic damage potentials into a single threat value.

NEO Asteroid 99942 : Apophis

Diameter : 886 ft / 270 m
2.9 x height Statue of Liberty [ 306 ft / 93.47 m ]
2 x height of Pyramids of Giza [ 449.5 ft / 137 m ]
Rotation : 30.4 h
Mass [ 59,524,810,800 lb / 27,000,000,000 kg ]
4.5 x Great Pyramids of Giza : 13,227,735,700 lb / 6,000,000,000 kg
519 x RMS Titantic : 114,640,376 lb / 52,000,000 kg
Caused a brief period of concern in December 2004 because initial observations indicated a small probability (up to 2.7%) that it would strike the Earth in 2029.
Additional observations provided improved predictions that eliminated the possibility of an impact on Earth or the Moon in 2029
However, a possibility remained that during the 2029 close encounter with Earth, Apophis would pass through a gravitational keyhole, a precise region in space no more than about a half-mile wide, that would set up a future impact on April 13, 2036.
This possibility kept the asteroid at Level 1 on the Torino impact hazard scale until August 2006, when the probability that Apophis will pass through the keyhole was determined to be very small.
Apophis broke the record for the highest level on the Torino Scale, being, for only a short time, a level 4, before it was lowered

NEO Asteroid 99942 : Apophis what DID NOT happen

Apophis Path of Risk
Energy Estimates were originally equivalent of 1480 megatons of TNT, but were later refined to estimate was 880 megatons, then revised to 510 megatons
- Barringer Crater or the Tunguska event are estimated to be in the 3–10 megaton range
Biggest hydrogen bomb ever exploded, the Tsar Bomba, was around 50 megatons
Krakatoa was the equivalent of roughly 200 megatons
Chicxulub impact has been estimated to have released about as much energy as 100,000,000 megatons
It was estimated that the hypothetical impact of Apophis in countries such as Colombia and Venezuela, which are in the path of risk, could have more than 10 million casualties

DAWN Spacecraft (https://dawn.jpl.nasa.gov/)

Science Payload that includes : Camera’s, Visible and Infrared Spectrometer, Gamma Ray and Neutron Spectrometer (GraND), and Gravity Science
Images of Vesta and Ceres in three colors and black and white
Full surface with mapping spectrometer
In three bands, 0.35 to 0.9 micron, 0.8 to 2.5 micron and 2.4 to 5.0 micron
Neutron and gamma ray spectra to produce maps of the surface elemental composition of each asteroid
Including the abundance of major rock-forming elements (O, Mg, Al, Si, Ca, Ti, and Fe), trace elements (Gd and Sm), long-lived radioactive elements (K, Th, and U), and light elements such as H, C, and N, which are the major constituents of ices.
Radio tracking to determine mass, gravity field, principal axes, rotational axis and moments of inertia.