Atomic hydrogen production rates for comet P/Halley from observations with Dynamics Explorer 1

Summary

The distribution of atomic hydrogen surrounding comet Halley is observed in resonantly scattered solar Lyman-α radiation with the imaging photometer for vacuum-ultraviolet wavelengths on the Earth-orbiting spacecraft Dynamics Explorer 1. Measurements are made of the total Lyman-α flux at Earth due to the cometary neutral hydrogen distribution and the hydrogen production rate determined as a function of heliocentric distance, r. Corrections are made for the finite field-of-view of the photometer and for the ~700-R background at the spacecraft arising from the presence of geocoronal, interplanetary and galactic hydrogen. The distribution of hydrogen surrounding the comet nucleus at time t is dominated by atoms released from the comet in parent molecules at about time t − τ, where the hydrogen lifetime is τ. Corrections are made for this time difference. For distances 1.5 to 0.68 AU before perihelion passage the hydrogen production rate varies as Q ₀ r ⁻ⁿ,where n= 2.30 and Q ₀ = 9.1 10²⁹ atoms/s. Using the same functional dependence in the post-perihelion period at distances 0.63 to 1.2 AU, preliminary values based on a limited data set are n= 1.62 and Q ₀ = 1.1 10³⁰ atoms/s. These production rates are consistent with in situ measurements from the Giotto and Vega spacecraft, and with remote observations using two sounding rockets and with the Pioneer-Venus and IUE spacecraft.