Agenzia Spaziale Italiana Research Articles

Proposed noncryogenic, nondrag-free test of the equivalence principle in space

Ever since Galileo scientists have known that all bodies fall with the same acceleration regardless of their mass and composition. Known as the Universality of Free Fall, this is the most direct experimental evidence of the Weak Equivalence Principle, a founding pillar of General Relativity according to which the gravitational (passive) mass m g and the inertial mass m i are always in the same positive ratio in all test bodies. A space experiment offers two main advantages: a signal about a factor of a thousand bigger than on Earth and the absence of weight. A new space mission named GALILEO GALILEI (GG) has been proposed (Nobili et al., 1995 [J. Astronautical Sciences, 43, 219]; GALILEO GALILEI (GG), PRE PHASE A REPORT, ASI (Agenzia Spaziale Italiana), September 1996) aimed at testing the weak Equivalence Principle (EP) to 1 part in 10 17 in a rapidly spinning (5 Hz) drag-free spacecraft at room temperature, the most recent ground experiments having reached the level of 10 −12 (Adelberger et al., 1990 [PhRvD, 42, 3267]; Su et al., 1994 [PhRvD, 50, 3614]). Here we present a nondrag-free version of GG which could reach a sensitivity of 1 part in 10 16. The main feature of GG is that, similarly to the most recent ground experiments, the expected (low frequency) signal is modulated at higher frequency by spinning the system, in this case by rotating the test bodies (in the shape of hollow cylinders) around their symmetry axes, the signal being in the perpendicular plane. They are mechanically suspended inside the spacecraft and have very low frequencies of natural oscillation (due to the weakness of the springs that can be used because of weightlessness) so as to allow self-centering of the axes; vibrational noise around the spin/signal frequency is attenuated by means of mechanical suspensions. The signal of an EP violation would appear at the spin frequency as a relative ( differential) displacement of the test masses perpendicularly to the spin axis, and be detected by capacitance sensors; thermal stability across the test masses and for the required integration time is obtained passively thanks to both the fast spin and the cylindrical symmetry. In the nondrag-free version the entire effect of atmospheric drag is retained, but a very accurate balancing of the test bodies must be ensured (through a coupled suspension) so as to reach a high level of Common Mode Rejection and reduce the differential effects of drag below the target sensitivity. In so doing the complexities of a drag-free spacecraft are avoided by putting more stringent requirements on the experiment. The spacecraft must have a high area-to-mass ratio in order to reduce the effects of nongravitational forces; it is therefore a natural choice to have three pairs of test masses (in three experimental chambers) rather than one as by Nobili et al. (1995) [J. Astronautical Sciences, 43, 219] and the mission called GALILEO GALILEI [PRE PHASE A REPORT, ASI (Agenzia Spaziale Italiana), September 1996]. The GG setup is specifically designed for space; however, a significant EP test on the ground is possible — because the signal is in the transverse plane — by exploiting the horizontal component of the gravitational and the centrifugal field of the Earth. This ground test is underway.

`Galileo Galilei' flight experiment on the equivalence principle with field emission electric propulsion

An experiment to test the equivalence of inertial to gravitational (passive) mass in space offers two main advantages: a signal about a factor of a thousand larger than on Earth and the possibility of exploiting the absence of weight. `Galileo Galilei' (GG) is a small satellite mission currently under study in Italy with the financial support of ASI (Agenzia Spaziale Italiana). The mission concerns a small, low Earth satellite ( total mass, altitude) with two objectives. One is scientific, in the field of fundamental physics, and the other technological within the framework of spacecraft propulsion and drag compensation. The scientific goal is to test the equivalence principle to one part in , four orders of magnitude better than the best ground results. The technological goal is a full, comprehensive test of FEEP (field emission electric propulsion) thrusters for accurate drag compensation, a technology developed in Europe by the ESA (European Space Agency) which is likely to become an essential component of all space experiments which require measurement of small forces. The GG experiment is based on novel concepts and does not require low temperatures.

