Frequently Asked Questions (FAQ)

Are we prepared to deal with this threat in Europe?

We are making good progress in different aspects of the problem, such as the coordination of the observation and cataloguing efforts, but we are not yet prepared to deal with an asteroid that might hit the Earth, in case one was detected. We have been working to understand what is the most viable and pragmatic approach to develop this capability. In our opinion the effort to prepare ourselves should be divided in stages, some concrete but realistic steps that are meaningful by themselves. Don Quijote would be the first one of them, as it would attempt to demonstrate our capability to interact with an asteroid and modify its trajectory in a controlled way.

How are ESA activities in this field being organised?

From the programmatic point of view, all the work has been funded by ESA's General Studies Programme. This work followed several recommendations by high level international organisations, such as the UN (Report of the 3rd United Nations Conference on the Exploration and Peaceful Uses of Outer Space (Declaration of Vienna), approved by the UN General Assembly in December 1999), the Council of Europe ( Resolution 1080 of 26 March 1996 ), and more recently, also the OECD (recommendations of the OECD's Global Science Forum resulting from the Workshop on NEOs: Risks, Policies and Actions, held in Frascati on January 2003).

Partly as a consequence of these recommendations ESA's long-term space policy (LSPC's Second Report "Investing in Space - The Challenge for Europe", ESA SP-2000, May 1999) also identified NEO research as a task that should be actively pursued by the Agency.

What were the recommendations from the UN conference in 1999 and the Council of Europe Resolution 1080?

Here is a relevant quote from the Report of the 3rd United Nations Conference on the Exploration and Peaceful Uses of Outer Space (Declaration of Vienna): Approved by the UN General Assembly, December 1999.

"[...] actions should be taken [...] to improve the international coordination of activities related to near-Earth Objects, harmonizing the worldwide efforts directed at identification, follow-up observations and orbit prediction, while at the same time giving consideration to developing a common strategy that would include future activities related to near-Earth Objects [...]"

Also, a relevant excerpt of the recommendations of the Council of Europe Resolution 1080 of 26 March 1996 on the detection of asteroids and comets potentially dangerous to humankind:

"The Assembly invites governments of member states and the European Space Agency (ESA) [.} to give the necessary support to an international programme which would:

  • further our understanding of the physical nature of NEOs, as well as the assessment of the phenomena associated with a possible impact, at various levels of impactor kinetic energy and composition;

  • contribute to a long-term global strategy for remedies against possible impacts."

Why do you think the approach ESA is following is realistic (in financial, technical, risk terms...)?

Because the Agency is very well aware that right now, and most likely also in the near future, there are more pressing needs and more urgent priorities that require immediate attention, but we also know that this is not an argument that should be used to postpone the start of any developments indefinitely - this issue is still too important to leave it aside and not to take any action. Therefore we propose following a realistic, stepped approach, involving concrete goals and relatively modest projects. We also suggest exploiting the synergies with activities having other motivations, such as technology development, international cooperation and scientific research. A concept like Don Quijote provides an excellent example of how all these aspects can be addressed in a project with clear boundaries, one that could enable ESA and its international partners to cooperate with a limited cost and risk, and at the same time be easily understandable by the general public and therefore have their involvement and support.

Do you think it makes sense to invest in a NEO spacecraft mission? Can't we just do some research or observations and research form the ground?

All the research and ground=based observation programmes are extremely important and should be supported, but they are not enough. Trying to move an asteroid without a spacecraft would be like trying to perform heart surgery without a scalpel. Spacecraft are our best tools to study an asteroid from close quarters and the only one that is available if we want to interact with them e.g. push them or seeing how they react when we hit them. So yes, we are convinced about it.

How good are we at detecting NEO's?

We are very good and we are getting better at it, but we are not doing so well that we can relax too much. Current survey programmes have been very efficient so far in detecting the large objects, above 1 km in size, but not all of them have been discovered yet. Considering that we that the current estimates suggest that there are about 1100 of these large objects, give or take 100, we have currently catalogues about 60%. It is also important not only to detect them, but to be able to determine its trajectory to find out whether they are dangerous, and these requires more observations separated in time; the more we have the more accurate the orbit determination will be and the more confident we will be that they do not represent a threat to our planet. Therefore, it should be a continuous effort and it will require continuous support.

