Neutral buoyancy test evaluation of hardware and extravehicular activity procedures for on-orbit assembly of a 14 meter precision reflector

A procedure that enables astronauts in extravehicular activity (EVA) to perform efficient on-orbit assembly of large paraboloidal precision reflectors is presented. The procedure and associated hardware are verified in simulated Og (neutral buoyancy) assembly tests of a 14 m diameter precision reflector mockup. The test article represents a precision reflector having a reflective surface which is segmented into 37 individual panels. The panels are supported on a doubly curved tetrahedral truss consisting of 315 struts. The entire truss and seven reflector panels were assembled in three hours and seven minutes by two pressure-suited test subjects. The average time to attach a panel was two minutes and three seconds. These efficient assembly times were achieved because all hardware and assembly procedures were designed to be compatible with EVA assembly capabilities

Results of the ACCESS experiment

All basic EVA space construction tasks included in the experiment were accomplished on-orbit successfully, and the construction task time shows good correlation with neutral buoyancy data. However, the flight assembly times were slightly longer than the best times obtained in the water tank. This result was attributed by the EVA astronauts to the new, tighter tolerance truss hardware used on-orbit as opposed to the well-worn training hardware used in the neutral buoyancy and was, thus, not a space related phenomenon. The baseline experiment demonstrated that erectable structure can be assembled effectively by astronauts in EVA. The success of ACCESS confirmed the feasibility of EVA space assembly of erectable trusses and played a role in the decision to baseline the Space Station as a 5 meter erectable structure

A 60-meter erectable assembly concept for a control of flexible structures flight experiment

A flight experiment which proposes to use a 60-m deployable/retractable truss beam attached to the Space Shuttle to study dynamic characterization and control of flexible structures is being studied by NASA. The concept requires a relatively complex mechanism for deploying and retracting the truss on-orbit. Development of such a mechanism having a high degree of reliability will be expensive. An alternative method for constructing the truss is discussed requiring no new technology development or complex mechanisms and has already been demonstrated on-orbit. The alternative method proposes an erectable truss beam which can be assembled by two astronauts in EVA. The EVA crew would have to manually assemble the beam from 468 struts and 165 nodes, and install 7 instrumentation platforms with signal and power cabling. The predicted assembly time is 3 hr and 23 min. The structure would also have to be disassembled and restowed following testing, thus 2 EVA days would be required. To allow 25 hr for data collection (probably a bare minimum to accomplish meaningful tests), current Shuttle operations policy dictates a 9-day mission. The design, assembly procedure and issues associated with the alternative concept are discussed

The versatility of a truss mounted mobile transporter for in-space construction

The Mobile Transporter (MT) evolution from early erectable structures assembly activities is detailed. The MT operational features which are required to support astronauts performing on-orbit structure construction or spacecraft assembly functions are presented and discussed. Use of the MT to perform a variety of assembly functions is presented. Estimated EVA assembly times for a precision segmented reflector approximately 20 m in diameter are presented. The EVA/MT technique under study for construction of the reflector (and the entire spacecraft) is illustrated. Finally, the current status of development activities and test results involving the MT and Space Station structural assembly are presented

Tests of an alternate mobile transporter and extravehicular activity assembly procedure for the Space Station Freedom truss

Results are presented from a ground test program of an alternate mobile transporter (MT) concept and extravehicular activity (EVA) assembly procedure for the Space Station Freedom (SSF) truss keel. A three-bay orthogonal tetrahedral truss beam consisting of 44 2-in-diameter struts and 16 nodes was assembled repeatedly in neutral buoyancy by pairs of pressure-suited test subjects working from astronaut positioning devices (APD's) on the MT. The truss bays were cubic with edges 15 ft long. All the truss joint hardware was found to be EVA compatible. The average unit assembly time for a single pair of experienced test subjects was 27.6 sec/strut, which is about half the time derived from other SSF truss assembly tests. A concept for integration of utility trays during truss assembly is introduced and demonstrated in the assembly tests. The concept, which requires minimal EVA handling of the trays, is shown to have little impact on overall assembly time. The results of these tests indicate that by using an MT equipped with APD's, rapid EVA assembly of a space station-size truss structure can be expected

Packaging, deployment, and panel design concepts for a truss-stiffened 7-panel precision deployable reflector with feed boom

A concept is presented for achieving a remotely deployable truss-stiffened reflector consisting of seven integrated sandwich panels that form the reflective surface, and an integrated feed boom. The concept has potential for meeting aperture size and surface precision requirements for some high-frequency microwave remote sensing applications. The packaged reflector/feed boom configuration is a self-contained unit that can be conveniently attached to a spacecraft bus. The package has a cylindrical envelope compatible with typical launch vehicle shrouds. Dynamic behavior of a deployed configuration having a 216-inch focal length and consisting of 80-inch-diameter, two-inch-thick panels is examined through finite-element analysis. Results show that the feed boom and spacecraft bus can have a large impact on the fundamental frequency of the deployed configuration. Two candidate rib-stiffened sandwich panel configurations for this application are described, and analytical results for panel mass and stiffness are presented. Results show that the addition of only a few rib stiffeners, if sufficiently deep, can efficiently improve sandwich panel stiffness

