How cost-effective are
efforts to detect
near-Earth-objects?
Toby Newberry

Global Priorities Institute | May 2021
GPI Technical Report No . T1-2021

How cost-effective are efforts to detect
near-Earth-objects?1*
1. Introduction
Near-Earth-objects (NEOs) include asteroids and comets with orbits that bring them into close proximitywith Earth. NEOs are well-known to have impacted Earth in the past, sometimes to catastrophic effect.2
Over the past few decades, humanity has taken steps to detect any NEOs on impact trajectories, and, in
doing so, we have significantly improved our estimate of the risk that an impact will occur over the next
century. This report estimates the cost-effectiveness of such detection efforts. The remainder of thissection sets out the context of the report, as well as its motivation. Section 2 explains the methodology
used for making estimates. Section 3 estimates the cost-effectiveness of past NEO detection efforts, andmakes some comments on the prospects for future efforts.
NEOs fall into two basic categories: asteroids and comets. Asteroids are essentially lumps of rock, andtypically orbit somewhere between Mars and Jupiter (in the region known as ‘the asteroid belt’). Whilethey range in diameter from one or two metres up to hundreds of kilometres, smaller asteroids are far
more common than larger ones, and there are only a handful with diameters greater than 10km. The total
population is in the millions, with some 20,000 or so having orbits that qualify as ‘near Earth’ (JPL, 2019).
Almost all NEO detection efforts so far have focused on asteroids.
Comets are mixed lumps of rock and ice. They typically follow highly elliptical orbits, which extend into
the outer planets, and only rarely come close to the Sun. Comets tend to be larger than asteroids, andmost have diameters in the hundreds of metres - although, once again, there are very few larger than
10km. The total population of comets is more difficult to assess than asteroids, but scientists haveidentified close to 800 near-Earth comets to date.
The destructive potential of a given NEO depends on its mass, composition, speed relative to Earth, andmaterial details of the site where it impacts (or whether it explodes in the air). Unsurprisingly, an impactor
that is larger and faster will tend to cause greater destruction. The composition of both the NEO itself,
and the impact site, have implications in terms of an impactor’s climatic effects. From the perspective of
this analysis, however, it makes sense to categorise NEOs by mass (as approximated by diameter) in thefirst instance. Of the various features mentioned, mass is the best proxy for destructive potential: theother features explain less of the variation in destructive potential and/or are harder to discern in advancethan an NEO’s diameter.3
This report builds directly on work by Jason Matheny (2007). As part of a broader inquiry into strategies
for reducing extinction risk, Matheny (2007) estimates the cost-effectiveness of a hypothetical plan to1*

I am grateful to Joao Fabiano for his assistance in fact-checking this report. I am also grateful to Will Macaskill,
Toby Ord, Carl Shulman, Matthew van der Merwe, and Anders Sandberg for comments on an earlier version.
Nevertheless, this is still a working draft, and likely contains errors (for which I take full responsibility). Please send
helpful comments to tobias.newberry@philosophy.ox.ac.uk.
2
Most famously, the Chicxulub impactor caused the extinction of around 75% of species, including allnon-avian
dinosaurs.
3This broad characterisation of different NEOs, as well as the factors that inform their destructive potential, comesprimarily from Ord (2020), informed by Stokes et al. (2017).

[April 2021 - Working draft - Toby Newberry]1detect and (if necessary) deflect all near-Earth asteroids over the next century. The aim here is to update
and amend Matheny’s estimate in three ways. First, this report incorporates more recent estimates forsome of the relevant data, including the risk of acatastrophic or existential impact over the next century.
Second, this report focuses primarily on estimating the cost-effectiveness of real-world detection efforts
over the past few decades, as opposed to hypothetical future proposals. Third, this report addresses
considerations of NEO-deflection (rather than detection) differently to Matheny. Where Mathenyincorporates the costs and benefits of deflection into his central estimate, this report focuses almost
exclusively on detection. We treat the prospect of deflecting or otherwise mitigating the negative effects ofan Earth-bound NEO as exogenous to the central estimate.
The human future has the potential to be unimaginably vast, and, as a result, is plausibly of enormous
moral importance. At the same time, it may seem unlikely that anything we might do in the present day
could reasonably be expected to have an effect over the very long term. The case of NEO impact
provides a counterexample to scepticism of this sort. By demonstrating that our efforts in this area have
had, and may continue to have, a robust and positive effect over the future of humanity, this report makes
the case that affecting the long-term future in predictable ways, and towards favourable ends, is wellwithin our power.

