#Planetary defence
Finding, tracking and, if ever needed, deflecting asteroids that could come close to Earth.
Planetary defence is the work of finding asteroids and comets that come near Earth, working out their orbits, and preparing ways to change the path of one if it were ever on course to hit us.
Telescopes around the world discover new near-Earth objects every night, and their orbits are calculated years ahead. Nearly all known large ones pose no risk for at least the next century.
In 2022 NASA's DART spacecraft deliberately struck the small moonlet Dimorphos and changed its orbit around its partner asteroid Didymos. It showed for the first time that hitting an asteroid can nudge its path.
Part of Meteorites across the sciences.
Courses (6)
- Shock, craters and deflecting an asteroidHypervelocity impacts generate shock pressures that transform minerals. This final lesson connects shock metamorphism, crater evidence and t
- A-level: The physics and chemistry of meteoritesTwelve lessons that apply A-level physics, chemistry and geology to meteorites: gravitational fields and Kepler's laws, impact and entry
- Impacts in Earth's historyImpacts have shaped Earth from its formation to the present. This lesson looks at craters, the end of the dinosaurs, modern airbursts and ho
- Hazard, risk and planetary defenceSmall rocks hit Earth every day; large impacts are very rare. This final lesson uses ideas of risk, probability and uncertainty to judge the
- Momentum: nudging an asteroidIn 2022 a spacecraft deliberately hit an asteroid moon to change its orbit. Momentum explains how a tiny spacecraft could move a body millio
- Missions to asteroidsSpacecraft have now visited asteroids, brought back samples and even pushed one off course.
Space missions (9)
- Chang'e 2A Moon mapper that went on to make China's first close flyby of an asteroid. Chang'e 2 was built as a backup for China's first M
- DARTThe first test of moving an asteroid: a spacecraft deliberately crashed into a small moon.
- HeraEurope's detective mission to study the asteroid moon that DART struck. DART showed that an asteroid can be nudged, but telescopes on Ea
- NEAR ShoemakerThe first spacecraft to orbit an asteroid, and the first to land on one. NEAR, the Near Earth Asteroid Rendezvous mission, was the first spa
- NEO SurveyorA planned telescope built to find most of the near-Earth asteroids large enough to devastate a region.
- NEOWISEAn infrared telescope that measured the sizes of tens of thousands of asteroids. WISE, the Wide-field Infrared Survey Explorer, was launched
- OSIRIS-APEXThe OSIRIS-REx spacecraft, sent on to meet Apophis just after its close pass of Earth in 2029.
- RAMSESA European mission planned to fly alongside Apophis as it passes close to Earth in 2029.
- Mission timeline213 milestones in the exploration of asteroids, comets, the Moon and Mars, from Luna and Apollo to Hayabusa2, OSIRIS-REx and Chang'e 6.
Glossary (19)
Questions and answers (15)
- Are comets dangerous to Earth?Comets can hit Earth, and a large one would be very dangerous, but such impacts are extremely rare.
- Can asteroids have moons?Yes. Many asteroids have moons. The first was Dactyl, found orbiting Ida in 1993, and DART's target Dimorphos is the moon of Didymos.
- Could an asteroid wipe out life on Earth?A very large impact could cause a mass extinction, as one did 66 million years ago, but sterilising the whole planet would need an object fa
- How can I teach momentum using the DART mission?DART's impact on Dimorphos is a real example of momentum transfer: students can estimate the change in speed and see why ejected debris
- How can I teach orbits and gravity using asteroids?Asteroids show how gravity shapes paths around the Sun, how resonances with Jupiter deliver meteorites, and how gravity assists and deflecti
- How do I talk about asteroid impacts without frightening pupils?Give accurate facts calmly, emphasise how rare large impacts are and how scientists are watching, and focus on curiosity and problem-solving
- How often does a Tunguska-sized event happen?Objects about 40 to 60 metres across, like the one that caused the Tunguska event of 1908, are thought to strike Earth roughly once every fe
- What are good discussion questions about meteorites?Questions about ownership, selling, Antarctica, planetary defence and evidence for life spark debate and develop reasoning skills.
- What happened with asteroid 2024 YR4?Asteroid 2024 YR4, about 60 metres across, briefly had a chance of hitting Earth in 2032 of up to about 3 per cent, before further observati
- What is a near-Earth asteroid?A near-Earth asteroid is one whose orbit brings it within about 1.3 times Earth's distance from the Sun, so it can pass relatively close
- What is Apophis and will it hit Earth?Apophis is an asteroid about 340 metres across that will pass very close to Earth on 13 April 2029.
- What is NASA doing about dangerous asteroids?NASA funds sky surveys that find and track near-Earth objects, calculates their future orbits, coordinates planetary defence through a dedic
- What is the closest a big asteroid will come to Earth soon?On 13 April 2029, asteroid Apophis, about 340 metres across, will pass about 32,000 kilometres above Earth, close enough to see without a te
- What is the Torino scale?The Torino scale rates the hazard of an asteroid or comet impact from 0, no hazard, to 10, certain global catastrophe, to help communicate r
- What was the DART mission?DART (Double Asteroid Redirection Test) was a NASA spacecraft that deliberately crashed into the small asteroid Dimorphos on 26 September 20
Impact events (1)
- Asteroids seen before impactSince 2008, telescopes have caught more than a dozen small asteroids hours before they hit.
Asteroids (12)
- 2024 YR42024 YR4 is a small asteroid that briefly became the most talked-about rock in the sky, and its story is a perfect lesson in how asteroid tr
- AmorAmor is a small asteroid, roughly a kilometre across, best known for the group of asteroids named after it.
- ApolloApollo was the first asteroid found whose orbit crosses Earth's. Karl Reinmuth discovered it at Heidelberg on 24 April 1932, when it pas
- ApophisApophis is an asteroid about 340 metres across that will make a spectacular but completely safe pass by Earth on 13 April 2029.
- AtenAten was the first asteroid found that spends most of its time closer to the Sun than Earth does.
- Didymos and DimorphosDidymos is a near-Earth asteroid about 780 metres across with a small moon, Dimorphos, about 150 metres wide, circling it at a distance of a
- ErosEros was the first near-Earth asteroid ever discovered. Gustav Witt found it from Berlin on 13 August 1898, and Auguste Charlois photographe
- FlorenceFlorence is one of the largest near-Earth asteroids, about 4.5 kilometres across. Schelte Bus discovered it on 2 March 1981 at the Siding Sp
- GanymedGanymed is the largest known near-Earth asteroid, about 35 kilometres across. It is not to be confused with Ganymede, the largest moon of Ju
- GolevkaGolevka is a small near-Earth asteroid, about 530 metres across, that made history in 2003.
- IcarusIcarus is named after the young man in Greek legend who flew too close to the Sun, and the name fits.
- ToutatisToutatis is an elongated asteroid about 4.5 kilometres long that does not spin neatly like most bodies.
People (4)
- Carolyn ShoemakerThe astronomer who began searching the sky in her fifties and discovered more comets than almost anyone, including the one that hit Jupiter.
- Krisztián SárneczkyThe Hungarian astronomer who discovered three small asteroids hours before they struck the Earth, including one that fell near Berlin in 202
- Richard BinzelThe MIT astronomer who found the family of small asteroids broken off Vesta and created the Torino scale for impact risk. Richard P.
- Richard KowalskiThe Catalina Sky Survey astronomer who discovered 2008 TC3, the first asteroid ever found before it hit the Earth. Richard A.