From Forecasts to Actions: Communicating Asteroid Warnings
- Sally Potter
- Mar 4
- 11 min read
Asteroid 2024 YR4 was discovered on 27 December 2024 by observers in Chile, and was notable due to it’s potential for future impact with Earth in December 2032. The likelihood of impact varied over a couple of months as more observations were made. After peaking around 2-3%, the probability is currently 0.0018% according to NASA, and has dropped to number 36 on the European Space Agency Near-Earth Objects Coordination Centre Risk List as of 4 March 2025.
While this particular asteroid has slipped down the priority list, there are constantly new Near-Earth Objects being identified and the impact probabilities calculated and refined. When a new space hazard becomes a high enough risk to alert the public, what should be said, and how should we mitigate the risk?
This post covers the following topics:
Space Speak: Defining the terms
What is the difference between an asteroid, a comet, and a meteor? What is a ‘Near-Earth Object’?
According to NASA, Near-Earth Objects are asteroids or comets that come within 195 million kilometers of the Sun, which means they can come through the Earth’s orbit.
Asteroids are small, rocky objects that orbit the Sun. They are mostly in the asteroid belt, which is a ring between Mars and Jupiter’s orbit.
Comets also orbit the Sun but are made of ice and dust. They vaporise when they get too close to the Sun, which is why they can have a tail.
A meteoroid is a small piece (typically pebble-sized) of an asteroid or comet. When it gets close to the Earth it is called a meteor, and when it comes through our atmosphere, it creates a shooting star. If it isn’t completely burned up and falls to the ground, it’s called a meteorite.
A bolide is an extremely bright meteor, especially one that explodes in the atmosphere. The brightness of stars, space objects and satellites are measured in ‘apparent magnitude’ (or ‘visual magnitude’), where the lower the number, the brighter the object is. For reference, Venus has an apparent magnitude of -4.2, and the faintest stars visible with the naked eye on the darkest night have an apparent magnitude of about +6.5. A bolide has an apparent magnitude of -4 or brighter, and a superbolide has an apparent magnitude of -17 or brighter. The Chelyabinsk meteor that exploded over Russia in February 2013 was a superbolide.
Space debris are artificial objects that are in Earth’s orbit, or that re-enter Earth’s atmosphere. The European Space Agency (ESA) states that moderate sized objects (of which there are modelled to be 1.1 million sized 1 to 10cm in orbit) re-enter Earth’s atmosphere about once per week, while smaller sized objects (130 million sized 1 to 10mm in orbit) re-enter almost daily.
While most objects burn up entirely during re-entry, some parts survive to reach land or water. The ESA says that the risk for any single individual “is several orders of magnitude smaller than commonly accepted risks, such as those encountered when driving a car, in day-to-day life”, but that with increasing numbers of satellites being deployed, and the potential for a runaway collision effect in future, the likelihoods are expected to grow.
Near-Earth Object alert level systems explained
I love alert level systems and natural hazard scales – I know it’s super geeky, but there are so many ways you can take continuous natural phenomena and split it up into categories for useful communication.
You can base it just on the severity of natural phenomena, such as how big an eruption is, or how severe the wind is. Or you can integrate consequences and risk, so that towns and cities would get a higher alert level than rural areas due to having more people, and therefore higher societal risk. But is that the right thing to do? You can base the alert level system on forecasts, or just on what is observed at the time. The number of levels needs to be decided, and how to label them (colours, numbers, words?). And so on and so forth.
Let’s take a look at a few scales that have been developed for asteroids and other Near-Earth Objects.
Palermo scale
The Palermo Scale is a continuous scale, and is aimed at specialists (e.g., astronomers) to quantify the level of concern relating to future potential impacts. It specifically is not intended for public communication of impact risks (Chesley et al. 2002).
The Palermo Scale has particularly been used for less threatening and more common Torino Scale 0 events to assess the risk, and has been designed to be calculated from an equation quickly for the many encounters that occur. This assessment then indicates how much attention or urgency should be given to the threat to guide the level of observations, monitoring and analysis that should take place.
The Palermo Scale takes into account the time until the predicted potential impact, the predicted energy of the impact, and the likelihood of occurrence. It measures each event in two ways: without considering the event’s time proximity or significance relative to the background threat, and in the context of expected risk from other objects over time. In other words, it takes into account the background rate of asteroids hitting the Earth (which is level zero on the scale), according to the size of the asteroid or comet. As it incorporates time until impact, even a very impactful asteroid or comet that is not due for thousands or millions of years will have a very low Palermo score.
