“Compared to conventional ground-based radiometric tracking, GIRO is expected to provide accuracy that is 10 to 100 times better,” Ryan Park, chief engineer at NASA’s Jet Propulsion Laboratory, said in Space.com. That bold declaration is not a pipe dream rather, it’s based on a basic shift in planetary scientists’ ability to peer through the surfaces of other planets.
The Gravity Imaging Radio Observer, or GIRO, is a tiny, battery-powered probe that will piggyback on an escort spacecraft, then break away and circle or fly by a target moon, planet, or asteroid. Its mission to measure the weak gravitational “lumpiness” that betrays the unseen structure of alien interiors. The technology for creating GIRO’s sleekly minimalist design is both technically difficult. As the probe and host vessel travel within a target body’s gravitational field, the minute gravity fluctuations from dense rock, hidden voids, or entrapped ice deflect their course by infinitesimally small amounts. While their displacements are too small to be detected by the naked eye, they result in Doppler shifts in radio signals that travel between probe and host.
Scientists can utilize the precise analysis of these Doppler signatures to produce gravity maps of stunning precision. Such maps do not simply follow the contours of a planet or moon they chart its patterns of density, indicate its makeup, and even document its geologic history. “These changes can be measured using the Doppler effect in the radio signals,” Park said, allowing scientists to not only infer mass and density, but also geologic or volcanic activity pivotal signs in the search for worlds supportive of life beyond Earth.
The GIRO concept is both technological and strategic innovation. Compared to previous gravity-mapping missions, like NASA’s GRAIL, that dispatched twin spacecraft to the Moon to chart its gravitational field with unprecedented precision, GIRO will do it or better with a single, compact, low-power device. GRAIL’s twin satellites, Ebb and Flow, utilized radio tracking to measure how far apart they were as they circled the Moon and detected mass concentrations mascons hidden in the surface that would otherwise have gone undetected. The maps of gravity derived revealed tectonic structure, volcanic terrain, and the shattered crust of the Moon, transforming lunar geology and the history of rocky worlds.
The ingenuity of GIRO is its economizing and versatility. The spin-stabilized, battery-driven design of the probe allows deployment in hostile or adverse conditions, such as the dense rings of Uranus or the region of small, irregular asteroids where conventional orbiters would be strained. Its compact size means that a number of probes could be flown on a single mission, each set to collect data in brief but precious windows of opportunity some dozens of flybys or ten days’ worth of battery life. Solar recharging can lengthen lifetimes for Sun-proximate missions.
Technical needs are daunting. To achieve high-accuracy gravity measurement, GIRO must be inserted into orbits most sensitive to gravitational anomalies and yet maintain a stable radio link to the host spacecraft. This is similar to the mission complexity involved with GRAIL, where the separation between the twin satellites was determined to sub-millimeter accuracy, and in recent planetary radio science experiments such as the PRIDE mission on ESA’s JUICE mission. PRIDE employs global networks of radio telescopes to perform very long baseline interferometry (VLBI) and Doppler tracking, measuring the position and velocity of spacecraft with unprecedented precision. The planning and scheduling of such observations selecting appropriate calibrator sources, coordinating with sets of telescopes, and removing atmospheric noise are critically important to the quality of resulting data, as demonstrated by recent PRIDE operations.
GIRO’s plan is to place these ground-breaking technologies into a small, self-contained package. The probe reflects radio waves off the host spacecraft, and resulting Doppler data are analyzed to reveal gravity anomalies. The method, borrowed from GRAIL and PRIDE, reduces complexity and cost by eschewing a complete set of ground-track stations or dual-spacecraft configurations. “This means gravity science can be conducted as part of broader exploration missions rather than requiring dedicated spacecraft,” Park explained.
But the promise of GIRO comes with its own engineering and operational challenges. The probe’s limited battery life historically capped at ten days for outer solar system flights is demanding in that it necessitates careful mission planning to ensure all measurements are made before power runs out. For spaceships with challenging environments or restricted times for data acquisition, the timing and geometry of probe release are of critical concern. Proper radio link, especially as the host spacecraft and probe separate in orbits, requires precise navigation and robust communications protocols.
Planetary protection is also a significant factor. International standards require probes to not contaminate unspoiled worlds, specifically those of possible life. This impacts GIRO’s orbits and disposal at mission end regarding prevention of unplanned impact or long-duration orbital debris.
The scientific payoff is high-potential. By making possible high-accuracy gravity mapping of small asteroids, distant moons, or even exoplanets, GIRO can potentially open new paths in planetary science. The possibility of accessing faint gravitational signals, such as those of concealed oceans or differentiated interiors, could redefine our knowledge of planet evolution and the needs for life. Their inclusion with sophisticated imaging devices, e.g., the Advanced Pointing Imaging Camera (APIC), would extend these missions further by providing topography and geophysical data at high resolutions to complement gravity measurements in order to permit detailed investigations of tidal flexing, rotational dynamics, and internal structure.
The road to GIRO’s debut is clear, though not simple. “The most important milestones before integration involve building and testing flight-like prototypes in environments that closely simulate actual mission conditions,” Park said. With successful testing of GIRO, it may soon join the payloads of missions to asteroids, moons, or the outer planets carrying a new era of gravity science that is both achievable and revolutionary to planetary exploration.
