Throughout history, large asteroids and comets have impacted the Earth with devastating consequences. The dinosaurs likely went extinct after a massive asteroid collided with our planet around 66 million years ago. More recently, in 1908, an asteroid estimated to be 190 feet wide exploded above Tunguska, Siberia with a force of around 15 megatons of TNT – flattening over 800 square miles of forest. While major asteroid impacts are rare on a human timescale, the risk they pose is very real. Scientists estimate impacts of objects capable of global consequences occur on average every 500,000 years or so.
With our increased understanding of the threats from near-Earth objects (NEOs), protecting our planet against the impact risk has become an area of serious concern and active research. This article explores the various risks posed by NEOs, the options available to mitigate those risks, as well as the costs associated with implementing a comprehensive planetary defense strategy.
Risk Assessment
NEOs are comets or asteroids whose orbits bring them within 30 million miles of Earth’s orbit. Of the over 26,000 NEOs that have been discovered to date, approximately 2,000 are larger than 0.6 miles in diameter. Objects of this size or greater have the potential to cause planetary scale damage if they were to impact Earth. Scientists assess the impact risk via both the statistical likelihood of impact as well as the potential consequences should one occur.
The Palermo Technical Impact Hazard Scale is used to characterize the threat of a future impact based on the estimated probability as well as the size of the impacting object. Objects with Palermo Scale ratings of -2 or lower are not considered a serious risk, while those of 0 or greater require monitoring and may warrant deflection efforts depending on the estimated size and trajectory. Currently there are no known impact threats with Palermo Scale ratings above 0 but continuing discovery efforts aim to identify potential future impactors well in advance.
In terms of impact consequences, an asteroid 0.6 miles wide or greater could have worldwide effects and lead to regional or even global climate changes resulting in mass extinctions if it impacted a populated area. Smaller impacts still pose very serious local threats. For reference, the 1908 Tunguska impact is thought to have been caused by an object around 200 feet wide, releasing energy equivalent to around 15 megatons of TNT. Thankfully, humanity has had the good fortune to avoid a civilization-ending impact since records began, but our luck will likely run out eventually without proactive planetary defense measures.
Detection and Tracking
The key to protecting Earth from impacts is having adequate early detection and tracking capabilities. Since 1998, NASA has operated the Spaceguard Survey with the goal of discovering 90% of Near-Earth Objects larger than 0.6 miles wide. As of 2021, it is estimated the survey has so far identified over 95% of these potentially catastrophic impactors. While substantial progress has been made, continued detection efforts are essential as some threatening objects could still remain undiscovered.
Once discovered, NEOs require ongoing monitoring and orbit determination using ground-based telescopes to ascertain their impact risk over long periods of time. The more observations, the better constrained their trajectory becomes. If an object is determined to be an impact threat decades into the future, early detection allows time for appropriate mitigation measures to be developed and implemented. The orbit of some NEOs, especially smaller objects, remain poorly known with high uncertainties however. Additional targeted follow-up observation campaigns are therefore needed.
Several space-based missions may help enhance global NEO detection and tracking capabilities going forward. The NEO Surveyor space telescope, set to launch in 2026 if funded, would boost discovery rates significantly. The Near-Earth Object Camera (NEOCAM) space telescope concept could achieve complete Spaceguard Survey completion within 10 years. Moreover, redirect observation resources to the highest priority impact risk NEOs requiring further characterization. Ensuring at least 95% survey completion will require an ongoing commitment to ground- and space-based NEO discovery and tracking for generations to come.
Mitigation Options
If an object is determined to be on an impact trajectory with Earth, various mitigation options exist to alter its course depending on the warning time. For impact threats over decades away, the preferred approach is generally slow pushing or tugging to nudge the object’s path enough to safely sail past Earth. One concept involves impacting the object with a “kinetic impactor,” either a high-speed spacecraft or explosive, to impart a small velocity change that accumulates over time into a missed encounter. Nuclear explosives could provide a more robust change but may not be politically or internationally acceptable.
