Asteroid 2002 NT7 by Nadia Paes and Kate Sloyan

On the 24th July 2002 the world was told, on the basis of 17 days of observations, that life on Earth may be destroyed as soon as February 1st 2019. Media coverage, coupled with images of Hollywood blockbusters Armageddon and Deep Impact , made us all aware of the devastation an asteroid impact could bring. But is it all true? Could Asteroid 2002 NT7 really bring about the end of mankind?

Astronomer John Rogers captured this image of Asteroid 2002 NT7 using a 0.3 meter telescope at the Camarillo Observatory.

But first we need to know what asteroids really are:

WHAT ARE ASTEROIDS?

Asteroids are rocky bodies, often containing fractions of metals or metallic compounds, that orbit the Sun. They are too small to be considered planets so are termed to be minor planets. They comprise of material left over from the formation of the solar system. Asteroids, as with all of the planets in the solar system, are made from material that originates from the death of stars (leading to the formation of atoms heavier than hydrogen). The majority of this material often goes on to coalesce into a planet. It is believed that most recorded asteroids are formed from the small fraction of left over material. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1500 kilometres in diameter which is less than half the diameter of the Earth’s moon.

Artist's impression of asteroid belt Diagram of the main asteroid belt

Asteroids range widely in size from Ceres, which has a diameter of approximately 1000 kilometres down to the size of pebbles. Of those asteroids catalogued, only sixteen are known to have a diameter of 240 km or greater. Their positions in the solar system range from within the Earth’s orbit to beyond Saturn's orbit. The majority, however, are contained within a single band across the solar system known as the main belt. The main belt is found between the orbits of our neighbouring planet Mars and the largest planet in the solar system, Jupiter. Although some, like asteroid 2002 NT7 have orbits that cross Earth's path, most do not pose any threat of impact. Some have hit the Earth in the past and there are many examples of impact craters across the world. The Earth’s climate is usually able to sustain any changes that the impact may produce. However there is a threshold point and when an asteroid exceeding this point is in mass impact with the Earth the consequences are devastating. It is believed that one such impact led to the extinction of the dinosaurs.

Asteroids which set on a collision course with Earth are called meteoroids. When a meteoroid passes through our atmosphere at a high velocity, friction between the molecule of gas being forced out of the way at great speed and the meteoroid's surface causes it to incinerate in a streak of light known as a meteor. If the meteoroid is not completely burnt up, what remains to strike the Earth is called a meteorite. Much of our understanding of asteroids comes from examining these pieces of space debris that strike the Earth.

CLASSIFYING ASTEROIDS

In general, the classification of asteroids can be separated into two categories:

COMPOSITION – what material they are made from.

ORBIT – the positioning of the asteroid’s orbit in the solar system.


Composition:

It is possible to classify asteroids into a number of types according to their spectra. By observing the spectra of sunlight reflected off an asteroid’s surface and observing the absorption bands, the minerals that form the asteroid can be identified. The different categories of asteroid composition are as follows:

§ C- type (where C indicates carbonaceous nature)
Approximately 75% of all asteroids visible from Earth fall into this category. C-type are extremely dark in colouration and are very similar to carbonaceous meteorites. These asteroids have the same chemical composition as the sun, minus hydrogen, helium and other volatiles and their spectra have relatively blue colours and are fairly flat and featureless. They are believed to be the most primitive material in the solar system because they appear to be unaltered since the planets' formations. They are thought to be unaltered because they are dark in colour, thus revealing their hydrocarbon content. This hydrocarbon content shows evidence of containing water that has not been melted since they first formed.

§ S-type (where S indicates silicaceous nature)
This type of asteroid represents about 17% of the total asteroid population. They have a metallic composition, containing nickel, iron and magnesium silicate compounds. The spectra relative to this class are reddish. It is believed that this category of asteroids is related to the stony-iron meteorites or the chondritic meteorites.

§ M-type (where M represents metallic nature)
This class includes the majority of the remaining asteroids. They are rare because they contain an pure nickel-iron alloy. They appear to be ‘melted’ asthere are indications of volcanic lava flow on the surface.

