The questions and answers on this page are related to the minor bodies of the solar system and space debris.
If and when a meteoroid passes close enough to the Earth to be captured by the Earth's gravity, it will enter the upper atmosphere, and a process of ablation will occur. In this process, the kinetic energy of the meteoroid is reduced by collision with the air molecules, heating up the outer layers of the meteoroid and causing them to vapourise and fragment. This process is visible to observers on the Earth as a meteor. At the same time as the light and heat are produced, the air is ionised (electrons are stripped from the air molecules), and this ionisation may be detected by a meteor radar.
It is most important to understand that the term meteor refers to the visible (or radio) phenomenon that the meteoroid produces in our upper atmosphere. The word meteor should not be used to refer to the physical body (i.e. a meteor is not the name for the meteoroid when it is in the atmosphere) In fact, the general definition of the term meteor is any phenomenon that occurs in the atmosphere - this was coined originally by meteorologists, but is rarely used in this general context any more - unless prefixed, as in the term hydrometeor.
Most of the time, the meteor process reduces the meteoroid to no more that a scattering of dust, and this slowly drifts down through the atmosphere over a period of a few years. Occasionally however, if the meteoroid is large enough, a remnant body will be left over when all the kinetic energy of the original meteoroid has been dissipated by the atmosphere. In this case, the remains of the meteoroid will fall to the Earth at terminal velocity (determined by the balance between the gravitational force and the air resistance forces acting on the mass), which is typically about 100 metres/second. When this reaches the surface of the Earth it is termed a meteorite.
So in summary, meteoroid is the name of a small body in space. Meteor is the term used to describe the phenomenon of the passage of a meteoroid through the atmosphere, and meteorite is the name given to any fragment of a meteoroid that reaches the surface of the Earth.
It is not possible to predict orbits of NEOs accurately for more than a few hundred years into the future. This is because of two factors. The first is due to the limited accuracy of the initial observations, and the second is due to perturbations or changes in the orbits of NEOs produced by other bodies in the solar system.
If you want to find out the current situation with regard to potential
Earth impacts by known asteroids or comets, there are two web sites,
one in the USA, and the other in Italy, which keep updated pages on any
known NEO hazards. These are:
JPL: neo.jpl.nasa.gov/ca/ - lists NEO close approaches and neo.jpl.nasa.gov/risk/ - lists NEO impact risks
NeoDys Risk Page: NeoDys
A cometary fragment about 100m across will generally not make it to the ground, but will produce an "airburst", equivalent to a large nuclear bomb detonated in the atmosphere. An asteroidal fragment of the same size will generally make it through the atmosphere, and if it impacts on land produce a crater at least one kilometre in diameter. In both cases the energy released is enough to destroy a large city, but only if the incident occurs near a large city. However, it is much more probable that such an impact will occur over the ocean with no noticeable effects, or if over land, it will be well away from large population centres.
A body one kilometre across, if impacting the ocean, may create a damaging tidal wave. If on land, it is likely to be much more damaging, laying waste a region the size of a large State.
A body ten kilometres across will undoubtedly create global effects, raising devastating tsunamis if in the ocean, or laying waste whole countries if on land. Consequent effects, such as vast dust clouds lofted into the atmosphere, could make conditions for most life around the world very difficult for several years on.
More detail on specific effects are provided by specific pages in the NEO section of the Australian Space Academyof web site. Check also our NEO references section.
Following the realisation that an impact threat does exist (something which we have only come to widely appreciate in the last two decades), the next step is to try and locate all potentially hazardous objects before they impact, and finally to try and take steps to avert the impact through space based means. As awareness of the potential impact threat has grown so has the activity to deal with it. Governments, universities, corporations and private individuals around the world have all become involved. Conferences dealing with these issues have been held regularly since around 1990.
For the many smaller sized bodies, which have a greater chance of escaping the observation network, but which also carry a much smaller damage bill, probably the best defence that any government can do is to ensure that the country has the best possible emergency service networks to deal with all the other natural (and man-made) hazards to which we are subject on this Earth.
The Australian Government has a plan in place to deal with any serious emergency that might affect a large number of people. This would apply equally to a populated area suffering from a small hypervelocity impact as it would to a city (such as Darwin) that had been devastated by a tropical cyclone. The plan is maintained by Emergency Management Australia and is one of four emergency management plans. This one is called COMDISPLAN, short for Commonwealth Government Disaster Response Plan.
