How can I teach momentum using the DART mission?

NASA's DART mission provides a real-world application of momentum for GCSE and A-level physics.

The facts. On 26 September 2022, the DART spacecraft, with a mass of about 580 kilograms at impact, struck Dimorphos, a small asteroid moon about 160 metres across, at about 6.1 kilometres per second. Dimorphos orbits a larger asteroid, Didymos. The impact shortened its orbital period by about 32 minutes, from about 11 hours 55 minutes.

Momentum calculation. Momentum equals mass times velocity. DART's momentum was about 580 times 6,100, roughly 3.5 million kilogram metres per second. Dimorphos's mass is estimated at about 4 to 5 billion kilograms. If the momentum were simply transferred, the change in Dimorphos's speed would be about 3.5 million divided by 4.3 billion, under 1 millimetre per second.

The ejecta bonus. The impact blasted out a large plume of rock and dust. As this debris flew off, it pushed Dimorphos in the opposite direction, like a rocket's exhaust. Scientists estimate that the total momentum change was several times larger than DART's momentum alone. They describe this with a factor called beta, estimated at roughly 2 to 5, depending on assumptions about Dimorphos's mass.

Discussion. Why did a small speed change make a measurable difference? Because it altered the orbit, and over many orbits the timing change adds up. Why is early warning important for deflection? Because small changes applied early have more time to grow.

Extension. Students compare other deflection methods, such as gravity tractors and ion beams.

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