Asteroid defense mission shifted the orbit of more than its target

Source: Ars Technica

# Hit One, Move Two

The binary asteroid’s orbit around the Sun was affected by the impact.

[Image: Italy's LICIACube spacecraft captured asteroids Didymos (lower left) and Dimorphos (upper right) a few minutes after the DART impact on September 26, 2022.]

*Credit: ASI/NASA*

On September 26, 2022, NASA’s **Double Asteroid Redirection Test (DART)** spacecraft [crashed into a binary asteroid system](https://arstechnica.com/science/2023/07/hubble-is-able-to-spot-boulders-blasted-loose-by-the-dart-impact/). By intentionally ramming a probe into the 160‑meter‑wide moonlet **Dimorphos**, the smaller of the two asteroids, humanity demonstrated that the kinetic‑impact method of planetary defense actually works. The immediate result was that Dimorphos’s orbital period around Didymos, its larger parent body, was [slashed by 33 minutes](https://arstechnica.com/science/2022/10/dart-mission-successfully-shifted-its-targets-orbit/).

Altering a moonlet’s local orbit might not seem sufficient to safeguard Earth from civilization‑ending impacts. However, long‑term observational data now show that DART did more than that: it changed the trajectory of the entire Didymos binary system, altering its orbit around the Sun.

Tracking Space Rocks

Measuring the orbital shift of a 780‑meter‑wide primary asteroid and its moonlet from millions of miles away isn’t trivial. When DART slammed into Dimorphos, it didn’t knock the binary system wildly off its trajectory around the Sun. The change in the system’s heliocentric trajectory was expected to be small—a minuscule nudge that would become apparent only after months or years of continuous observation. By analyzing enough painstakingly gathered data, a global team of researchers led by Rahil Makadia at the University of Illinois Urbana‑Champaign has now determined the consequences of the DART impact.

How the tiny deviation was measured

Makadia’s team relied primarily on a technique called stellar occultation. When an asteroid passes in front of a distant star from the perspective of an observer on Earth, the star briefly blinks out. By precisely timing these blinks as they sweep across the globe, astronomers can pinpoint an asteroid’s position with astonishing accuracy.

Data set

“Once we had enough measurements before and after the DART impact, we could discern how Didymos’ orbit has changed,” Makadia said.

Impact of the DART collision

When the vending‑machine‑sized DART probe crashed into Dimorphos at > 22 000 km h⁻¹, it decreased the along‑track velocity of the entire Didymos system by roughly 11.7 µm s⁻¹.

“When you do it early enough, even a small impulse can accumulate over years and cause a meaningful shift,” Makadia explained.

Other forces at play

The DART impact was not the only force that altered Didymos’ orbit; natural perturbations (e.g., solar radiation pressure, gravitational interactions with other bodies) also contribute to the long‑term evolution of the system.

The Ejecta Engine

The pure kinetic energy of a 500‑kilogram spacecraft hitting an asteroid at hypersonic speeds is impressive, but on its own it would not significantly slow a large body. When DART struck Dimorphos, it blasted pulverized rock and dust into space.

“The material kicked up off an asteroid surface acts like an extra rocket plume,” — Makadia

Scientists refer to this effect as the momentum‑enhancement factor, denoted by the Greek letter β. If the spacecraft impact transferred only its own momentum and no debris was ejected, β would be exactly 1.

Because Dimorphos orbits Didymos, some of the ejecta remained trapped in the binary system, altering the mutual orbit of the two rocks. A crucial fraction, however, achieved escape velocity from the entire system. The momentum carried away by this escaping debris ultimately contributed to shifting the center of mass of the Didymos–Dimorphos pair.

“In our case, we found that the β parameter due to the DART impact was around 2,” — Makadia

The debris that left the Didymos system gave the asteroids a push roughly equal to the initial impact of the spacecraft itself.

Determining Momentum Transfer

To calculate how momentum was transferred, Makadia and his colleagues first needed precise masses for Didymos and Dimorphos. By linking the heliocentric deflection to the previously measured changes in Dimorphos’s local orbit, they performed a clever mathematical trick that revealed the bulk densities of both bodies. This analysis uncovered an unexpected result:

“Most studies assumed that both asteroids have equal density—turns out that assumption was not correct,” — Makadia

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Key take‑aways

• β ≈ 2 for the DART impact, meaning the ejecta roughly doubled the effective momentum transfer.
• A portion of the ejecta escaped the binary system, providing the dominant contribution to the overall push.
• The densities of Didymos and Dimorphos differ, challenging prior assumptions of equal density in the system.

A Rubble Pile

Based on Makadia’s calculations, Didymos, the primary body, is relatively solid. It has a bulk density of around 2.6 t m⁻³, which aligns with standard estimates for siliceous asteroids. Dimorphos, however, tells a different story. Its density is a surprisingly low 1.51 t m⁻³, implying that the smaller asteroid targeted by DART is essentially a fluffy, loosely bound agglomeration of boulders, rocks, and dust, with empty voids between the rubble.

“This was a real surprise,” Makadia said. “We previously didn’t know anything about the density of Dimorphos.”

The contrast in density reveals how this binary system likely formed.

Formation via the YORP Effect

1. Uneven heating and solar radiation over billions of years cause an irregularly shaped asteroid like Didymos to spin faster—a phenomenon known as the YORP effect (Yarkovsky‑O’Keefe‑Radzievskii‑Paddack).
2. As Didymos spun up, centrifugal forces eventually overcame its self‑gravity at the equator.
3. Loose material was shed from the equatorial region.
4. The ejected debris coalesced in orbit, gently clumping together to form the porous, fragile moonlet we now know as Dimorphos.

Mass Ratio and System Inertia

• Didymos is ≈ 200 times more massive than its smaller companion.
• This large mass disparity explains why shifting the entire binary system requires an enormous amount of force.
• The sheer inertia of Didymos means that the barycenter deflection of the whole system was only a tiny fraction of the local deflection experienced by Dimorphos.

Planetary Defense

Makadia’s findings confirm the models we used to estimate the consequences of the DART impact: the Didymos system still poses zero threat to Earth for at least the next 100 years.

“The pre‑DART condition was that the closest the Didymos system can get to Earth was around 15 lunar distances, and this has not changed appreciably,” Makadia explained.

The primary goal of DART was to move planetary‑defense research out of the realm of computer models and give us hands‑on, practical experience. Makadia believes we succeeded:

“Our work proves that hitting the secondary asteroid is a viable path for deflecting a binary system away as long as the push is large enough. This wasn’t the goal of DART, but we can always design a bigger spacecraft.”

The experience applies both to binary asteroid systems like Didymos and to single objects:

“Our results definitely help us in all sorts of future kinetic‑impact endeavors,” Makadia added.

The final verification of DART’s consequences will come in late 2026, when ESA’s Hera spacecraft arrives at the Didymos system. By performing independent, in‑situ measurements (e.g., density of Didymos and Dimorphos), Hera will provide precise gravitational and physical data that Makadia hopes to use to refine his calculations.

“It’s a high‑fidelity instrument that hopefully will give us confirmation of what we believe. Plus, there are always new things to be found out when we visit an asteroid. I’m very excited about when Hera gets there.”

Science Advances, 2026. DOI: 10.1126/sciadv.aea4259

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Author

Jacek Krywko – freelance science and technology writer covering space exploration, artificial‑intelligence research, computer science, and engineering wizardry.

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