Extravehicular activity compatibility evaluation of developmental hardware for assembly and repair of precision reflectors

This report presents results of tests performed in neutral buoyancy by two pressure-suited test subjects to simulate Extravehicular Activity (EVA) tasks associated with the on-orbit construction and repair of a precision reflector spacecraft. Two complete neutral buoyancy assemblies of the test article (tetrahedral truss with three attached reflector panels) were performed. Truss joint hardware, two different panel attachment hardware concepts, and a panel replacement tool were evaluated. The test subjects found the operation and size of the truss joint hardware to be acceptable. Both panel attachment concepts were found to be EVA compatible, although one concept was judged by the test subjects to be considerably easier to operate. The average time to install a panel from a position within arm's reach of the test subjects was 1 min 14 sec. The panel replacement tool was used successfully to demonstrate the removal and replacement of a damaged reflector panel in 10 min 25 sec

Results of EVA/mobile transporter space station truss assembly tests

Underwater neutral buoyance tests were conducted to evaluate the use of a Mobile Transporter concept in conjunction with EVA astronauts to construct the Space Station Freedom truss structure. A three-bay orthogonal tetrahedral truss configuration with a 15 foot square cross section was repeatedly assembled by a single pair of pressure suited test subjects working from the Mobile Transporter astronaut positioning devices (mobile foot restraints). The average unit assembly time (which included integrated installation of utility trays) was 27.6 s/strut, or 6 min/bay. The results of these tests indicate that EVA assembly of space station size structures can be significantly enhanced when using a Mobile Transporter equipped with astronaut positioning devices. Rapid assembly time can be expected and are dependent primarily on the rate of translation permissible for on-orbit operations. The concept used to demonstate integrated installation of utility trays requires minimal EVA handling and consequentially, as the results show, has little impact on overall assembly time

Three-dimensional super-resolution microscopy of the inactive X chromosome territory reveals a collapse of its active nuclear compartment harboring distinct Xist RNA foci

Background: A Xist RNA decorated Barr body is the structural hallmark of the compacted inactive X territory in female mammals. Using super resolution three-dimensional structured illumination microscopy (3D-SIM) and quantitative image analysis, we compared its ultrastructure with active chromosome territories (CTs) in human and mouse somatic cells, and explored the spatio-temporal process of Barr body formation at onset of inactivation in early differentiating mouse embryonic stem cells (ESCs). Results: We demonstrate that all CTs are composed of structurally linked chromatin domain clusters (CDCs). In active CTs the periphery of CDCs harbors low-density chromatin enriched with transcriptionally competent markers, called the perichromatin region (PR). The PR borders on a contiguous channel system, the interchromatin compartment (IC), which starts at nuclear pores and pervades CTs. We propose that the PR and macromolecular complexes in IC channels together form the transcriptionally permissive active nuclear compartment (ANC). The Barr body differs from active CTs by a partially collapsed ANC with CDCs coming significantly closer together, although a rudimentary IC channel system connected to nuclear pores is maintained. Distinct Xist RNA foci, closely adjacent to the nuclear matrix scaffold attachment factor-A (SAF-A) localize throughout Xi along the rudimentary ANC. In early differentiating ESCs initial Xist RNA spreading precedes Barr body formation, which occurs concurrent with the subsequent exclusion of RNA polymerase II (RNAP II). Induction of a transgenic autosomal Xist RNA in a male ESC triggers the formation of an `autosomal Barr body' with less compacted chromatin and incomplete RNAP II exclusion. Conclusions: 3D-SIM provides experimental evidence for profound differences between the functional architecture of transcriptionally active CTs and the Barr body. Basic structural features of CT organization such as CDCs and IC channels are however still recognized, arguing against a uniform compaction of the Barr body at the nucleosome level. The localization of distinct Xist RNA foci at boundaries of the rudimentary ANC may be considered as snap-shots of a dynamic interaction with silenced genes. Enrichment of SAF-A within Xi territories and its close spatial association with Xist RNA suggests their cooperative function for structural organization of Xi

Activation of Estrogen-Responsive Genes Does Not Require Their Nuclear Co-Localization

The spatial organization of the genome in the nucleus plays a role in the regulation of gene expression. Whether co-regulated genes are subject to coordinated repositioning to a shared nuclear space is a matter of considerable interest and debate. We investigated the nuclear organization of estrogen receptor alpha (ERα) target genes in human breast epithelial and cancer cell lines, before and after transcriptional activation induced with estradiol. We find that, contrary to another report, the ERα target genes TFF1 and GREB1 are distributed in the nucleoplasm with no particular relationship to each other. The nuclear separation between these genes, as well as between the ERα target genes PGR and CTSD, was unchanged by hormone addition and transcriptional activation with no evidence for co-localization between alleles. Similarly, while the volume occupied by the chromosomes increased, the relative nuclear position of the respective chromosome territories was unaffected by hormone addition. Our results demonstrate that estradiol-induced ERα target genes are not required to co-localize in the nucleus