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 4×10
¹⁹
/m
³
. Critical measurement techniques will be validated using results from the May 20, 2011, Lunar Occultation of New Horizons spacecraft.