2. Methodology
Cost-effectiveness estimates depend on two pieces of information: the cost of what is being estimated,
and the benefit it provides. For NEO-detection efforts, the relevant cost is just the time and money spenton scientific equipment, personnel, and so forth. The relevant benefit is more difficult to make precise,but can be glossed as ‘the expected value of learning the results of a given detection programme’. Here,
we use the tools of expected utility theory, together with relevant empirical evidence, to inform an
estimate of the same.
We make two versions of this estimate, using two different assumptions - where the assumptions differ inwhether or not NEOs are taken to present a non-negligible risk of human extinction. According to the‘catastrophic impact assumption’, NEOs present a risk of catastrophic loss of life, as well as many othernegative effects, but do not genuinely threaten human extinction. In this case, the amount we would be
willing to pay for NEO detection efforts depends primarily on the number of lives we expect to be lost in
the immediate aftermath of impact, and in the years to decades that follow. Following NASA (1992), thisreport defines a ‘catastrophic impact’ as the impact of an NEO with diameter greater than 1km, but lessthan 10km.4
According to the ‘existential impact assumption’, NEOs that are sufficiently large present a non-negligiblerisk of causing human extinction outright. In this case, the amount we would be willing to pay for NEO
detection efforts depends primarily on the expected size of the entire human future. Even though an
impact that could cause human extinction would also have catastrophic shorter-term effects, the expected
size of the future is so large that it ends up dominating these in the estimate. This report defines an

4

Of course, under this assumption an impactor of diameter10km or greater would also have catastrophic (but notexistential) effects. The reason we restrict this category to the 1-10km range is largely pragmatic, since the best
recent estimates of impact risk use this categorisation (e.g. Ord, 2020). At the same time, including 10km or greater
NEOs here would not affect the estimate very much, since NEOs of this size are extremely rare, and, on thecatastrophic impact assumption, would not have effects that are all that more significant than those in the 1-10km
range.

[April 2021 - Working draft - Toby Newberry]2‘existential impact’ as the impact of an NEO with diameter 10km or greater (for similar definitions see,e.g. Ord, 2020; Matheny, 2007).
The estimates below that correspond to these two assumptions should be interpreted as follows. For thecatastrophic impact assumption, there is a clear scientific consensus that a 1-10km impactor would havetruly devastating effects. Because of this consensus, the relevant estimate should be interpreted as a robust
lower bound on cost-effectiveness: everyone agrees that impactors of 1km diameter or greater would atleast cause widespread destruction and significant loss of life. For the existential impact assumption, there
is no such consensus. Despite the salience of asteroids as a vector for human extinction in popular
culture, there are reasons to think that even a very large impactor would not mean the loss of the entire
human future: humanity thrives in an extraordinarily diverse set of ecosystems, and has made numerous
technological advances that might aid in surviving a major impact event. As a result, the relevant estimate
should be interpreted only as a tentative upper bound on cost-effectiveness: NEO detection efforts could
be this cost-effective, but only if we think NEOs really do pose a non-negligible extinction risk. Here, we
assume that an impactor of diameter 10km or greater has at least a 1% chance of causing humanextinction.

