Planetary Defense Explained: DART and What Comes Next

What planetary defense covers

NASA established its Planetary Defense Coordination Office (PDCO) in 2016 to manage the agency's work on near-Earth objects, the asteroids and comets that come within 48 million km (30 million miles) of Earth's orbit. The office does three kinds of work, summarized below.

Warnings follow a clear rule. If a hazardous object has a 1 percent or greater chance of impacting Earth over the next 50 years, the PDCO prepares notification messages for NASA to send to the Executive Office of the President, Congress and other agencies. The office also works with the Federal Emergency Management Agency (FEMA) and with international partners through the International Asteroid Warning Network and the Space Mission Planning Advisory Group, both endorsed by the United Nations Committee on the Peaceful Uses of Outer Space.

Most of that work is detection. The article on near-Earth asteroids explains how objects are found and counted. That statement covers objects larger than 140 m. Smaller ones are harder to find, and NASA's counts show far more of them still undiscovered.

The idea of a kinetic impactor

A kinetic impactor is a spacecraft that changes an asteroid's motion by hitting it. The change in speed is small, but a small change early can grow with time. NASA scientists describe it this way: a tiny nudge to an orbit, given enough time, can become a significant deflection. NASA adds that detecting hazardous objects early enough is the key to making this work. A hit also needs precise navigation to meet a small target, something sample missions such as OSIRIS-REx at Bennu have also had to do.

NASA's mission page describes DART as the first mission dedicated to investigating and demonstrating one method of asteroid deflection. The Didymos pair suited a test because Dimorphos orbits Didymos, so the effect of the hit could be seen as a change in the moonlet's orbital period.

The DART test in numbers

What DART showed

The orbit of Dimorphos around Didymos became 32 minutes shorter. NASA announced the result in October 2022, with a margin of uncertainty of about plus or minus 2 minutes. NASA had set a minimum success level of a 73-second change, so the result was roughly 26 times that threshold. A March 2026 NASA summary of later research cites a shortening of about 33 minutes.

A large part of the push came from the debris thrown off by the impact. NASA describes the ejecta as the many tons of asteroidal rock displaced and launched into space, and says the recoil from that debris substantially enhanced DART's push. The March 2026 summary gives a momentum enhancement factor of about two, meaning the debris roughly doubled the push from the spacecraft alone.

That summary also reports a new measurement: the Didymos pair's orbit around the Sun changed by 0.15 seconds in period. The study, published March 6, 2026 in Science Advances, combined radar and ground-based observations with 22 stellar occultations recorded by volunteer astronomers between October 2022 and March 2025. A stellar occultation happens when an asteroid passes in front of a distant star and briefly blocks its light, a method explained in the article on stellar occultations. The change is tiny, but it is real and measured.

What comes next

• October 7, 2024: ESA's Hera spacecraft launches toward Didymos.
• November 2026: ESA lists Hera's rendezvous with the Didymos pair for this month.
• 2026 and 2027: Hera carries out a detailed survey of the asteroids after the impact.
• No earlier than September 2027: NASA's NEO Surveyor space telescope launches.

ESA says Hera will measure the precise mass and structure of Dimorphos, work out why the deflection was larger than simulations predicted, and check how the impact changed the asteroid, for example whether it left a crater or altered its spin or shape. The spacecraft carries 12 payloads and two CubeSats. ESA moved the arrival a month earlier than first planned because the spacecraft's propulsion performed well. The latest news item on ESA's Hera page, dated October 27, 2026, says Hera had resolved the two asteroids separately for the first time from about 197 million km (122 million miles) away. The page does not yet report arrival, so check ESA's mission page for the current status.

NEO Surveyor, per NASA's page updated November 3, 2026, will use infrared light to help find potentially hazardous asteroids and comets. A different chance to learn arrives in April 2029, when the asteroid Apophis passes very close to Earth, as described in the guide to the Apophis flyby.

Common mistakes and myths

Myth: DART was a rescue from a known threat. It was a demonstration. NASA's overview says no known asteroid larger than 140 m has a significant chance of impact in the next 100 years. Smaller objects can still reach Earth, as the February 15, 2013 airburst over Chelyabinsk, Russia showed, and that event is covered in the article on the Chelyabinsk meteor of 2013. DART was built to test a method, not to answer a known object.

Myth: DART blew up the asteroid. The mission changed the orbit of Dimorphos around Didymos, and that orbit is what NASA measured. ESA's Hera is designed to check what the hit did to the moonlet's shape, spin and surface.

Myth: one success proves any asteroid can be stopped. DART tested one target. ESA says Hera's task is to help turn the kinetic impactor method into a reliable technique, and it points to open questions such as why the push was bigger than simulated.

Myth: planetary defense means mostly fighting asteroids. NASA's overview lists finding, warning and mitigation, and most of what it describes is search, orbit refinement and notification. Deflection studies are one part of the program.

Frequently asked questions

Did DART make Earth safer?

NASA says the Didymos orbit result supports kinetic impact as a way to defend Earth. It was one test on one target, and NASA stresses that finding hazardous objects early is key.

Is a hazardous asteroid heading for Earth now?

NASA's planetary defense overview says there is no known asteroid larger than 140 m with a significant chance of impacting Earth in the next 100 years. Agencies update that as observations improve, so check NASA's pages for the current statement.

How much did DART change the orbit of Dimorphos?

It shortened the orbit by about 32 minutes, from about 11 hours 55 minutes to about 11 hours 23 minutes, according to NASA's first announcement in October 2022. Later analysis refined the figure to about 33 minutes.

What is Hera?

Hera is ESA's mission to survey Didymos and Dimorphos after the impact. It launched October 7, 2024, and ESA lists its rendezvous for November 2026.