L. Iess

A test of general relativity using radio links with the Cassini spacecraft

According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass. The bending and delay are proportional to γ + 1, where the parameter γ is unity in general relativity but zero in the newtonian model of gravity. The quantity γ - 1 measures the degree to which gravity is not a purely geometric effect and is affected by other fields; such fields may have strongly influenced the early Universe, but would have now weakened so as to produce tiny—but still detectable—effects. Several experiments have confirmed to an accuracy of ∼0.1% the predictions for the deflection and delay of photons produced by the Sun. Here we report a measurement of the frequency shift of radio photons to and from the Cassini spacecraft as they passed near the Sun. Our result, γ = 1 + (2.1 ± 2.3) × 10-5, agrees with the predictions of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models.

Gravity Field, Shape, and Moment of Inertia of Titan

Titan Through to the Core Gravity measurements acquired from orbiting spacecraft can provide useful information about the interior of planets and their moons. Iess et al. (p. 1367; see the Perspective by Sohl) used gravity data from four flybys of the Cassini spacecraft past Saturns moon, Titan, to model the moons gravity field and probe its deep interior structure. Their analysis implies that Titan is a partially differentiated body with a core consisting of a mix of ice and rock or hydrated silicates. Analysis of gravity data reveals that Saturn’s moon Titan has a partially differentiated internal structure. Precise radio tracking of the spacecraft Cassini has provided a determination of Titan’s mass and gravity harmonics to degree 3. The quadrupole field is consistent with a hydrostatically relaxed body shaped by tidal and rotational effects. The inferred moment of inertia factor is about 0.34, implying incomplete differentiation, either in the sense of imperfect separation of rock from ice or a core in which a large amount of water remains chemically bound in silicates. The equilibrium figure is a triaxial ellipsoid whose semi-axes a, b, and c differ by 410 meters (a – c) and 103 meters (b – c). The nonhydrostatic geoid height variations (up to 19 meters) are small compared to the observed topographic anomalies of hundreds of meters, suggesting a high degree of compensation appropriate to a body that has warm ice at depth.

Titan's Rotation Reveals an Internal Ocean and Changing Zonal Winds

Cassini radar observations of Saturns moon Titan over several years show that its rotational period is changing and is different from its orbital period. The present-day rotation period difference from synchronous spin leads to a shift of ∼0.36° per year in apparent longitude and is consistent with seasonal exchange of angular momentum between the surface and Titans dense superrotating atmosphere, but only if Titans crust is decoupled from the core by an internal water ocean like that on Europa.

The Gravity Field and Interior Structure of Enceladus

Inside Enceladus Saturns moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturns largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep. The saturnian moon is differentiated and likely hosts a regional subsurface sea at its southern pole. The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1σ) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.

The Tides of Titan

Getting to Know Titan Gravity-field measurements provide information on the interior structure of planets and their moons. Iess et al. (p. 457; published online 28 June) analyzed gravity data from six flybys of Saturns moon, Titan, by the Cassini spacecraft between 2006 and 2011. The data suggest that Titans interior is flexible on tidal time scales with the magnitude of the observed tidal deformations being consistent with the existence of a global subsurface water ocean. Gravity measurements by the Cassini spacecraft suggest that Saturn’s moon Titan hosts a subsurface ocean. We have detected in Cassini spacecraft data the signature of the periodic tidal stresses within Titan, driven by the eccentricity (e = 0.028) of its 16-day orbit around Saturn. Precise measurements of the acceleration of Cassini during six close flybys between 2006 and 2011 have revealed that Titan responds to the variable tidal field exerted by Saturn with periodic changes of its quadrupole gravity, at about 4% of the static value. Two independent determinations of the corresponding degree-2 Love number yield k2 = 0.589 ± 0.150 and k2 = 0.637 ± 0.224 (2σ). Such a large response to the tidal field requires that Titan’s interior be deformable over time scales of the orbital period, in a way that is consistent with a global ocean at depth.

Jupiter’s interior and deep atmosphere: The initial pole-to-pole passes with the Juno spacecraft

Juno swoops around giant Jupiter Jupiter is the largest and most massive planet in our solar system. NASAs Juno spacecraft arrived at Jupiter on 4 July 2016 and made its first close pass on 27 August 2016. Bolton et al. present results from Junos flight just above the cloud tops, including images of weather in the polar regions and measurements of the magnetic and gravitational fields. Juno also used microwaves to peer below the visible surface, spotting gas welling up from the deep interior. Connerney et al. measured Jupiters aurorae and plasma environment, both as Juno approached the planet and during its first close orbit. Science, this issue p. 821, p. 826 Juno’s first close pass over Jupiter provides answers and fresh questions about the giant planet. On 27 August 2016, the Juno spacecraft acquired science observations of Jupiter, passing less than 5000 kilometers above the equatorial cloud tops. Images of Jupiter’s poles show a chaotic scene, unlike Saturn’s poles. Microwave sounding reveals weather features at pressures deeper than 100 bars, dominated by an ammonia-rich, narrow low-latitude plume resembling a deeper, wider version of Earth’s Hadley cell. Near-infrared mapping reveals the relative humidity within prominent downwelling regions. Juno’s measured gravity field differs substantially from the last available estimate and is one order of magnitude more precise. This has implications for the distribution of heavy elements in the interior, including the existence and mass of Jupiter’s core. The observed magnetic field exhibits smaller spatial variations than expected, indicative of a rich harmonic content.

