The short answer to why Dimorphos now has a 10,000 km long tail, some two weeks after NASA intentionally slammed a space probe into it, is that the solar wind dragged the debris with it into space.
If this is all there is to the story, we should expect the tail soon to disconnect from the asteroid and become an elongated cloud, separate from it. However, if this doesn't happen, we're looking at something more complex, namely a comet of sorts.
Comets have tails that persist over time, not because they're dirty snowballs, but because the environment comets travel through is constantly changing in charge density. Going towards the Sun, comets have to adjust for higher charge density. Going away from the sun, they have to adjust to lower charge density. This adjustment is achieved through the shedding of material through electrochemical processes and possibly nuclear fission. Hence, the tail of comets.
Planets don't have tails like comets do because planets have near circular orbits. There's no need for readjustments when it comes to charge density. The difference between a comet and a planet is therefore due entirely to the shape of their orbits. When orbits are circular, there're no tails. When orbits are oblong, there're tails.
If Dimorphos' tail proves persistent, we can use the above to explain the reason for this: Before the impact, Dimorphos had no tail because it was orbiting in a circle around Didymos. After the impact, Dimorphos acquired a persistent tail due to its new and oblong orbit.
Didymos is no sun, but it has around it a charge density of its own, and Dimorphos is a heap of rubble from which dust can easily be dislodged. As such, the two asteroids represent a miniature system comparable to the solar system.
My thesis when it comes to orbits is that they are more stable than generally believed. I've based this on the fact that orbits are governed by gravitational attraction and electrical repulsion, with gravity acting from the centre of bodies and electric repulsion acting from the surface of bodies. When these forces combine, we get a shock absorber effect that steadies orbits of bodies hit by an external force.
In the case of Dimorphos, we can add an extra source of stability, namely the solar wind which acts like an external power supply. This power supply may have importance to where the ideal orbit of Dimorphos should be relative to Didymos. If so, we have a chance of seeing the disturbed orbit not only steady into a circle quicker than most would expect but restore itself completely back to its original.
What we're about to witness might turn out to be a miniature version of what some believe to have happened some 10,000 years ago, when legend has it that Venus settled into its current orbit after a turbulent journey from Jupiter to where it's currently located.
Venus is everywhere in the world depicted as either a goddess with long flowing hair or a god with a long beard, indicating that it had a tail relatively recently. However, this tail disappeared once Venus settled into her current orbit. Venus went from being a comet to a planet in less than 10,000 years.
If Dimorphos steadies into a circular orbit quicker than expected, we'll have supporting evidence for the Venus as a comet theory. If Dimorphos retains its tail until its orbit is near circular, we have additional evidence for this theory, and if the orbit gets completely restored, the evidence becomes even stronger.
NASA's experiment may turn out to be more revealing than anyone had thought.
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| Dimorphos 285 hours after impact |
By NASA
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