Planetary defence

Also known as Planetary defense, Asteroid deflection, Asteroid impact mitigation

Planetary defence means finding asteroids that could hit Earth early enough to act, then changing their path. NASA's DART mission showed in 2022 that a spacecraft impact can measurably change the orbit of a small asteroid moon, but no system is in place yet to find and deflect a dangerous object on demand.

Research and prototypesDemonstratedReviewed 22 Oct 2026
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

In plain language

Planetary defence is the work of finding asteroids and comets whose paths could bring them close to Earth, and of changing their course before they arrive. Survey telescopes scan the sky, track each object to pin down its orbit, and estimate whether it could hit. If a threat is found with enough warning, a spacecraft could nudge the object onto a slightly different path so that it misses.

The main test so far is NASA's DART mission. In 2022 DART struck Dimorphos, a small moon orbiting the asteroid Didymos, and Earth-based observations showed that the moon's orbit around Didymos became about 33 minutes shorter. The European Space Agency's Hera spacecraft is planned to reach the Didymos system in November 2026 to study the aftermath of the impact. NASA's NEO Surveyor telescope is planned to launch no earlier than September 2027, and the Vera C. Rubin Observatory began its Legacy Survey of Space and Time on 30 June 2026.

Technical explanation

Deflection works by changing an asteroid's velocity by a small amount early on, so that the change grows into a large distance at the planet's position. A kinetic impactor transfers momentum by collision. Thomas and colleagues (Nature, 2023) report a shortening of the orbital period of Dimorphos of 33.0 plus or minus 1.0 minutes (3-sigma), from two independent methods using lightcurve and radar observations. The authors attribute a substantial share of the extra momentum to material thrown out by the impact. Cheng and colleagues (Nature, 2023) report an instantaneous reduction in Dimorphos's along-track orbital velocity of 2.70 plus or minus 0.10 mm per second. They give a momentum enhancement factor, beta, between 2.2 and 4.9, depending on the mass of Dimorphos, assuming equal densities of 2,400 kg per cubic metre.

Detection is the other half of the problem. Early orbit estimates for a newly found object can change as more observations arrive. NASA's page on the asteroid 2024 YR4 (last updated 5 March 2026) reports that later Webb observations ruled out any chance of an impact with the Moon on 22 December 2032, and that the asteroid poses no significant impact risk to Earth in 2032 or beyond. The same page shows an earlier lunar impact probability of 3.8 per cent, dated 2 April 2025, which shows how much estimates can move with new data. Rubin's early operations catalogued more than 11,000 previously unknown asteroids in six weeks, including 33 near-Earth objects, according to Astronomy magazine's report of the NOIRLab release, but an orbit needs repeated observations before it is reliable.

What the evidence says

Each statement is typed by the strength of its evidence and linked to the source that supports it.

Demonstrated Shown in a published experiment, test or shipping product.

NASA's DART spacecraft struck Dimorphos, a small moon of the Didymos binary asteroid system, in 2022.

Sources: [3] · Hera mission background

Earth-based lightcurve and radar observations, analysed by two independent methods, measured the orbital period of Dimorphos around Didymos to be shortened by 33.0 plus or minus 1.0 minutes (3-sigma).

Sources: [1] · Abstract

The authors attribute a substantial share of the momentum transfer to material thrown out by the impact.

Sources: [1] · Abstract · medium confidence

Cheng and colleagues report an instantaneous reduction in Dimorphos's along-track orbital velocity of 2.70 plus or minus 0.10 mm per second, and a momentum enhancement factor between 2.2 and 4.9, depending on Dimorphos's mass, assuming equal densities of 2,400 kg per cubic metre.

Sources: [2] · Abstract · medium confidence

NASA reports that later Webb observations ruled out any chance of 2024 YR4 impacting the Moon on 22 December 2032, and that the asteroid poses no significant impact risk to Earth in 2032 or beyond.

Sources: [4] · Status section

Reported, unverified Stated by a company or team but not independently confirmed.

ESA's Hera spacecraft launched on 7 October 2024 and, according to ESA's October 2025 update, is planned to arrive at the Didymos system in November 2026, one month earlier than originally planned.

Sources: [3] · Article text · medium confidence

NASA's NEO Surveyor infrared space telescope has a launch target of no earlier than September 2027.

Sources: [5], [6] · Launch section · medium confidence

Congress directed NASA to find 90 per cent of the estimated 25,000 near-Earth objects larger than 140 metres, and The Planetary Society reports that 43 per cent of that goal has been met so far.

Sources: [6] · Mission goal · medium confidence

The Vera C. Rubin Observatory began its Legacy Survey of Space and Time on 30 June 2026. In its first six weeks of early operations, it catalogued more than 11,000 previously unknown asteroids, including 33 near-Earth objects, according to Astronomy magazine.