There are a dozen or so other rare types of asteroids, the most well known of which are listed below:
§ A type
Very few have ever been discovered. They contain an abundant amount of olivine (a large constituent of S-type asteroids also) which is an olive green magnesium-iron silicate.

§ P-type
These asteroids have a reddish tinge and their composition remains a mystery to scientists.

§ D-type
These are redder in colouration than P-type asteroids and are once again unknown in composition.

§ E-type
These asteroids are rich in the compound enstatite which is a whitish, brown and occasionally pale green magnesium silicate.

Without direct evidence it is hard to say what type of asteroid 2002 NT7 is. However, based on the figures above it is probable that asteroid 2002NT7 is either a C-type or S-type asteroid.


Orbits:

Bias involved with observation methods of asteroids (e.g C-type asteroids are darker in colouration and therefore more difficult to see), the percentages of each composition category above may not be representative of the true distribution of asteroids. For this reason there are other methods of classification for asteroids. An asteroid’s orbit position in the solar system is another method of classification:

Main Belt Asteroids
The main belt is the region that lies between Mars and Jupiter where the majority of the asteroids in our solar system are located. The main belt is divided into further sub-groups; Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas. Each sub-group is named after the main asteroid in the group. Between the main concentrations of asteroids are regions known as the Kirkwood gaps, which are relatively empty. In these regions an object's orbital period would be very small in comparison to that of Jupiter so would be likely to be accelerated by Jupiter into a different orbit.

Trojans
Trojans are located near Jupiter’s Lagrange points, these are points where two asteroids are 60 degrees ahead and behind Jupiter respectively in its orbit. Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable:

Diagram of Lagrange points

Asteroids that lie at Lagrange points are known as Trojans. Several hundred such asteroids have been discovered but it is estimated that there may be a thousand or more altogether. There are many more in the leading Lagrange point (L4) than in the trailing one (L5).

Near-Earth Asteroids (NEAs)
NEAs are asteroids that closely approach the Earth. There are three key types:
· Atens – asteroids that have orbits smaller than that of the Earth’s.
· Apollos – asteroids that intersect the Earth’s orbit.
· Amors – asteroids that intersect Mars' orbit.

Centaurs
Centaurs are asteroids in the outer solar system. Their orbits may lie anywhere between Jupiter to beyond Neptune.

ASTEROID 2002 NT7

Orbital diagram of inner solar system objects

Although we cannot know many of the features of 2002 NT7 for certain, it is likely that it shares many of the characteristics of other minor planets. It is probably made primarily of silicate, hydrocarbons, iron/nickel, or a mixture of the three. Unlike the majority of bodies that orbit the sun in the same plane, 2002 NT7 travels at an angle of 42.35 degrees to the Earth’s orbit at a distance of between 0.82 and 2.66 AU. Because of this, the asteroid only crosses the path of the Earth once in its period (2.29 years), usually when the Earth is elsewhere in its orbit.

People begin to panic, however, when the possibility of the two colliding occurs. The most remarkable feature of this object is its size; estimated to be around two kilometres in length. An object of this magnitude would release approximately 1 million megatonnes of energy on collision, causing not only massive local destruction but also severe climate change. The large amounts of debris injected into the atmosphere could block out sunlight, depressing temperatures around the globe. Because of this, first plants, then herbivores and ultimately predators such as humans would die.

ASTEROID IMPACTS

An example, calculated by Richard W Tinus and David J Roddy
If Asteroid 2002 NT7 were to collide with the Earth it would hit at a speed of 28km/s. An impact of this magnitude could lead to a drop in temperature of up to 10 degrees C. Unused to the extreme cold, 50% of forests in the Northern Temperate Zone would die within days. The dead trees would then dry out, creating huge amounts of dry biomass that would then act like kindling. A single lightning strike could ignite it, leading to massive forest fires and 100 million tonnes of atmospheric soot.

