Spacecraft Experiments

ACE Acoustic Composition Experiment

Professor Fegley is the principal investigator of a NASA grant to build a sound velocity instrument, which is called ACE for Acoustic Composition Experiment, to measure the ortho to para ratio of hydrogen gas in the atmospheres of the Jovian planets. The ACE instrument is being designed to fly on future entry probes into the atmospheres of Jupiter, Saturn, Uranus, and Neptune.  The measurement of the ortho to para ratios is important because at low temperatures, the ortho (parallel proton spins) and the para (antiparallel proton spins) forms of hydrogen gas have distinct physical properties which affect the atmospheric thermal structure of the Jovian planets, especially Uranus and Neptune.  As part of this work, Professor Fegley is directing construction of the breadboard ACE instrument by a commercial contractor.

Rotational Heat Capacity for HydrogenFigure 1. The rotational heat capacity (joules per mole per degree) for hydrogen as a function of temperature. The curves show data for para hydrogen, ortho hydrogen, equilibrium hydrogen, and normal hydrogen. Para hydrogen has antiparallel nuclear spins and even rotational quantum numbers while ortho hydrogen has parallel nuclear spins and odd rotational quantum numbers. Equilibrium hydrogen is the equilibrated o-p hydrogen mixture at any temperature (see Figure 2). Normal hydrogen is a 3:1 mixture of ortho (75%) and para (25%) hydrogen that is the equilibrium composition at high temperatures (i.e., about 298 K and above). From Professor Fegley’s April 1996 presentation to the NASA Outer Planet Science Working Group (OPSWG).

Composition of Equilibrium H2Figure 2. The composition of equilibrium hydrogen as a function of temperature. The molar percentage of para hydrogen as a function of temperature is plotted. From Professor Fegley’s April 1996 presentation to the NASA Outer Planet Science Working Group (OPSWG).


Sound Velocity in H2-He mixturesFigure 3. The sound velocity in hydrogen – helium mixtures as a function of the heat capacity ratio and mean molecular weight. Lines of constant helium content (the nearly horizontal lines inside the sound velocity triangle) and lines of constant ortho/para ratio (the nearly vertical lines inside the sound velocity triangle) are shown along with contours of constant sound velocity. From Professor Fegley’s April 1996 presentation to the NASA Outer Planet Science Working Group (OPSWG).