For shorter warning times of several years, more energetic deflection methods may be needed such as mounting a solar sail and beaming solar radiation on it via directed energy propulsion. An alternative is launching explosive charges near an asteroid to impart an impulse via the blast. Precision guidance would be needed to hit small surface targets on the object. Gravitational tractors, or spacecraft that use their own ion engines to gravitationally tug an asteroid over time also hold promise.
As a last resort defense for impacts with only months of warning or less, if the resource has already been developed, a standoff nuclear explosive device could potentially be the swiftest means of altering an object’s path fractionally to avoid impact. However, major technical and policy challenges exist for implementing any planetary defense system relying on nuclear detonations. For ethical, safety, and diplomatic reasons it may prove impossible or inadvisable to weaponize space in this manner.
Considerable research and technology development are still needed to characterize specific mission and system designs as well as the capability of each deflection method on objects of varying sizes, compositions, and impact warnings. Computer modeling and ground experiments augmenting ongoing NEO impact mitigation research will help evaluate the viability of different options for responding to future credible impact threats when they arise.
Cost Estimates
The total costs of implementing a robust and sustained planetary defense program encompass both recurring expenses as well as one-time infrastructure and technology development investments. Building and maintaining a global NEO detection and tracking network, including support for ground observatories, targeted follow-up observations, and potential space-based sentinels will entail ongoing operational costs measured in hundreds of millions of dollars per year.
Obtaining adequately deep coverage of the entire sky on a continual basis to detect over 90% of large NEOs down to sizes as small as possible is the long-term objective. Shorter term, filling remaining gaps and focusing resources on objects deemed highest priority impact risks will require an incremental investment escalating to around $250-300 million annually according to Planetary Society estimates. Additional future missions such as NEO Surveyor would carry multi-billion dollar price tags for manufacturing, launching and operating new space telescopes.
For actual deflection efforts, costs will vary greatly depending on the type of mission required, target asteroid characteristics like size and composition, warning time, and degree of change in trajectory demanded. Kinetic impactors may require a $300-500 million unmanned spacecraft while advanced solar sail concepts or orbital deflection missions could reach $1-2 billion or more. Standoff nuclear options would likely far surpass that figure including the costs of readying and delivering a nuclear device into space which presents huge political and international law considerations.
Given the potential consequences of unmitigated impacts, experts argue the costs of comprehensive planetary defense, while substantial, are dwarfed by the damages that would ensue should a catastrophe occur. A single medium-sized impact could threaten global crop harvests and cause a mass extinction event that eliminated a large fraction of Earth’s biodiversity. Such a scenario could plunge civilization into a long-term crisis. When considered in light of risks to human civilization and life itself on our planet, funding an robust and ongoing planetary defense effort emerges as a prudently responsible choice.
Conclusion
Protecting Earth from the impact hazard posed by near-Earth objects requires sustained efforts across several fronts including detection, tracking, characterization and preparing mitigation capabilities for credible impact scenarios. While substantial progress has been made detecting the largest and most dangerous NEOs thanks to initiatives like NASA’s NEO survey program, fully achieving Spaceguard survey completeness and accurately tracking all observable threatening asteroids will remain an ongoing task for coming generations.
Survey telescopes will need to push the limit of achievable sizes well into the 100 meter range and continue systematic searches of the entire sky for new discoveries. Emerging space-based options show promise increasing detection rates. Once a confirmed impact threat is identified, researchers are developing promising deflection strategies from slow asteroid nudge to more robust options that may become viable with further testing and maturing of technologies. Overall costs associated with comprehensive planetary defense amount to hundreds of millions annually to over a billion dollars depending on the scope and types of missions undertaken.
However, the still underestimated hazard posed by even a single civilization-changing event argues strongly for continued allocation of resources to this important global security issue as a prudent investment. With dedicated work across both governmental and non-governmental organizations, our planet and species stand a realistic chance of gaining the knowledge, capabilities and advance warning necessary to protect Earth from disasters that have struck in the deep past and remain scientifically inevitable on longer future timescales. Planetary defense thus merits maintained focus and support as one of humanity’s highest long-term priorities.