Even today we can see the effects of soot and debris on the global climate. Forest fires in Southern Asia, coupled with atmospheric pollution, has lead to the creation of the “Asian Black Cloud”. Primarily consisting of carbon particles from forest fire smoke and unburned hydrocarbons from vehicles and industry, the man-made cloud has shown us the devastating effects on the environment and human health caused by atmospheric debris.

But although the effect of such an impact would be catastrophic, would the human race be rendered extinct? It would not be the first time an asteroid has struck, nor the first time the dominant species has apparently been destroyed. Many believe that the dinosaurs became extinct after an asteroid similar to NT7 hit the Earth, causing climate change to which the giant reptiles could not adapt. The 180- to 300-kilometer-diameter Chicxulub crater in Mexico is believed to be the scar of the asteroid or comet that struck Earth about 65 million years ago, whose impact may have lead to the death of many prehistoric species. Evidence of changing sea levels and quantity of iridium in rock samples from the period have been viewed by some as evidence for the catastrophe theory (see right). Evidence that many marine organisms died suddenly around this point has been found at a number of sites, including Gabbio, Italy (Alvarez et al) and Brazon River, USA (Hansen et al). This again can be interpreted as indicative of changing climate, as the combination of changing sea levels, temperature and levels of sunlight could affect marine biology.

However, there are many other theories relating to the dinosaurs' extinction than the impact of this one body. Volcanic/seismic activity and natural climate change could both affect sea and iridium levels to the same extent as an impact. Many other species – between 25 and 50% – also survived the Cretaceous-Tertiary boundary (the period in which the impact is believed to have occurred). If the catastrophe theory is correct, the effect of an impact devastating enough to destroy the dinosaurs should have destroyed most forms of life simultaneously. However fossil evidence has shown that many species had become extinct long before the believed impact, and that throughout the Late Cretaceous the climate was becoming more unstable and seasonal. Perhaps then it was progressive environmental deterioration that began the downfall of the dinosaurs, with an impact rendering them completely extinct.

However, an asteroid like NT7 could not only bring destruction. Without the extinction of the dinosaurs, caused in part by a catastrophe, other forms of life may not have been allowed to become sophisticated. Mammals in particular benefited from the decrease in predator numbers and have been able to evolve beyond primitive shrew-like creatures. Many scientists believe that an impact may have brought the building blocks of life – carbon-based molecules and water ice – to Earth. Geological evidence shows that for the first billion years of its existence the Earth was bombarded by asteroids and comets, rendering the surface too hot for the first life to develop. There is also no evidence to suggest that the large amounts of the carbon-based molecules required for life existed. However, there is evidence to show that biological activity began to occur around 3.8 billion years ago, at the end of a period of less intense bombardment. It is possible that the building blocks of life, abundant in many asteroids but absent on early Earth, were brought to the planet by an impact. We may owe our existence to a comet or asteroid.

COMPARISON


This may not be a comfort, however, when we learn that we are to be hit by an asteroid. Recent events have shown how easy it is for a threatening NEA to only be detected at the last minute. On the 14th June 2002 asteroid 2002 MN passed the Earth at a distance of only 0.0008AU, or about 120,000 km. This is less than a third of the distance between the Earth and the Moon. Like NT7, MN is a minor planet orbiting the sun probably made primarily of silicate, metals, hydrocarbons or a mixture of the three. It has an absolute magnitude of 23.32 and therefore an estimated diameter of about 120m, just over half that of NT7. Because of this, the implications of a possible impact by MN are not as serious as those of NT7. Had it hit the Earth, it is likely that it would have exploded in the atmosphere with a force in the region of 10 megatonnes. Local devastation would occur, as it did in 1908 in Tunguska, Siberia when the detonation of an asteroid similar to MN flattened 2000 square kilometres of forest, but the impact would not be an extinction level event. This is little damage compared to the possible 1 million megatonnes of damage that an impact by NT7 would cause. MN also travels in a similar plane to the majority of solar system objects, whereas NT7 is inclined by about 42 degrees.