For some details, here is a relevant paragraph of the report produced by the NEO Mission Advisory Panel (NEOMAP), the panel of NEO experts appointed by ESA:

"The current consensus amongst the impact-hazard community is that future NEO search telescopes should be made sensitive enough to achieve near completion for objects significantly smaller than 1 km (the "civilization-threatening" threshold in the original "Spaceguard Goal" set by the US Congress in 1994). According to Harris (2004), systems such as the US Pan-STARRS (Panoramic Survey Telescope And Rapid Response System), the DCT (Discovery Channel Telescope) and LSST (Large Aperture Synoptic Survey Telescope) should be capable of discovering 90% of the population of NEAs with diameters of around 200 m or more after 10 years of operation. However, some of these facilities will serve general astrophysical research and will not be dedicated to NEO searches. It is not clear how much of their time will be made available for NEO searching or what operational constraints search programmes will be subject to."

The current concerns in the community are mainly related to this smaller objects, down to between 200 and 100 m in size, that are much more numerous than the very large ones. Most of them are still undetected, and some we see only when they have zoomed past the Earth.

Both ground and space systems should help to keep them under control in the future. Here is another quote from the NEOMAP report, providing a recommendation on the survey strategy (ground -based vs Space based).

"It was concluded that at the present time a space-based NEO discovery mission, within the scope of those considered here, is not the highest priority given the combined efforts of the various ground-based surveys likely to be productive over the coming decade. A reasonable approach may be to re-consider a space-based NEO observatory mission at a later stage, once the residual hazard from NEOs not accessible to the ground-based surveys has become better defined".

This meaning that the planned surveys should accomplish the task of discovering a large majority of the objects - say 90%) for the moment, provided that they are finally funded and they work as planned. However there will be a point in 10 or 20 years time at which these ground-based telescopes will just not be capable of finding and tracking more objects (especially because there is a certain type that appears mostly sunwards in the sky and thus are very difficult to see from the Earth, in the same way as we cannot see the stars during the day). These object would be easier to detect from space. Then space-based observatories might become the only way to reduce the risk even further. How many NEO are threatening the Earth?

There are different estimations based on different models. These are sometimes based on observational data on e.g. Moon craters and other times on assumptions on the origin of NEOs. These assumptions are needed as we are still far from discovering all of the objects. One of these studies, "Understanding the distribution of Near-Earth Objects", was funded by ESA's General Studies Programme.

There have been updates but the values have not varied too much. The accompanying plot gives information on the distribution of the number of objects as function of their size.

It can be seen that for 1km-objects the estimated number (with some dependence on the model that is used) is approximately 10^3 i.e 1.000 objects (the figure is actually estimated to be 1090 +/-180). For the object with 100 m= 0.1 Km we have from 10^4= 10.000 to 10^5=100.000. But the total accumulated number of all objects (1000, 200, 300 m and so on) is actually much larger.

How many threatening NEOs fall on Earth every year / or maybe every 100.000 years? - How often do smaller NEO's (100m-1000m) collide with the Earth?

The impact probability depends on the size range. For instance, every year there are hundreds, even thousands of small fragments that fall and disintegrate in the Earth's atmosphere. As the size of the objects grows the event become more unlikely. For instance, there is only one possibility out of six that an asteroid as large as 1 Km in size will hit the Earth in the Earth every 100.000 years, killing millions of people and probably having global effects on the environment and climate. The following table, extracted from the Final Report of ESA's NEO Mission Advisory Panel, provides figures for impact probability i.e. the estimated number of objects -for a given size- that actually hit the Earth in a given period of time.

Impactor size (m)

Mean impact interval (yr)

Energy released (megatons TNT)

Crater diameter (km)

Possible effects/
comparable event





Fireball, shock-wave, minor damage.




= 1

Tunguska explosion or small crater.





Largest H-bomb detonation.





Destruction on national scale.





Destruction on European scale.





