This paper assumes a requirement for an unmanned multiunit satellite to be assembled in orbit. The requirement to be met is to bring the satellites together so tha t they do not collide but actually rendezvous. The equations of motion of the rendezvous satellite in a relative coordinate system are derived and used to compute a final injection velocity which would effect collision after a time r. The velocity is corrected periodically by a command guidance system and just before impact retrothrust is applied. A terminal infrared homing sj^stem is required to actually accomplish physical contact and joining of the satellites. The first satellite placed in orbit is the "control satellite" and controls all the satellites to be assembled and contains the ccmputer, command guidance equipment, precision orientation equipment, and other features necessary to effect rendezvous. The succeeding satellites contain a propulsion system, a rough at t i tude control system, and a command receiver plus whatever scientific equipment they carry to perform their basic mission. This paper presents the following:

Terminal Guidance System for Satellite Rendezvous

https://icesat.gsfc.nasa.gov/icesat/publications/pubs/ICESatZwallyetalCor.pdf

ICESat's laser measurements of polar ice, atmosphere, ocean, and land

We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. Wc focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon satellite altimetry. thermal expansion is the dominant contribution to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rale of thermosteric rise during the 1990s. Topex/Posei don shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly related to ocean warming.

/pdf/present-day-sea-level-change-observations-and-causes-1kvgf9bs2o.pdf

Present‐day sea level change: Observations and causes

This paper concerns heteroclinic connections and resonance transitions in the planar circular
restricted 3-body problem, with applications to the dynamics of comets and asteroids and the design
of space missions such as the Genesis Discovery Mission and low energy Earth to Moon transfers.
The existence of a heteroclinic connection between pairs of equal energy periodic orbits around two of
the libration points is shown numerically. This is applied to resonance transition and the construction
of orbits with prescribed itineraries. Invariant manifold structures are relevant for transport between
the interior and exterior Hill’s regions, and other resonant phenomena throughout the solar system.

/pdf/dynamical-systems-the-three-body-problem-and-space-mission-48b7chyrqv.pdf

Dynamical Systems, the Three-Body Problem and Space Mission Design

Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year-approximately 7% of the observed rise.

/pdf/greenland-ice-sheet-high-elevation-balance-and-peripheral-3qmhjr5qcs.pdf

Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning

A new method for computing near-optimal, minimum-time Earth-orbit transfers for solar electric propulsion spacecrafthasbeen developed. Adirect optimization approach is utilized to solvetheoptimalcontrol problem. The optimalthrustdirection timehistoryiscomputedby obtainingtheoptimalblendofthreeextremalfeedbackcontrol laws. Thisapproach isapplied to long-duration orbit transfers, and theeffects of Earth shadowing, oblateness, and solarcell degradation areincluded in the simulation. The optimal trajectories computed using the direct approach exhibit a very close match (within 1%) with the corresponding optimal trajectories obtained with a calculus of variations approach. The direct method also exhibits robust convergence properties and is insensitive to the initial guess of the optimization variables.

Direct Approach for Computing Near-Optimal Low-Thrust Earth-Orbit Transfers

Solar electric propulsion technology is currently being used for geostationary satellite station keeping. Analyses show that electric propulsion technologies can be used to obtain additional increases in payload mass by using them to perform part of the orbit transfer. Three electric propulsion technologies are examined at two power levels for geostationary insertion of an Atlas IIAS class spacecraft. The onboard chemical propulsion apogee engine fuel is reduced in this analysis to allow the use of electric propulsion. A numerical optimizer is used to determine the chemical burns that will minimize the electric propulsion transfer times. For a 1550-kg Atlas IIAS class payload, increases in net mass (geostationary satellite mass less wet propulsion system mass) of 150-800 kg are enabled by using electric propulsion for station keeping, advanced chemical engines for part of the transfer, and electric propulsion for the remainder of the transfer. Trip times are between one and four months.

Advanced Propulsion for Geostationary Orbit Insertion and North-South Station Keeping

Minimum-fuel, three-dimensional trajectories from a circular low Earth parking orbit to an inclined circular low lunar parking orbit with a fixed thrust-coast-thrust engine sequence are computed for a low-thrust spacecraft. The problem is studied in the context of the classical restricted three-body problem. The minimum-fuel transfer to a polar lunar orbit is obtained by successively solving a sequence of fixed lunar inclination problems. Minimumfuel three-dimensional transfers to a polar lunar orbit with meridian plane constraints are also computed. The minimum-fuel trajectory problems are solved using a "hybrid" direct/indirect method. The hybrid method utilizes the costate time histories to parameterize the three-dimensional thrust steering angle time histories. Numerical results are presented for the optimal three-dimensional Earth-moon trajectories.

Optimal low-thrust three-dimensional Earth-moon trajectories

Seasat and Geosat satellite altimeter measurements for the Greenland ice sheet (south of 72 degreesN latitude) show that surface elevations above 2000 meters increased at an average rate of only 1 5 +/- 05 centimeters per year from 1978 to 1988 In contrast, elevation changes varied regionally from -15 to +18 centimeters per year, seasonally by +/-15 centimeters, and interannually by +/-8 centimeters The average growth rate is too small to determine if the Greenland ice sheet is undergoing a long-term change due to a warmer polar climate

https://science.sciencemag.org/content/sci/279/5359/2086.full.pdf

Elevation Change of the Southern Greenland Ice Sheet

Minimum-fuel, planar trajectories from a circular low Earth parking orbit to a circular low lunar parking orbit with a fixed thrust-coast-thrust engine sequence are computed for a low-thrust spacecraft. The problem is studied in the context of the classical restricted three-body problem. Since a low-thrust trajectory is a long-duration transfer with slowly developing spirals about Earth and the moon, the minimum-fuel Earth-moon trajectory is obtained by formulating and successively solving a hierarchy of three subproblems. This three-stage approach presents a systematic and effective method for solving the complex and numerically sensitive minimum-fuel, lowthrust, trajectory problem. The first subproblem is to obtain several optimal continuous-thrust Earth-escape and moon-capture trajectories. The second subproblem is to compute a suboptimal, all-coasting, translunar trajectory between boundary conditions provided by the first subproblem. Finally, the complete minimum-fuel trajectory problem is solved using a "hybrid" direct/indirect method. The hybrid method utilizes the costate time histories to parameterize the thrust steering history. Numerical results are presented for the optimal Earth-moon trajectories.

Craig A. Kluever

Papers

Direct Approach for Computing Near-Optimal Low-Thrust Earth-Orbit Transfers

Advanced Propulsion for Geostationary Orbit Insertion and North-South Station Keeping

Optimal low-thrust three-dimensional Earth-moon trajectories

Elevation Change of the Southern Greenland Ice Sheet

Three-stage approach to optimal low-thrust Earth-moon trajectories