However, the most significant difference between MN and NT7 is the date when they were discovered. NT7 was discovered in July 2002, 17 years before coming close enough to Earth to constitute a threat. MN was only spotted three days after it passed 120,000km from Earth, on June 17th. Although the difference in size makes it easier to spot asteroids such as NT7, it is chilling to think that an NEA can pass so close and yet go undetected. The first warning we may get of a devastating impact may be the sight of a burning rock exploding in the sky

When the story broke, 2002 NT7 had only been observed for 17 days. At the time, the likelihood of an impact was only 1 in 60,000, and subsequent observations have shown the current probability is only 1 in 250,000. Although there are many minor planets orbiting the Sun both they and the Earth are tiny, cosmically speaking. While bodies such as Asteroid 2002 NT7 could potentially wipe out mankind, February 2019 is probably safe.

ACTIVITY

The following group activity involves three short experiments that help to demonstrate how low the probability of an asteroid collision with Earth actually is.

Equipment needed:

Steel ball bearing

2 large magnets

Sandpaper

A meter rule

A flat even surface

Classroom activity

BIBLIOGRAPHY

General information about asteroids:

http://neo.jpl.nasa.gov/torino_scale.htm

http://ca.geocities.com/spacephysicsisu/asteroid.html

http://www.solarviews.com/eng/asteroid.htm

http://neo.jpl.nasa.gov/glossary/h.html

Microsoft Encarta 97

Information about classifying asteroids:

http://spaceguard.ias.rm.cnr.it/tumblingstone/dictionary/class-asteroid.htm

http://seds.lpl.arizona.edu/nineplanets/asteroids.html

Information about Asteroid 2002 NT7:

http://www.klet.cz/2002nt7.html

http://www.rasnz.org.nz/MinorP/Collider.htm

http://curious.astro.cornell.edu/question.php?number=144

http://www.camarilloobservatory.com/pr01.htm

Information about asteroid impacts:

http://neo.jpl.nasa.gov/images

http://news.bbc.co.uk/1/hi/sci/tech/2211402.stm

http://dsc.discovery.com/news/afp/20020819/meteor.html#

http://neo.jpl.nasa.gov/neo/life.html

Global Catastrophes in Earth's History , a collection of papers by various authors including Karl W Flessa, Peter Douglas Ward, Richard W Tinus and David J Roddy. Published by the Geological Society of America

Information about Asteroid 2002 MN:

http://www.spacedaily.com/news/020620121339.dc05vk7l.html

http://neo.jpl.nasa.gov/news/news130.html

http://neo.jpl.nasa.gov/cgi-bin/neo_ca

  !     How do we know?

What are they made of?
Asteroids comprise mainly of rock with traces of metals and metallic compounds. Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials.
Scientists are interested in the composition of meteorites because they can tell us about the history of the solar system and its structure during its infancy. Until recently very little information was obtained from sources external to Earth based observations. However, in the 1990’s, through interaction between spacecrafts such as Galileo with the asteroids 951 Gaspra and 243 Ida and the spacecraft NEAR with 253 Mathilde more in-depth information about composition and high-resolution images could be taken. 951 Gaspra and 243 Ida are examples of S-type asteroids composed of metal-rich silicates and 253 Mathilde was an example of a carbon rich C-type asteroid. The material that an asteroid is composed of is one means of classifying its nature.

Describing the eventuality of an impact
The ‘Torino impact hazard scale’ is used to measure likelihood of an asteroid impact and the extent of the damage it might cause. The scale serves to provide an international language with which to describe the potential danger of an asteroid so that a global understanding of the danger can be obtained. The scale divides the asteroids into five coloured zones, with each progressively more hazardous than the one before. The last three zones, yellow orange and red, are split further into sub-categories outlining the extent of damage that would be caused by the impact. The scale used ranged from 0-10. Asteroid 2002NT7 was rated as a risk of “1”:

"Events Meriting Careful Monitoring (Green Zone) 1: The chance of collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades."

