Planetary Defence (PD) - the most complete effort to address a cosmic threat to date: a short review

By MaxSJC @ 2026-07-18T12:20 (0)

Planetary defence - the effort to detect, track and intercept asteroids that might impact the Earth - is the most complete (and only?) effort to tackle a cosmic threat (asteroids/comets, though way more asteroids than comets) by far. Unlike most cosmic threats which remain a topic of speculation, planetary defence has:

Policy support: 

• In contrast to the funding cuts in the FY2026 President's Budget Request for NASA, PD funding actually increases.
• A number of PD-dedicated organisations such as the International Asteroid Warning Network (IAWN) and the Space Mission Planning Advisory Group (SMPAG) has been set up to quickly assess and mitigate any emerging asteroid threat. (IAWN operations was first put to the test by the asteroid 2024YR4 in late 2024 - early 2025.)

Scientific effort:

• Numerous current astronomical surveys (which scans large parts of sky to detect and study objects) are dedicated to find new asteroids and track already discovered ones. Upcoming surveys such as the Vera C. Rubin Observatory (just started its operations!) and NEO Surveyor (a main reason for the FY2026 budget increase) is expected to expand the effort drastically.
• The Double Asteroid Redirection Test (DART) was launched in 2022 to test out a method to deflect asteroids and the results from the mission is still being analysed till this day - in fact, the ESA HERA mission is expected to do follow-up observations later this year!

This resulted in huge progress made in characterising the threat from asteroid impacts and developing interventions. So far:

• over 90% of the asteroid which poses existential threats (size larger than 1 km) has been found
• we launched multiple probe missions to asteroids, studying their structure and composition: asteroid properties are important in knowing the effectiveness of interventions!
• we have a tested intervention to stop an asteroid of medium size (hundreds of meters)
• we tested our 'action plan for a potentially harmful asteroid' on the asteroid 2024YR4: It is discovered on 27 December 2024. Multiple observations are made with major observatories in the coming weeks and months. We ruled out the impact possibility by March 2025.

Future work seems to lie in these directions:

• finding at least 90% of asteroid of sizes over 140 m: these can deal at least regional damage
• continue tracking found asteroids: they need to be continuously tracked over centuries, we still cannot predict their long-term orbits very well!

However, current PD efforts are not focused on comets. I have not gained much understanding so far but the biggest problem with comets so far is that unlike asteroids, they come from the outer regions of the solar system, which means:

• they come into Earth's orbit at a higher velocity, meaning we can only detect a harmful comet a short time before impact
• they do not do multiple orbits within a human lifetime, making it hard (or perhaps useless) to determine their orbits
• they consist of volatile substances which can only form far from the Sun; as they get closer to the Sun, these substances boils and outgas, pushing the comet like a rocket, making their orbits even harder to determine!

Seems like the only way to deal with a harmful comet is to find it as early as possible and come up with interventions as it comes towards us. Thus the bottleneck seems to be detection capabilities and perhaps a further understanding of their near-Sun behaviour. 

SUMMARY

• the effort to mitigate risks from asteroid impacts are well-funded and attended unlike other cosmic threats
• we have identified most of the extinction-causing asteroids, now working on intervention development, continuous tracking and identifying asteroids that cause at least regional damage
• the effort to mitigate risks from comets remains an open question, perhaps with detection capabilities most needed