Many millions dead, global effects.


20 million

10 million


Billions dead, global climate change.


100 million

80 million


Destruction of human civilization.

Note: The energy release estimates assume a density of 3500 kg m-3 (stony body) and an impact velocity of 20 km s-1.

How do you quantify the impact hazard of a NEO?

To quantify the impact hazard of a NEO scientists have used for years the Torino scale, a classification similar to the Ritcher scale for earthquakes. This classification has been introduced, for the first time, at an International Conference on Near-Earth objects held in June 1999 in the city of Torino, as a revised version of the "Near-Earth Object Hazard Index" created by Professor P.Binzel of the MIT.

The Torino scale is a two parameters scale: it utilizes numbers form 0 to 10 to indicate the chance of a collision, while the color is used as a second parameter to give an information about the danger of the event (going from white, non dangerous bodies, to red, catastrophic events).The parameters that lead to the classification of an object, aside to its position and its dimension, are its kinetic energy and its collision probability. This means that, an object that will make several different close approaches of Earth, will have a different Torino scale value for each approach. Normally, only the highest of these values is considered to identify an object.

Another important thing about the Torino scale value, is that it will change in time. This change will be the result of better measurements of the object's orbit. Scientists are also experimenting with a new scale called Palermo technical scale, presented at the Palermo meeting by Chesley et al. In this new scale the time of the possible impact is taken into account. Thus in this new formula the shorter the time span, the larger the relative risk: understandably, a possibility of impact in two years results in a comparatively large value, and this well reflects our intuitive feeling, that such an immediate risk is worse than one remote in time.

How many of the NEOs are comets and how dangerous are they?

A relevant quote from the from the Final Report of ESA's NEO Mission Advisory Panel:

"Comets can also collide with the Earth. Due to their porous and fragile structure, short warning times of long-period comets, and the relatively large potential impact velocities, mitigating against them is generally beyond the capability of current technology. However, observations and dynamical computation [...] show that far fewer comets than asteroids make close approaches to the Earth. [..]".the threat from all long-period or short-period comets whether active or dormant, is about 1% the threat from the N[ear]E[arth]A[steroid] population".

In other words, the probability of impact is comparatively much smaller.

On the issue of whether this has happened in the past, comet impacts have indeed happened early in the during the Earth's history when all planets were being formed and impacts (of all types of bodies) were common.

I read that certain types of NEO could provoke tsunamis and millions of casualties. What is the diameter of these objects?

Most of the object with sizes above 100 or 200 meters falling on the sea / ocean would, the effects would of course greatly depend on the size and specific circumstances of the impact (depth, distance to the shore, etc).

If a major asteroid or comet was heading towards the Earth, what could we do to change its path?

We still do not know for sure. There have been many deflection strategies have been proposed but there is not general consensus on the most practical , reliable or efficient way of changing the orbit of an asteroid. ESA's aim is to choose the most viable and pragmatic approach to develop this capability, and Don Quijote is the first step in this direction, as it would constitute the simplest possible attempt to demonstrate our capability to interact with an asteroid and modify its trajectory in a controlled way.

A mission like Don Quijote would provide us with critical information for us to judge on which one of the many different deflection strategies that have been proposed so far is more realistic. So far these proposals are at best guesses that are based on assumptions both on the characteristics of the objects and our technical capability that need to be confirmed.

Why is the mission proposed by ESA called Don Quijote? Where do the names "Sancho" and "Hidalgo" come from?

The name comes from one of the most famous scenes in Cervantes' renowned book, during which Don Quijote's overexcited imagination leads him to believe that a group of windmills is a group of giants that he should fight against. Sancho, his loyal and much more pragmatic companion in his travels tries unsuccessfully to convince him otherwise, and he remains at a safe distance, like the orbiter does in the mission. Meanwhile Don Quijote, a Spanish "Hidalgo", (a member of minor nobility of the period) unhearing the recommendations of his friend smashes into one of the windmills and ends up hurting himself badly. A description of this episode can be found here.

What are the differences between ESA's Don Quijote and other space missions e.g. NASA's Deep Impact?