As an example, 2024 YR4 has a Palermo scale of -3.75 as of 4 March 2025. It peaked at -0.18 on 18 February 2025.
The scale is called ‘Palermo’ after the observatory in Palermo that first discovered an asteroid in 1801, and it was the location of the conference where this scale was first presented.
Torino Scale
The Torino scale uses an integer scale from 0 (lowest) to 10 (highest). It has been developed for communication with the public, and to enable assessment for asteroid and comet impact and hazard predictions that are due to occur within the next 100 years.
The scale takes into account the predicted energy of the impact (the spatial scale of devastation), and the likelihood of collision. The scale itself does not take into account the length of time until predicted impact. So, an asteroid due to hit Earth in 10 days or 10 years would receive the same score. However, the interpretation description on the right side of the table indicates some aspects of time until collision. When the potential impacts are small, or the probabilities are very low (even of a large-impact event), or the collision is over 100 years away, a score of zero is assigned.
My own interpretation of this scale is that it is heavily weighted towards probability of collision, rather than of the potential size of the impacts. As such, a global catastrophe that is still a little uncertain gets a score of 6 or 7, whereas a more certain but localised impact gets a score of 8. An uncertain threat of global catastrophe (score 6) will rightly focus the attention of astronomers, but contingency planning only “may be warranted”. To be honest, I would hope that some serious coordinated contingency planning is going on well it reaches before this level.
The maximum Torino score for 2024 YR4 was 3, between 27 January and 20 February 2025. As of 4 March, the score is 0.
Broomfield Hazard Scale
Another scale, called the ‘Broomfield Hazard Scale’, appears to have been proposed and discussed by the International Asteroid Warning Network (IAWN) around 2013-2014. However, there is no evidence to suggest that this scale was taken up and implemented. The idea behind the scale was to be non-probabilistic and characterise the object size, energy potential (in tons of TNT equivalent), and extent of area devastated.
The scale has numbered ‘classes’ and a colour scale that both unfortunately could cause confusion and apparent inconsistencies with the Torino scale. As such, I am hesitant to put an image of the scale’s table into this post – see the link above for more details on this proposed scale.
Asteroid Warning: What information should be communicated?
As I’ve described in an earlier post, there are several elements that need to be included in a warning message, shown in the image below. I’m assuming here that the warning is being issued by the emergency management sector, and the audience is the general public. The situation may call for more tailored messages to different audiences.
Let’s apply this to the context of a Near-Earth Asteroid and explore what each of the elements in a warning could look like*. This also touches on some of the key questions we can investigate in advance.
Source – who is the official agency that is responsible for communicating about space hazards? Each country is likely to have one, as well as any international body (e.g. the European Space Agency Near-Earth Objects Coordination Centre). Ensure roles and responsibilities are well defined now. The agency issuing the warning would put their name at the start of the warning, avoiding acronyms if possible.
Risk information (hazard, impacts, likelihood) – start with the hazard(s), providing a plain language explanation of the terms if needed. For example, the hazards could be an asteroid hitting the Earth, a shock wave, bright light, ground shaking, fires, and/or a tsunami. The hazards to include will depend how far away the impact location is.
Describe the impacts, including cascading impacts, such as flying glass, building damage, critical infrastructure outages (e.g., power outages, water supply disruption), danger to life, disruption to supply chains, impacts to agriculture/horticulture/ fisheries and long-term food supply from climate impacts (the asteroid would have to be huge for this!). Research the impacts of previous similar-sized asteroids/meteorites to find out what could happen (e.g., the Tunguska asteroid that hit Siberia in 1908).
Describe potential tsunami heights and link to hazard maps that include the right size of expected tsunami for each location (from modelling). Describe how widespread and severe the impacts could be, especially relative to the distance to the expected impact location. For example, if the asteroid is not expected to hit anywhere near New Zealand, but could hit the Pacific Ocean, describe what the potential hazards and impacts from a tsunami would be to people in New Zealand.
Integrated with all of this, communicate likelihoods effectively – the likelihood of asteroids hitting the Earth as a whole will be much higher than the likelihood of any one area (or person) being directly impacted. We may not have that level of detailed modelling for our country right away though, so we will need to describe the uncertainties in the likelihoods and impacts. This could be done by stating the known likelihood of the asteroid hitting Earth (e.g. ‘1% chance) within a time frame, then describing the lower likelihood of a certain area being hit using verbal terms (‘extremely unlikely’). Giving ranges in numbers (e.g., 1 – 3% chance) when possible is a good way to communicate uncertainty.