It is described as the highest impact risk asteroid discovered to this day.

Absolute Magnitude
Magnitude is the term used by astronomers to define the brightness of an object, and is based on the scale devised by Ptolemy. The brightest objects have the lowest magnitude and fainter objects have a higher one, with the faintest star so far observed being of the 23rd magnitude. Absolute magnitude is “the apparent magnitude of the object when it is 1 AU from both the sun and the observer”, and is used to determine the intrinsic magnitude of a body rather than the magnitude observed on Earth.
The absolute magnitude and albedo (a measure of how reflective a body is) can be used to estimate of the diameter of an object. Because the albedo of an object is usually unknown, a value is assumed. In the case of asteroids this is between 0.25 and 0.05 so in general, the larger an objects absolute magnitude the larger the object is. Asteroid 2002 NT7 has an absolute magnitude of 16.4, and so is estimated to be around 2km in diameter.

Radio astronomy
Astronomers use various methods to obtain information about celestial objects. The transmitter sends out a series of pulses of radio energy, the modulation characteristics of which are known. Those of the signal returned from the object are compared to obtain the travel time of the signal to and from the object. By using the known velocity of radio waves, the distance to the object can be calculated.
The wavelength difference between the transmitted and received signal is carefully measured. By applying the Doppler effect, the difference can be used to find the velocity of the object relative to the earth. If the object is rotating, the signals returned from various parts of it will be changed in wavelength for that reason. After studying the object for a time the direction in which the object is rotating about its axis can be determined.

Crater at Manicouagan, Canada

Looking at the Moon
We believe that the Earth has been struck many times by large minor planets, and that it may even be a regular event. Craters from previous impacts still exist all over the world, the geology around which may be studied and the age of the crater and therefore the impact determined. On the Moon, where there are none of the atmospheric conditions that lead to the erosion of craters, there is widespread evidence of impacts. The surface of the moon is covered with craters - the remains of impacts that have occurred throughout its history. Because it is unlikely that the Moon would have been hit many times while the Earth remained virtually untouched, the Moon's surface is taken as evidence to support the idea that the Earth has been the subject of bombardment by minor planets.

Crater at Yucatan, Mexico

Looking at the climate
A picture of the climate at various points in the Earth's history can be built up by analysing rocks that formed through the build up of sediments. These are composed of both biogenic (organic) and terrigenous (inorganic) materials, analysis of which records may reveal information about past water temperatures, composition and nutrient availability, continental humidity-aridity variations, and the intensities and directions of winds.

Iridium
Iridium is a metal that does not occur in large quantities on Earth, but is found in greater amounts in many asteroids. Elevated iridium levels are seen by some as evidence of an asteroid impact as larger amounts are unlikely to result from a terrestrial source.


Spherules
A recent study by researchers from the Stanford and Louisiana State Universities has published evidence of an impact billions of years ago. ‘Spherules’, Small particles of impact debris, were found at sites in Barberton (South Africa) and Pilbara (Australia), thousands of miles apart. An instrument called the ‘Sensitive High-Resolution Ion MicroProbe Reverse Geometry’ was used to determine the age of zircon particles in the rock, and therefore the age of the impact. Further analysis has found high levels of iridium, a metal often found in meteorites. This evidence supports the idea that a body 20km in diameter hit the Earth 3.47 billion years ago. A crater has not been found, suggesting that the asteroid may have hit the ocean, tearing up the sea floor and causing massive tidal waves

Durable zircon particles

Something to think about
What would be the best way to stop an asteroid heading towards Earth? Should we send nuclear bombs to detonate it, possibly creating a series of smaller pieces that are just as deadly? Should we try to blow it off course, and if so how? What would happen if the rocket carrying the bombs exploded in the atmosphere? Or can you think of another way?

Image of Asteroid 2002 NT7

This image of Asteroid 2002 NT7 was observed by J. Ticha and M. Tichy at the Klet Obsevatory on 19th July 2002. It was taken using the 0.57-m f/5.2 reflector and CCD camera SBIG ST-8.