3. Estimates
3.1 What we have achievedSince the mid 1990s, international spaceguard programmes have been working to track NEOs, with theaim of identifying any on impact trajectories. To date, scientists working as part of this collaboration havetracked over 95% of asteroids of diameter 1km or morein near-Earth orbit, including, with highlikelihood, all asteroids of diameter 10km or more. Here, we estimate the cost-effectiveness of the
programme under each of the two assumptions stated earlier. The total cost of the programme wasestimated at around $70 million (USD) in 2013 (Mainzer et al., 2011; U.S. House of Representatives(2013)).5 A reasonable estimate of the total cost to date, incorporating more recent figures, is around $600
million (USD) (Dreier, 2019).
To estimate the benefit provided by the programme under each of our two assumptions, we follow an
identical set of steps. First, we ask the high-level question: how much would we be willing to pay to learn
the results of the programme? This question can be answered using the tools of expected utility theory,
where the programme effectively has two possible outcomes: either it finds an asteroid on an impact
trajectory, or it doesn’t.6 Second, we estimate the probabilities of these two outcomes, using available
empirical evidence, but excluding evidence provided by the programme itself. Third, we estimate theutility values of the two outcomes, as determined by a similar set of empirical claims. Together, these latter
two estimates allow us to answer the high-level question, which, when combined with the cost estimate
given in the previous paragraph, gives an estimate of the overall cost-effectiveness.
Under the catastrophic impact assumption, the probability of the programme finding an asteroid on an
impact trajectory is just the risk of catastrophic impact over the next century, excluding evidence from theprogramme itself. This figure is relatively well-characterised as being around 1 in 5000, which is both the5Note that this figure may not capture all relevant costs. The subsequent revision of this report will include a closer
investigation of the cost-estimate.
6
We ignore the possibility that it finds more than 1 such NEO, since this is highly unlikely, and, in any case, would
have minimal effect on the final estimate.

[April 2021 - Working draft - Toby Newberry]3number that NASA used in its own calculations prior to significant NEO detection efforts (see, e.g.
NASA, 1992), as well as our current best-guess at the average risk per century (Ord, 2020). It derives fromthe historical track-record of impacts of differentsizes, and does not incorporate information about thenext century in particular. Similarly, the equivalent probability under the existential impact assumption isjust the risk of a 10km (or larger) NEO colliding with Earth over the next century, which is also wellcharacterised at around 1 in 1.5 million (Ord, 2020). The probability of the programme finding no such
Earth-bound NEO, for each assumption, is just the relevant complement.
The utility values of the different outcomes, under each assumption, can be estimated using two pieces of
information: the expected damage caused by the relevant category of impactor, and the extent to which
detecting an Earth-bound impactor in advance would militate against that damage. For example, one
might think that an impactor in the 1-10km range would cause an expected 2 billion deaths, but that
advance detection would give us a 50% chance of deflecting the impactor, or otherwise reducing the
expected death toll to near-zero.7 In this example, the value of the outcome where we detect the asteroid
in advance is very high, since it saves 1 billion lives in expectation. By contrast, the value of the otherpossible outcome, where no Earth-bound asteroid is found, is close to zero under both assumptions (atleast as far as the effects of impact are concerned).
Based on the above, we can express the benefit provided by past NEO detection efforts as follows:
𝐵𝑒𝑛𝑒𝑓𝑖𝑡 = 𝑃 × 𝑈
Where P is the probability of an impact of the relevantkind taking place in the next century, and U is theutility of detecting this impact in advance. This is just a standard expectation value, where the terms
corresponding to the second possible outcome drop out (since the associated utility is zero). We canfurther express U as the product of the expected damage due to impact D, and the chance of averting thisdamage due to advance detection A. The overall cost-effectiveness (CE), in dollars per life saved, is thengiven by:𝐶𝑜𝑠𝑡

