REX (instrument)

REX (instrument)
Name	REX
Mission	New Horizons
Operator	NASA / Johns Hopkins University Applied Physics Laboratory
Launch	July 19, 2006
Spacecraft	New Horizons (spacecraft)
Type	radio science experiment
Target	Pluto, Charon, Kuiper belt

REX (instrument) is a radio science experiment flown aboard New Horizons (spacecraft) designed to probe planetary atmospheres, ionospheres, surfaces, and the interplanetary medium using radiometric and occultation techniques. Built and operated by teams at Johns Hopkins University Applied Physics Laboratory, NASA and partner institutions, REX exploited the spacecraft's telecommunications system to perform precision measurements during occultations of Pluto, Charon, and other Kuiper belt objects, contributing to studies by groups associated with Southwest Research Institute, European Space Agency, and university research centers.

Overview and mission objectives

REX was conceived as a low-resource, high-impact instrument to meet the strategic goals of the New Horizons mission: characterize atmospheres, determine surface thermal properties, and test fundamental radio science in the outer Solar System. Its objectives included measuring atmospheric temperature and pressure profiles of Pluto and Charon, detecting or constraining ionospheric electron densities, determining surface dielectric properties of Kuiper belt objects, and providing precise spacecraft radio occultation geometry for astrometry relative to catalogs like Hipparcos and later Gaia. REX data supported broader investigations by teams engaged in studies of planetary atmospheres, space plasmas, and comparative planetology involving bodies such as Triton and Iapetus.

Instrument design and components

REX is an experiment implemented primarily in firmware and software within the New Horizons (spacecraft) telecommunications subsystem, minimizing mass and complexity by leveraging the existing high-gain antenna and radio hardware. Core components included a dedicated onboard receiver chain, a precision frequency reference tied to the spacecraft's ultra-stable oscillator, digital signal processing modules, and onboard storage for collected radiometric and occultation data. The design integrated with the spacecraft's command and data handling architecture developed at Johns Hopkins University Applied Physics Laboratory and used heritage from radio science experiments on missions such as Voyager 2 and Cassini–Huygens. The team collaborated with instrument builders and institutions including SwRI, university laboratories, and commercial contractors for hardware, firmware, and calibration.

Measurement principles and capabilities

REX employed radio occultation and radiometry by receiving and analyzing X-band carrier and telemetry signals transmitted between New Horizons (spacecraft) and large ground stations such as the Deep Space Network. During occultations, refractive bending and amplitude/phase changes of the downlink were converted into profiles of atmospheric refractivity, from which temperature, pressure, and electron density profiles could be derived using inversion techniques applied by teams familiar with methods from missions like Mars Reconnaissance Orbiter and Cassini–Huygens. REX also performed passive radiometry by measuring thermal emission from surfaces with the high-gain antenna acting as a radiometer, enabling constraints on brightness temperature and dielectric constants analogous to measurements from instruments like Galileo (spacecraft) and Juno (spacecraft). REX's sensitivity allowed detection of pressure levels down to microbar regimes and temperature resolution sufficient to resolve thermal gradients in tenuous atmospheres.

Calibration and data processing

Calibration for REX combined preflight laboratory characterization with in-flight cross-calibration using known sources and telemetry signals from Deep Space Network antennas. The team used references including the spacecraft's ultra-stable oscillator and calibration tones embedded in downlink telemetry to remove instrumental phase and amplitude biases. Data processing pipelines developed by scientists at Johns Hopkins University Applied Physics Laboratory and partner institutions applied radio occultation inversion algorithms, radiometric deconvolution, and noise analysis techniques previously validated on missions like Voyager 2, Cassini–Huygens, and Magellan (spacecraft). Ancillary data from spacecraft navigation teams, including ephemerides tied to JPL solutions and timing from NASA operations centers, were essential for converting raw Doppler and phase observables into geophysical parameters.

Flight operations and results

During the 2015 encounter, REX performed downlink radio occultations of Pluto and a calibration occultation opportunity with Charon, recording changes in carrier phase and amplitude as the spacecraft passed behind the target relative to Earth. Operations required coordination with Deep Space Network scheduling, trajectory design by Jet Propulsion Laboratory navigation teams, and mission sequencing by Ames Research Center and APL operations. Post-encounter, REX continued passive radiometry observations of Pluto's nightside and was used during extended mission flybys of Kuiper belt objects, including the flyby of 2014 MU69 (Arrokoth), to assess surface thermal properties. The data sets were archived and analyzed by teams across universities and research institutes, leading to numerous peer-reviewed publications and presentations at conferences such as AGU and EPSC.

Scientific findings and legacy

REX produced definitive measurements of the lower atmosphere of Pluto, yielding surface pressure and near-surface temperature constraints that resolved competing models from ground-based stellar occultations and Hubble Space Telescope observations. REX found a shallow thermal inversion and confirmed a surface pressure near tens of microbars, informing volatile transport and atmospheric escape studies connected with work on solar wind interaction and exospheres studied by missions like MAVEN. Radiometric results constrained nighttime brightness temperatures and surface dielectric properties of icy terrains, contributing to geological interpretations together with imaging from the spacecraft's visible and infrared instruments and comparative analyses with Triton and Kuiper belt bodies. REX's legacy includes demonstrating the effectiveness of software-centric radio science on resource-limited missions, influencing instrument planning for future outer-planet and small-body missions by teams at NASA centers, ESA, and academic institutions worldwide.

Category:Instruments aboard New Horizons