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Could a massive asteroid that hit Mars or the moon affect Earth? How big would it need to be, and what would happen?
Benjamin Fernando
Johns Hopkins University, Maryland, US
Yes, absolutely! When asteroids or comets hit rocky bodies such as the moon or Mars, they form impact craters that displace a large volume of material from the surface. Some of this material can be ejected upwards at speeds exceeding a given object’s escape velocity, meaning it is no longer gravitationally bound to the planet or moon and escapes into space. Exactly how big an impactor is needed depends on factors such as what it is made of and the composition of the surface it hits.
Once in space, the ejecta’s inertia and gravitational interactions with other bodies can spread the material around the solar system. In the moon scenario, a fraction of this ejecta could end up on a collision course with Earth. Of these, the smallest fragments will burn up in the atmosphere as shooting stars, or meteors. Larger fragments might survive to impact the surface, creating new craters on land or tsunamis in the ocean. Mars is further from Earth, has a higher escape velocity and has an atmosphere that will slow down any escaping material, so any debris ejected from the Red Planet is less of a threat to Earth.
Thankfully, impacts of the size needed to eject substantial volumes of debris into space are rare in the present day, but were probably much more common in the early solar system. In fact, this process is a possible route to achieving “panspermia”, the idea that microscopic life can spread around the galaxy carried along with impact ejecta.
John Woodgate
Rayleigh, Essex, UK
If a massive asteroid hit the moon, Earth would certainly be visited by debris, perhaps in large chunks, quite quickly. Our planet’s orbital motion and day length might even be affected. If Mars were hit, debris would probably take up to some years to arrive at Earth in force. It would depend on the orbital positions of Mars and Earth at the time of collision. The result might be anything from an interesting display of meteors to a barrage of meteorites for quite a few days, perhaps repeating at varying intervals in years.
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