The rotation of LAGEOS

In view of the need of an accurate modelling of nongravitational forces on laser-tracked satellites, it is important to understand their rotational dynamics, which determines the temperature anisotropy and the ensuing radiation recoil effects. We propose a model of the torques acting on LAGEOS due to eddy currents and gravity gradient. The electromotive forces induced in the spacecraft by its rotation in the magnetic field of the Earth dissipate angular momentum and produce a precession of the spin axis; the oblate spacecraft will precess in the gravitational field of the Earth at a rate proportional to the rotation period. Therefore the gravitational torques become more and more important with time and eventually may produce a chaotic dynamics. The predicted evolution of the spin period agrees very well with the few experimental data available and corresponds to an approximately exponential growth rate of about 3 years.

Stability and control of electrodynamic tethers for de-orbiting applications

Abstract Electrodynamic tethers provide a very promising propulsion system for de-orbiting of spent upper stages or LEO satellites. In this application, the Lorentz force generated by the interaction between the current in the wire and the geomagnetic field produces an electrodynamic drag leading to a fast orbital decay. The attractiveness of tether system lies especially in their capability to operate with uncontrollable satellites and in the modest mass requirement. The need for significant along-track forces leads however to the onset of an undesirable torque which, if not controlled, may drive the system into a dangerous instability. The electrodynamic torque determines in-plane and out-of-plane librations whose amplitude depends upon the current in the wire, mass distribution and system dimensions. Even more important, this torque is modulated along the orbit due to the changing magnetic field and ionospheric plasma density, giving rise to forced oscillations. The counteracting (and stabilizing) gravity-gradient torque is generally to small to ensure stability in typical, strongly non-symmetrical mass distributions, where a massive satellite or upper stage is attached at the lower end and a light electron collecting device (or passive ballast mass) is deployed a few kilometers above. Reducing the electron current or increasing the mass at the upper end are both unattractive solutions. In this paper we show how the electrodynamic torque pumps energy into the system (finally leading to large librations angles) and indicate that many proposed configurations are intrinsically unstable. Our results point out the need for a control strategy. Fortunately, the librations amplitudes can be limited by acting on the current flowing in the wire. Our model of a rigid, conductive tether shows that a control based upon timely current switch-off, using energy criteria, is indeed effective and simple to implement. The resultant duty-cycles are satisfactory and affect only marginally the de-orbiting times.

Quantum tests of the Einstein Equivalence Principle with the STE-QUEST space mission

We present in detail the scientific objectives in fundamental physics of the Space-Time Explorer and QUantum Equivalence Space Test (STE-QUEST) space mission. STE-QUEST was pre-selected by the European Space Agency together with four other missions for the cosmic vision M3 launch opportunity planned around 2024. It carries out tests of different aspects of the Einstein Equivalence Principle using atomic clocks, matter wave interferometry and long distance time/frequency links, providing fascinating science at the interface between quantum mechanics and gravitation that cannot be achieved, at that level of precision, in ground experiments. We especially emphasize the specific strong interest of performing equivalence principle tests in the quantum regime, i.e. using quantum atomic wave interferometry. Although STE-QUEST was finally not selected in early 2014 because of budgetary and technological reasons, its science case was very highly rated. Our aim is to expose that science to a large audience in order to allow future projects and proposals to take advantage of the STE-QUEST experience.

Advanced radio science instrumentation for the mission BepiColombo to Mercury

Abstract Radio science experiments of BepiColombo will provide a detailed mapping of Mercurys gravity field and important information about its deep internal structure. The global orbital solutions, obtained from precise radio metric data, entail also very accurate tests of General Relativity and other metric theories of gravity. The classical tests of the solar gravitational deflection and the precession of perihelion could improve the measurement of the post-Newtonian parameters β and γ by 2–3 orders of magnitude, to a value in the range 10−6–10−5. At these levels, violations of General Relativity due to scalar fields, remnant of the inflation age, could occur. In order to achieve the scientific objectives in geophysics and fundamental physics, a suitable radio frequency instrumentation both for onboard and ground equipment is needed. The target two-way accuracy is 20– 30 cm for range and 3×10 −4 cm / s for range rate (at 1000– 10,000 s integration time). This precision requires the capability of transmitting and receiving at multiple frequencies (to reduce plasma noise) and larger modulation bandwidths for improved ranging performances. We propose an architecture of the onboard and ground radio frequency subsystems which combines minimization of mass and power, technological feasibility, and adequate phase stability and ranging accuracy.