The Radioscience Experiment on New Horizons

REX is the Radioscience Experiment in the payload on the New Horizons spacecraft en-route to its encounter with Pluto in July of 2015. REX will obtain the temperature and pressure profiles of Pluto´s tenuous atmosphere while measuring radiometric temperature, gravitational moments and ionosphere structure. Additional targets of opportunity, at lower priority, include the search for an atmosphere at Charon, improved gravity precision and a bistatic surface scattering on Pluto. For all but radiometry, these measurements take advantage of a high-power, X-band uplink transmitted from the earth, received on the spacecraft with the aid of a high-gain antenna, a low-noise X-band receiver and an ultrastable oscillator (USO), as a frequency reference. This combination enables REX to sense Pluto´s atmosphere with precision of ~0.1 Pa (1 μbar), and ~3 K, and a neutral number density of 4×10¹⁹/m³. Critical measurement techniques will be validated using results from the May 20, 2011, Lunar Occultation of New Horizons spacecraft.