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Dust grains may be accelerated to relativistic speeds by radiation pressure,diffusive shocks, and other acceleration mechanisms. Such relativistic grainshave been suggested as primary particles of ultrahigh energy cosmic rays(UHECRs). In this paper, we first revisit the problem of acceleration byradiation pressure and calculate maximum grain velocities achieved. We findthat grains can be accelerated to relativistic speeds with Lorentz factor$\gamma < 2$ by powerful radiation sources, which is lower than earlierestimates showing that $\gamma$ could reach $\sim 10$. We then investigatedifferent destruction mechanisms for relativistic grains traversing a varietyof environments. In solar radiation, we find that the destruction by thermalsublimation and Coulomb explosions is important. We also quantify graindestruction due to electronic sputtering by ions and grain-grain collisions.Electronic sputtering is found to be rather inefficient, whereas theevaporation following grain-grain collisions is shown to be an importantmechanism for which the $a \sim 0.01- 1\mu$m grains would be destroyed aftersweeping a gas column $N_{Coul}\sim 5\times 10^{19}-5\times 10^{20}cm^{-2}$.Relativistic dust in the interstellar medium and intergalactic medium (IGM)would be disrupted by Coulomb explosions due to collisional charging aftertraversing a gas column $N_{Coul} \sim 10^{17}cm^{-2}$ unless grain material isvery strong. We show that photoelectric emission by optical and ultravioletbackground radiation is also significant for the destruction of relativisticdust in the IGM. The obtained results indicate that relativistic dust fromgalaxies would be destroyed before reaching the Earth's atmosphere and unlikelyto account for UHECRs.

On origin and destruction of relativistic dust and its implication for ultrahigh energy cosmic rays