Sources: [7] · Article text · medium confidence

Limitations and open problems

  • DART tested one kinetic impact on one small moon of a binary asteroid. The size of the orbit change depends on the target's density and structure, which are still uncertain, so the result cannot be scaled directly to a larger or differently built asteroid.
  • The momentum boost from the impact depended on ejecta, and the momentum enhancement factor depends on the mass of Dimorphos assumed in the analysis (Cheng and colleagues, 2023).
  • Survey coverage is incomplete. The Planetary Society reports that 43 per cent of the goal of finding 90 per cent of near-Earth objects at least 140 metres across has been met so far. The page is undated, so this figure may be out of date.
  • Warning depends on detection and follow-up. Objects can be too faint to observe for long periods, and Rubin's early finds need repeated observations before their orbits are confirmed.
  • NASA says NEO Surveyor will find asteroids approaching Earth from the Sun's direction. The telescope is not yet launched, so this capability is not yet demonstrated in orbit.
  • The sources reviewed do not cover the cost, the decision process, or the international arrangements needed to act on a real threat.

Possible future uses

Speculative

Ideas, not predictions. None of these has been demonstrated.

  • A gravity tractor, in which a spacecraft hovers near an asteroid and changes its path slowly through gravitational pull. This has been proposed as a way to deflect objects without a collision, but it has not been tested in space.
  • Pre-positioned spacecraft ready to intercept a newly found threat, shortening the time needed to launch a response.
  • Deflection of a larger asteroid over many years, using a series of smaller impacts or a slow-acting method, if warning times are long enough.

Timeline

  1. 2022NASA's DART spacecraft strikes Dimorphos, the small moon of the Didymos binary asteroid system. Sources: [3]
  2. 1 Mar 2023Nature publishes the Dimorphos orbital period change (a shortening of 33.0 plus or minus 1.0 minutes) and the momentum transfer analysis of the DART impact. Sources: [1], [2]
  3. 7 Oct 2024ESA's Hera spacecraft launches to study the DART impact site. Sources: [3]
  4. 30 Jun 2026The Vera C. Rubin Observatory begins its Legacy Survey of Space and Time. Sources: [7]
  5. Nov 2026Hera is planned to arrive at the Didymos system, one month earlier than originally planned (planned, not yet reached). Sources: [3]
  6. Sept 2027NASA's NEO Surveyor infrared space telescope is planned to launch, no earlier than this month. Sources: [5]

Futures this could help bring about

AI illustration
How far away?

Deflecting an asteroid headed for Earth

Engineering prototypes · 4 of 9 steps done

Related breakthroughs

Glossary

Near-Earth object (NEO)
An asteroid or comet whose orbit brings it close to Earth's orbit around the Sun.
Kinetic impactor
A spacecraft that hits an object at high speed to change its velocity by transferring momentum.
Binary asteroid
Two asteroids in orbit around each other, such as Didymos and its moon Dimorphos.
Momentum enhancement factor (beta)
The ratio of the momentum transferred to an asteroid by an impact to the momentum the spacecraft brought, with extra momentum coming from ejected material.

Sources

  1. peer reviewed paper
    Orbital period change of Dimorphos due to the DART kinetic impact
    Cristina A. Thomas, Harrison F. Agrusa. Nature, 1 Mar 2023. DOI: 10.1038/s41586-023-05805-2. Accessed 22 Oct 2026.
  2. peer reviewed paper
    Momentum transfer from the DART mission kinetic impact on asteroid Dimorphos
    Andrew F. Cheng, Giovanni Zanotti. Nature, 1 Mar 2023. DOI: 10.1038/s41586-023-05878-z. Accessed 22 Oct 2026.
  3. government report
    ESA's Hera targets early arrival at Didymos asteroids
    European Space Agency, 7 Oct 2025. Accessed 22 Oct 2026.
  4. government report
    2024 YR4
    NASA Science, 5 Mar 2026. Accessed 22 Oct 2026.
  5. government report
    NEO Surveyor
    NASA Science, 6 May 2026. Accessed 22 Oct 2026.
  6. other
    NEO Surveyor, protecting Earth from dangerous asteroids
    The Planetary Society, 2026. Accessed 22 Oct 2026.
  7. news
    Vera C. Rubin Observatory begins its Legacy Survey of Space and Time
    Astronomy magazine, 30 Jun 2026. Accessed 22 Oct 2026.

Revision history

3 changes since this profile was created.

Changes on 22 Oct 2026
Published on owner instruction (owner requested space, food and water topics)dc4cd89
Sources and claims re-checked against originals409337d
Initial profile drafted from primary sourceseaac9eb