Burned

After three months of investigation, NASA and the Agenzia Spaziale Italiana (ASI) have concluded in a report that the tether connecting the TSS‐1R satellite to the space shuttle Columbia broke because unexpected electrical arcing severed it. Presenting its findings on June 4, the independent review panel led by Kenneth Szalai of NASA's Dryden Flight Research Center noted that something punctured the tether and allowed electrical current to breach the insulation and jump to a nearby electrical ground.

Mini-pressurized logistics module for space station freedom

The Italian Government, acting through Agenzia Spaziale Italiana, has selected Alenia Spazio to develop a small logistics module, the Mini-Pressurized Logistics Module (MPLM). This module is to be the only carrier for pressurized cargo during the Man Tended Phase of Space Station Freedom. Once permanent manned capability is achieved, the MPLM will continue to be used to supplement the capability of the other Freedom logistics elements. This paper describes the development of MPLM requirements. Design implementation is discussed including the structure, the subsystems, the major trade studies and the operational strategy for use of the module. Unique aspects of the program such as the management structure, hardware commonality, accommodation of national priorities, verification and integration and support of the hardware are presented. Due to late authorization of this program, schedule has become a prime program driver. Methods to accommodate and meet these requirements are discussed. Boeing, the National Aeronautics and Space Administration Work Package One contractor, has been designated by Marshall Space Flight Center to serve as NASA's representative in providing requirements for MPLM design and oversight of its development and utilization. This unique management arrangement has been implemented and is working effectively.

Methodology and results of a space station education pilot programme in the primary school

Potential users of the Space Station Freedom are now still in the Primary School. Subject studies 1 have shown that a robust familiarization programme has to be developed in order to increase public awareness on the microgravity environment and its capabilities to perform unique science. At the same time, several surveys 2 have demonstrated that elementary school students are showing the greatest interest and enthusiasm in space related activities among all school students. With these boundary conditions, a pilot programme, aimed at verifying the capabilities of young primary school pupils (aged between 10 and 12) in understanding why one performs research in space, has been conceived. In order to overcome the lack of space training of school teachers, an expert in space operations joined a group of elementary teachers to activate this program: merging the necessary didactic and technical capabilities. Consequently, the aim of the program becomes two folded: • •generate critical thinking and problem solving capacities as well as inventiveness in children making them aware on the use of space to improve life on Earth. • •identify the key issues for the definition of a robust space utilization educational programme. The programme has been managed by MARS Center. the Italian User Support Center for the Space Station utilization, and the institute “Speranzas” in the nearby of Naples, Italy. MARS Center, in particular, is responsible towards the national agency ASI, Agenzia Spaziale Italiana, of the execution of the promotional activity towards all the possible target groups: young students are among these groups. This programme started in late 1992 and is currently ongoing. The objective of this paper is to provide a description of the methodology and the reasons of such a programme with a snapshot on the preliminary results and future trends. Means used as supporting tools, such as films, posters and role plays are herein depicted as well as statistics on the pupils apprehension level.