𝐶𝐸 = 𝑃 × 𝐷 × 𝐴
As noted, the total cost of the programme has been estimated at $600 million (USD), and the relevant
values for P under each assumption are stated above. It remains to fill in values for the expected damage
due to impact, and the chance of averting this damage due to advance detection.
In their 1992 report on the Spaceguard Survey, NASA estimated that an impactor in the 1-10km range
could cause the deaths of one quarter of Earth’s population.Depending on when in the next century the
impact occurs (dated from 1992), this is one quarter of somewhere between 5 and 10 billion people. On
the assumption that an impact is equally likely to occur in any given year, we can approximate this as
about 1.9 billion people (¼ of 7.5 billion). This can then serve as our estimate of D under the catastrophic
impact assumption - though it is worth noting that the figure is highly uncertain, that ‘expected deaths’
does not capture all the effects of impact, and that using a point estimate, rather than a probability
distribution, is also a simplification. More generally, the correlation between an NEO’s diameter and its
destructive potential is somewhat noisy, meaning that any claim of the form ‘an impactor of diameter X
would cause the deaths of Y people’ should be understood as illustrative, rather than authoritative.

7

This might include constructing extensive safety bunkers, evacuating the areas likely to suffer the worst effects, andso on.

[April 2021 - Working draft - Toby Newberry]4Under the existential impact assumption, the expected damage due to impact is 1% of the entire humanfuture, since the existential impact assumption involves the claim that an impactor of diameter 10km or
greater has at least a 1% chance of causing human extinction. If we further assume, conservatively, thathumanity remains Earth-bound over the long-run, this is 1% of somewhere in the vicinity of 1014 lives(Newberry, 2021). If we instead assume that humanity’s future is bounded by the size of the Solar System,
the relevant figure is 1% of around 1027 lives.8
The chance of averting the damage due to impact A is determined by factors like the viability ofdeflecting an NEO, surviving an impact winter, and other strategies for enduring an impact event. Rather
than entering into a detailed discussion of these factors, this report simply assumes that this chance is
non-negligible, and uses a place-holder value of at least 5%, or 1 in 20, in the estimates below. The
reasons for taking a light touch to this question are (1) that the evidential base for precise claims about A
is relatively poor9, and (2) that questions of deflection, in particular, come with attendant concerns about
information hazards (see, for example, Ord, 2020, p68).
The table below summarises the key information from the preceding paragraphs:
Catastrophic Impact
Assumption

Existential Impact
Assumption

Cost of detection efforts$600 million (USD)$600 million (USD)

Probability of impact (P)1 in 5000

1 in 1.5 million

Damage due to impact (D)

1.9 billion lives lost in
expectation

1012 lives lost in expectation(Earthbound scenario)
1025 lives lost in expectation(Solar System-bound scenario)

Chance of averting impact
due to advance detection (A)1 in 20

1 in 20

We can now calculate the cost-effectiveness of the programme under each of our two assumptions, using
the formula given earlier. Under the catastrophic impact assumption, the programme involves paying $600
million for a 1 in 5000 chance of a 1 in 20 chance of saving 1.9 billion lives in expectation. This is acost-effectiveness of approximately $31,600 (USD) per life saved in expectation. Under the existential
impact assumption, the programme involves paying $600 million for a 1 in 1.5 million chance of a 1 in 20
chance of saving at least 1012 lives in expectation. This is a cost-effectiveness of approximately $18,000(USD) per life saved in expectation. If we instead use the assumption that humanity’s future will bebounded by the size of the Solar System, rather than just the Earth, then an investment of just $1 (USD)
could be expected to save around 500 million future lives. This number may seem astonishing, but reflects
the genuinely astronomical scale of what is at stake: it should not be especially surprising that protecting
the entire future of our species turns out to be a valuable investment.
For comparison, it is common for government programmes to value individual life-years at more than$100,000, and put the value of a statistical life at around $9 million. Moreover, it costs only $3,500 or so tosave a life by donating to the most effective traditional charities. This suggests that NEO detection efforts
8

This is also conservative, in that it ignores the prospects of interstellar or intergalactic settlement, as well as digital
persons. Each of these would dramatically increase the expected size of the future (see Newberry, 2021).
9E.g. no impact event has ever successfully been averted.

[April 2021 - Working draft - Toby Newberry]5have been considerably more cost-effective than standard government evaluations, and may have been
many orders of magnitude more cost-effective - if we think there is a reasonable chance that a large
enough impact could directly cause human extinction.