Deep Impact is a very interesting and exciting mission, but it is driven by completely different motivations to Don Quijote's. The objectives of the two missions do not overlap at all and the results could very complementary.

The most obvious difference between the missions is the type and size of the target object, which is totally different: a small rocky asteroid of a few hundred meters in the case of Don Quijote, as opposed to a massive icy comet 6 kms in size (comet Temple 1) for Deep Impact; While no appreciable deflection can be expected in the case of such a massive body (even if the impact crater will be large) we do expect that the deflection to be apparent in the case of the small asteroid.

Even more importantly, Deep impact is a science mission; as slamming a probe against a comet serves a purely scientific purpose of uncovering fresh, "unweathered" material, a material that has not been exposed to the space environment and has probably not been altered since the times when the planets were formed. This is not a deflection technology demonstration mission. Actually, carrying out the measurements that are Don Quijote's main objectives (orbital deflection and possibly probing of the asteroid's interior by seismic experiments) would simply not possible in the Deep Impact mission because the main spacecraft is only performing a fly by of the comet (after releasing the impact probe) at a comparatively large distance (500 Km as opposed to 3 Kms) and very high relative speed. Thus it can only observe the object from a quite significant distance, though a short one astronomic terms, for a few hours at best.

Nevertheless, to be able to measure any deflection we would need to pinpoint the asteroid position with very high accuracy -which cannot be achieved by ground based observations in a reasonable time- over an extended period of time, probably months. In the Don Quijote mission one of the spacecraft - Sancho - would become the companion of the asteroid in its constant journey around the Sun, and this spacecraft could be accurately tracked from the ground over all this period, enabling the detection of tiny variations of the trajectory of the asteroid resulting from the impact. Finally, Sancho would also have plenty of time to carry out seismology experiments and other type of observations at close quarters.

What if we could not detect any variation in the orbit of the asteroid after Hidalgo's impact?

As a matter of fact, it is interesting to note that if the mission went as planned and after the impact of Hidalgo we were not detect any change in the asteroid trajectory, we would still gain valuable information. Why? Well, this would tell us that capability of an asteroid of a certain type to dissipate energy might be larger than expected, and would teach us something on the way we should go about moving one of the dangerous objects, in case we needed to.

Actually even in the case the mission did not go according to the plans we would learn something - if after jumping over the first hurdle we trip and fall, maybe we need to try harder than we expected in order to make sure that we finish the race!

Would it be dangerous i.e. could we get it wrong and put the target asteroid in a collision course with the Earth?

No. We just need to choose a "test asteroid" whose orbit is far from the Earth's. Actually this might seem a contradiction but there are many objects that, even fall within the strict definition of "Near Earth Object", move actually far for the Earth's orbit and represent absolutely no hazard for our planet. In order to tell this category of "harmless" objects from the ones that are potentially more dangerous, the latter ones are called Potentially Hazardous Objects (PHOs). This are the ones we should avoid paying around with!

NB Objects with MOID < 0.05 AU are called Potentially Hazardous Objects (PHOs), where MOID stands for Minimum Orbit Intersection Distance, which is the closest possible approach distance between the (osculating) orbits of two objects (the definition does not imply that the objects themselves need to be at the closest orbital locations at the same time). AU stands to astronomical unit and is equal to the average distance between the Earth and the sun, roughly 150 million Kms

What else can the observations and experiments on NEO help us understand?

What we are trying to understand at the moment is not only on whether our assumptions on the characteristics of NEOs are correct, but also what are the limits of the current space technologies when applied to an asteroid deflection mission.

Many people believe that we are very close to having the capability to deflect a threatening object, in case we had to, but no one knows for sure, and the truth is that this might even be just an optimistic assumption. That is why we need to perform test, and while we do so we will obtain other information as an additional benefit.

Indeed, the benefits in terms of space technology can be very significant, as some of the techniques being used in the Don Quijote mission would be applicable to other space missions. Obviously, the scientific benefits can also be enormous. Research on NEOs can provide us with information on how the Solar System and the planet Earth or the Moon formed, it can even give us clues on how life appeared on our planet.

Last update: 18 May 2006

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