Questions and Answers

The questions (with accompanying answers) on this page, are in reply to some of the requests for information that we have received, and to clarify some commonly held misconceptions about the minor bodies of the solar system and space debris.

(Please note that due to staff and time limitations, we are generally unable to answer individual queries directly, but we will attempt to reply on this page if we feel your query is of general interest. Technical queries are more likely to be dealt with here than others.)

Q1 What is the difference between a meteor, a meteorite and a meteoroid?
ANSWER - A meteoroid is the name given to any small object moving in the solar system. The upper size for a meteoroid is not well defined, but is often regarded to be around 10 metres. Meteoroids may be chunks of stone or iron (in which case they probably originated from asteroids), or they may be much lower density friable material (in which case they probably originated from a comet). Meteoroids are more common the smaller they are. That is, meteoroids the size of a grain of sand are much more common than meteoroids 1 metre across. Even more common are meteoroids only a few microns (millionths of metre) across.

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.

Q2 How does one find out what bodies are on a collision course with the Earth?
ANSWER - As of the end of 2003, the Minor Planet Center listed over 500 asteroids with the designation PHA or Potentially Hazardous Asteroids. This does not mean however, that any of these asteroids are on a collision course with the Earth. It does however, indicate both that these bodies could possibly collide with the Earth in the distant future, and that if they did so, considerable damage to life on Earth might occur.

Again, as of the end of 2003, no signficantly sized asteroid or comet was known to be on a collision course with the Earth in the immediate future. 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: - lists NEO close approaches and - lists NEO impact risks
NeoDys Risk Page:

Q3 Why was this project named Wormwood?
ANSWER - The name was chosen from an ancient text as purely descriptive. It has no significance beyond that, and no deeper meaning should be read into the name. The staff of the project have no special scientific insights into any future NEO orbital behaviour! There is also no classified activity associated with the project.

Q4 What damage is likely to be caused by the impact of an NEO on the Earth?
ANSWER - This depends enormously on the size of the object, where on the Earth it hits, and what it is made of.

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 in the information section of this web page and the references contained therein. Check also our web links section.

Q5 What should governments be doing to prepare for and protect their citizens against NEO impacts?
ANSWER - NEO impacts of significant size are extremely rare (a rough estimate is that a 10km object, which has a global impact, might be expected about every 100 million years). An impact of a body such as this would overwhelm any conceiveable civil defence plan that any government could put in place. Because of this it is generally conceded that the only possible defence is to try and stop such an impact in the first place.

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.

Q6 Is it possible for an individual to assist in Planetary Defence?
ANSWER - Yes, this is certainly possible. Many amateur astronomers have equipment that can detect the brighter asteroids and comets, and the ability to submit very useful reports to the Minor Planet Center. If you are unable to individually purchase such equipment, you might consider joining one of the many amateur astronomical societies in Australia and around the world. Even if your local astronomical society does not currently have a program dedicated to searching and performing follow-up observations on NEOs, you may be able to lobby them to do so. The first step is to read all you can about NEOs and astronomical measurement techniques in general (and there is no dearth of information of this kind around at present). Use the links from this web site as a starting point.

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

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.

Q7 Are any asteroids visible to the naked eye?
ANSWER - Most asteroids are very small, and they orbit the Sun between the orbits of Mars and Jupiter, which puts them a long way from the Earth. Because of these two factors, almost no asteroids are routinely visible from the Earth without optical aid (eg a telescope). Even the largest known main belt asteroid, Ceres, at 900 km in diameter is always invisible to our eyes. However, there is one exception, and that is Vesta. Vesta is not among the four largest asteroids, but it is the most highly reflective asteroid known, and at times of opposition (when it is closest to the Earth), it just becomes a naked eye object. Not that it is easy to see. You will need a dark sky location well away from city lights, and a good star atlas, to help you distinguish it from the many magnitude 6 stars around. Sometimes the popular astronomy magazines run a feature on observing Vesta when it is at opposition.

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.

Q8 When are comets visible to the naked eye?
ANSWER - At any one time there are probably several dozen comets that are visible in a large telescope. However, only a very few of these will ever become bright enough to be visible to the naked eye. The brightness of a comet depends both on its distance from the Sun and its distance from the Earth. If these were the only two factors involved then a comet's brightness could be predicted as accurately as that of an asteroid. However, it is the third factor, cometary activity, which also affects the brightness, and as this is unpredictable, so we find that predictions of a comet's brightness are also unreliable.

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.

Q9 What is the chance of being hit by a meteorite or a piece of space debris?
ANSWER - Very low indeed! In fact, books about meteors up until the internet age used to quote time after time that "no one has ever been killed by a meteorite". The internet has now put historians in contact with meteorite researchers, and most will agree that throughout recorded history, there probably are several cases where people have in fact been killed by meteorite strikes.

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.

Q10 Does the Australian Government have a plan in place to deal with re-entering space debris?
ANSWER - Satellites in low Earth orbit are subject to orbital decay due to atmospheric drag. After a time such satellites may re-enter the lower levels of the Earth's atmosphere where they will be mostly consumed; burnt up due to the high temperatures encountered in the re-entry process. Occasionally very large pieces or very refractory structures may not be totally detroyed, and they may fall to the ground.

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

Q11 Where can one find a list of all asteroids?
ANSWER - The Minor Planet Center maintains lists of asteroids. These are broken up into sub-lists which contain specific types of asteroids (eg potentially harzardous asteroids, named asteroids in alphabetical order, etc). If you want to download the complete orbital information for all named and numbered asteroids, be prepared for a very large file!

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