3.2 What might yet be achieved
The work of spaceguard programmes is a clear example of how NEO detection efforts have been highly
cost-effective in the recent past, but there is also some reason to believe such interventions remain
relatively cost-effective today. This section considers two ways in which the existing work could be
extended: detecting and tracking any remaining asteroids, or extending detection efforts to include comets
as well.
As noted earlier, over 95% of near-Earth asteroids with diameter >1km have already been tracked. In fact,
if the best recent models of total asteroid populationare accurate, existing spaceguard programmes haveactually tracked all such asteroids: the remaining uncertainty, and corresponding risk, stem from the model
itself. We might therefore improve our risk estimate by continuing to test and improve the model, andtracking any asteroids that may have fallen through the cracks.
The best recent estimate of catastrophic impact over the next century, incorporating evidence fromexisting spaceguard programmes, is around 1 in 120,000 (Ord, 2020). Using the same estimate of thedamage due to a catastrophic impact as in the preceding section, this risk represents close to 16,000 lives
lost in expectation over the next century. Theoretically, work to improve the model, and track remaining
asteroids, could reduce this number to near zero. If we assume that this work could be achieved foraround $1.2 billion10 (USD), then its estimated cost-effectiveness works out to around $75,000 per life
saved in expectation.
The best recent estimate of an asteroid impact leading to human extinction over the next century isaround 1 in 150 million (Ord, 2020). Making a similar set of steps to those in the preceding paragraph,this works out to around 670,000 lives lost in expectation. If we again assume this work could becompleted for $1.2 billion (USD) or less, the associated cost-effectiveness works out to just under $1,800
per life saved in expectation.
A second way that existing NEO-detection work could be extended concerns comets, rather thanasteroids. The total risk of catastrophic impact posed by comets is plausibly similar to that posed by theremaining asteroids, at around 1 in 120,000 over the next century (Ord, 2020). As a result, we canbench-mark the cost-effectiveness of this work in exactly the same way as for the risk of catastrophic
impact from asteroids. However, there is some reason to think that comets would be significantly more
expensive to track than asteroids: comets spend only a small fraction of their orbital periods near the Sun,and are extremely difficult to detect at other times. As a result, comet detection is likely to be significantly
less cost-effective than asteroid detection, in the absence of major advances in detection techniques.

10

This figure comes from an extrapolation of costs to date, based on Dreier (2019).

[April 2021 - Working draft - Toby Newberry]64. References
Dreier, C. (2019). ‘How NASA's Planetary Defense Budget Grew By More Than 4000% in 10 years.’ The
Planetary Society.
JPL (2019). Discovery Statistics - Cumulative totals. http://cneos.jpl.nasa.gov/stats/totals.html
Mainzer, A. et al (2011). ‘NEOWISE Observations of Near-Earth Objects: Preliminary Results.’ The
Astrophysical Journal, (743)2.
Matheny, J. G. (2007). ‘Reducing the Risk of Human Extinction,’ Risk Analysis, 27(5).
Morrison, D. (1992). ‘The Spaceguard Survey: Report of the NASA International Near-Earth-Object
Detection Workshop,’ NASA.
Newberry, T (2021). ‘How many lives does the future hold?’ GPI Technical Report.
Ord, T. (2020). The Precipice. London: Bloomsbury.
Schulte, P., et al. (2010). ‘The Chicxulub Asteroid Impact and Mass Extinction at theCretaceous-Paleogene Boundary,’ Science 327(5970).
Stokes, G. H. et al (2017). ‘Update to Determine the Feasibility of Enhancing the Search andCharacterisation of NEOs.’ Near-Earth Object Science Definition Team.
U.S. House of Representatives (2013). Threats from Space: A Review of U.S. Government Efforts to Trackand
Mitigate Asteroids and Meteors (Part I and Part II) [Hearing]. U.S. Government Printing Office.

[April 2021 - Working draft - Toby Newberry]7