Guidance – what can people do to minimise the risk? This obviously depends a lot on how far in advance the warning is, and how far away the impact is likely to be from the person receiving the warning. I couldn’t spot any mitigation messaging for asteroids in the New Zealand CDEM consistent messages webpage, so have drawn on international sources here. Mitigation actions could include:
staying away from windows and heading down to a cellar/basement
holiday planning in advance to avoid riskier areas around the world
local evacuation closer to the time
storing water and stocking up on food and fuel
charging devices
considering crops to grow/not grow (for self-reliance or income).
If there is plenty of time before impact, exercises could be run, including with the public, and key messages referred to afterwards in the warnings.
Location – describe in words and show maps of which areas will potentially be directly affected, and also where the cascading hazards and impacts could occur. How far away from the firing line would people have to go to be safe? What are the uncertainties, and how can they be refined and communicated over time?
Be careful with words that imply spatial scale – stating ‘city-sized’ impacts expected is likely to be perceived by some people as only those in cities being affected. Likewise, ‘regional’ impacts can cause confusion when regions vary so widely in size, the asteroid won’t stick to political boundaries, and this term is sometimes used for a group of countries.
Some sort of live map tracking the most likely locations as time and scientific efforts reduce uncertainties would be a helpful global resource. Tsunami maps and other information showing indirect impact locations will also be useful.
Time – when exactly is the window of risk, and how long will the impacts be felt for? When will updates be issued, or the uncertainty reduced? When do people have to act by, and when will they know that they are safe?
Use 12-hour time (as opposed to 24-hour clocks) as more people understand them. Be clear on what time zone is being referred to – while international agencies will likely use UTC, this will need to be converted to the local time zone for your audience (you can use online tools such as this one).
What are you doing about it? It would be useful to describe what responsible agencies, critical infrastructure agencies, response agencies, and anyone else relevant are doing about the risk. Describe a deflection mission if that is the case, but remember to also describe all the other measures that are occurring, even if they are a bit less dramatic.
If the situation is just being monitored for awareness, what are the thresholds for further action? What modelling or scientific activities are occurring (such as tsunami modelling and hazard zone mapping, impact and risk modelling), and what decisions will the outcomes inform? What are the central and local government agencies doing to prepare our communities?
Finally, include a link to more information. Quite a bit of the above information will probably need to be in supplementary information. Include how to access the key sources of information for updates up until the time of expected impact. Frequently asked questions could be addressed here too, such as ‘could the asteroid hit the Moon or satellites, and what would happen then?’. Terms could be defined, seeing as ‘TNT equivalent’ seems to be bandied about by astronomers for energy release, but doesn’t mean much when you don’t work with explosives.
General communication principles apply, such as issuing useful, specific, clear, effective, consistent and timely warnings. See this post for more details on these. Consistency across countries will be an interesting challenge, when there are multiple asteroid observation agencies giving slightly different likelihoods and science advice. Politics will no doubt play a role, but I hope that information resources can be shared.
Conclusion
The likelihood of an asteroid or meteor impacting us in our lifetimes is very small. However, it is a warnable hazard, which means we can do something about it in advance to minimise the risk. To do this, we need to think ahead and make sure we know how to act on the scientific forecasts that are produced – including providing effective information. As has been highlighted in the research literature, there is a need for better communication planning and coordination (Billings, 2015).
By prioritising clear and coordinated communication strategies, we can ensure that society is well-prepared to respond effectively to any potential asteroid threats.
Notes and references
Billings, L. (2015). Words matter: A call for responsible communication about asteroid impact hazards and plans for planetary defense. Space Policy. 33 (2015): 8-12.
Chelsey et al. (2002). Quantifying the risk posed by potential earth Impacts. Icarus, volume 159, issue 2. Pp 423-432. https://www.sciencedirect.com/science/article/pii/S0019103502969101
Note: To view the full paper, go to Google Scholar, paste in the title of the paper, and click on ‘pdf’ on the right.
*I intentionally don’t provide an example asteroid warning in case it is used out of context. You can see example warnings for various hazards in ‘Potter, S. H. (2018). Recommendations for New Zealand agencies in writing effective short warning messages. GNS Science Report 2018/02. http://dx.doi.org/10.21420/G20H08’
Finally, while I have tried my best to provide as accurate and useful information as possible, Canary Innovation or the author are not liable for any actions taken using this information. Please refer to links and official sources as needed.
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