Abstract. The time-scales of import with respect to the physical survival of planet-crossing bodies (asteroids comets, meteoroids, dust) in the inner solar system are considered, and characteristic times for different masses reviewed. These physical lifetimes range from 103-105 yr for dust (masses 10-12-t0 -6 g), and 105-106 yr for small meteoroids (masses 10-6-1 g), to 107-108 yr for larger bodies; bodies with aphelion distance > 4 AU may have dynamical lifetimes lower than these figures due to ejection from the solar system by Jupiter. Values of terrestrial impact velocities and probabilities are given for characteristic orbits of long-and short-period comets, Earth-crossing asteroids, and near-Earth dust. Zodiacal dust particles, which are predominantly in near-circular, low-inclination orbits, have sufficiently low arrival velocities (< 20 km sec -1) at the Earth to make organic survival plausible. Alternatively larger objects with short periods, perihelia near 1 AU, and i ~ 20 °, will also impact at < 20 km sec -1, but their impact probabilities are smaller. This argues for organic delivery predominantly from dust rather than directly through meteoroids, asteroids or comets. Such dust may have delivered the amino acids deposited over at most ~ 105 yr at the K/T boundary: this is also the appropriate time-scale for the hierarchical disintegration of a giant comet and its daughter products, and the accumulation by the Earth of the dust produced, but is too short for deposition by discrete large bodies produced in the disintegration of a large comet. This supports the conjecture that some organics arrived in the first ~ 109 yr of the planet's history as constituents of cometary dust gently decelerated in the atmosphere, allowing survival to the surface.