Observations of ionosphere heating in the TSS‐1 subsatellite presheath

The first flight of the Tethered Satellite System (TSS‐1) was to investigate the mechanical and electrical dynamics of a conducting satellite deployed from the orbiter via a tether whose core was a conducting wire [Dobrowolny and Melchioni, 1993; Dobrowolny, 1987; Dobrowolny and Stone, 1994). In the TSS‐1 system the satellite deployed from the orbiter radially away from the Earth. The relative motion between the tether and Earth's magnetic field generated an electromotive force (EMF) that is the product of orbiter velocity, Earth's magnetic field, and the length of the deployed tether. This EMF drove a current through the tether. Electrons were collected on the satellite's electrically conductive skin and traveled through the tether to the orbiter, where they either went to orbiter structural ground or were emitted into the ionosphere via active electron emission. During TSS‐1 this electron emission was accomplished mainly by the 100 mA, 1‐keV fast pulsed electron gun (FPEG) of the shuttle electrodynamic tether system (SETS) (Williamson et al., 1988; Banks et al., 1994; V. M. Aguero et al., manuscript in preparation, 1994]. The FPEG electron emission was much higher than either ambient ion collection at the orbiter end or electron collection at the satellite. Potentials of the orbiter with respect to the ambient plasma were obtained from measurements from the Shuttle Potential and Return Electron Experiment (SPREE) (Oberhardt et al., 1993a, b, 1994), the SETS tether current voltage monitor (Thompson et al., 1993), and the Agenzia Spaziale Italiana deployer and satellite core equipment (Bonifazi et al., 1988, 1994). Despite the limited tether deployment length of 268 m the TSS‐1 system proved capable, during certain events, of generating satellite potentials sufficient to illuminate a previously unexplored aspect of plasma physics: that of an ion repelling, electron attracting, moving probe in a magnetoplasma. During such events the satellite boom‐mounted Langmuir probe flown as part of the Research on Electrodynamic Tethers experiment (Dobrowolny et al., 1994) measured an increase in the electron plasma temperature in the quasi‐neutral ionospheric region beyond the satellite sheath. This observed heating of the presheath electrons was distinctly different from the acceleration of the electrons in the sheath, which was also observed when the sheath expanded such that the probe was completely in the sheath. We show that the observed elevated electron temperatures are consistent with the formation of a Bohm stable electron collecting sheath.

Space: Growing pains

The Italian space budget increased by an order of magnitude during the 1980s. In 1980 it was a little over $100m; by 1990 it had swollen to almost $1000m (L1200bn), making Italy Europe's third largest space power, behind France and Germany. But this impressive growth, and plans for a near doubling to $1.8bn by 1994, conceal a range of problems. This year's budget for the Italian space agency (Agenzia Spaziale Italiana; ASI) will be L50bn less than last year's L850bn and the ASI Science Committee is conducting a bitter and very public squabble about spending on basic research. Italy is also finding it difficult to receive an adequate industrial return on its substantial investment (about L570bn last year) in the European Space Agency (ESA).

The cooperation betweenNASA andASI on the Cassini mission

The updated status of the cooperation between NASA and ASI on the Cassini mission is here presented. This outstanding scientific mission is devoted to study the Saturnian system with a NASA orbiter and a ESA probe. The Italian involvement, managed by the Agenzia Spaziale Italiana (ASI), concerns the design and development of important Cassini Orbiter subsystems. Promotion of the Italian science community’s interests and of the national industry’s technological capabilities are the rationales for the cooperation.

An introduction to the Cassini-Huygens mission

Cassini-Huygens, a large mission sponsored by NASA and ESA, will fully explore the Saturnian system at the beginning of next century. In view of its great scientific interest and the involvement of the European Space Agency and the Agenzia Spaziale Italiana, Il Nuovo Cimento is publishing five papers dealing with the main features of the mission. This is a general introduction, with some attention to the planning of interplanetary space mission.

A Re-Entry Capsule Control System Design for Microgravity Experiments

Abstract CARINA is a new retrievable unmanned capsule designed by ALENIA SPAZIO under ASI (Agenzia Spaziale Italiana) contract. This paper deals with the operative attitude acquisition phase and the operative phase of its life cycle. In the operative phase the control system must assure microgravity requirements. Two different type of actuators are used for the two different phases: 20N hydrazine thrusters for the first, magnetic coils and a reaction wheel for the second, and, consequently, two control systems techniques are used. The former is based on a new discrete time modeling of the system due to the PWM-like behaviour of the thrusters and results in a discrete controller able to guarantee robustness against parameter uncertainties. The latter is a scheduling technique based on a LOR approach and guarantees the fulfilling of microgravity requirements. All the simulations, performed on the ESA-MIDAS dynamics simulator, take into account atmospheric drag, solar pressure and gravity gradient disturbances, and show the effectiveness of the adopted approach.