There is another opportunity you may like to consider, and this involves only a high speed connection to the internet. The Spacewatch Project of the Lunar and Planetary Laboratory of the University of Arizona has now set up a project in which anyone can join in the detection of fast moving objects (FMOs) from the images that they acquire. You do need a fast internet connection (the images involved are very large), a good eye, and an appropriate amount of time. For more information go to Spacewatch-FMO.
Several dedicated amateurs also peruse data from the SOHO satellite instrument LASCO (a solar white light coronagraph) to find Sun grazing comets. In fact, through their activities, SOHO is now credited with more comet discoveries than any other single individual, team or program. The LASCO web site is located at lasco-www.nrl.navy.mil.
NASA also has a program called Skymorph, by which individuals have discovered comets using only a Personal Computer and the internet.
And, if you read the answer to the last question, you will realise that by contributing your time and effort to any emergency service organisation (such as volunteer fire brigade, ambulance, state emergency service or rescue organisation), you are also helping prepare your country (and yourself) for any possible future Planetary Defence scenario.
There is also a very very slight chance that once in a while, a Near Earth Asteroid might pass close enough to be able to observe visually. However, NEA's are all much smaller than the large main belt asteroids, and they appear to move very rapidly when close to Earth, so that you would probably be very lucky to see a single NEA within your lifetime. And you will probably have to be closely watching all the MPC (Minor Planet Center) and JPL (Jet Propulsion Lab) web sites to learn of such an observing opportunity.
Sometimes a comet will be observed at a very great distance from the Sun, and astronomers may then predict that this comet should become very bright, to the point where it will be visible to the naked eye, as it approaches perihelion (the closest point in its orbit to the Sun), and as it passes near the Earth. Generally a comet becomes more active, throwing off gas and dust, as it gets closer to the Sun, and the Sun's heat causes sublimation (turning from a solid to a gas) of the volatile material of which comets are composed. This materials forms the well known tail behind the comet. It also determines the comet's reflectivity and thus brightness.
In one year it may be possible to see two or three comets with the naked eye. There may then be a stretch of several years in which no comets are visible. Because comets are generally small objects (no more than a few tens of kilometres across) they cannot be seen far from the Sun where they are inactive in the coldness of space, and so it is not possible to predict years ahead when a naked comet will be visible.
Then again, there are some exceptions to this rule. Some comets have well known periodic orbits (such as Halley's comet) of small enough period that it is possible to predict their return with good accuracy. What it is never possible to predict is exactly how bright they will be when they return. This again is because of the variability in their activity.
In the last decade, there are also a few recorded instances of damage to people and property, although no deaths. A car has been dented, and one holed, a house roof has been breached, and a person hit, amongst others. In 2003, the town of Park Field in Illinois suffered multiple damages from a meteorite "shower" (probably fragments from the one meteoroid that entered the Earth's atmosphere moments before).
It should be noted that the above incidents are from meteorites that are travelling relatively slowly (about 100 metres per second). They have been decelerated by the Earth's atmosphere from the initial meteoroid speed of maybe 20 kilometres per second. They form no crater when they hit the ground.
Despite the above records, you are much much more likely to be killed in a car accident than to be non-lethally struck by a meteorite. And if you do happen to have such luck, with the current fetching price of meteorites, you can probably reap enough financially to pay any hospital bills with some left over (except in states such as Western Australia where all meteorites have been declared the property of the State).
The probability of being hit by a piece of man-made re-entering space debris (satellite fragment) is even lower. While we know that a few pieces of space debris make it to the ground each year (without being totally burnt up in the atmosphere - which most are), there are no recorded incidents of anyone being hit by such an object, despite some of these pieces being recovered every now and then by enthusiastic prospectors.
The Australian Government has set up a plan to deal with such re-entering space debris that is termed AUSCONPLAN-SPRED, the Australian Contingency Plan for Space Re-entry Debris. This is maintained by Emergency Management Australia. If any of the debris is known or suspected to contain radioactive material (and in fact this is really the only kind of debris that is worth worrying about), then the Government agency ARPANSA (Australian Radiation Protection and Nuclear Safety Agency) will be involved www.arpansa.gov.au/arlspace.htm.
Australian Space Academy