Walter Hohmann's contributions towards space flight: an appreciation on the occasion of the centenary of his birthday

In 1923 Hermann Oberth published his book “Die Rakete zu den Planetenräumen” (The Rocket into Planetary Space), in 1924 Max Valier's book “Der Vorstoss in den Weltenraum” (The Advance into Space) appeared while in the U.S.A. already in 1919 Robert H. Goddard reported on his rocket experiments. Altogether different from the publications just mentioned was a book entitled “Die Erreichbarkeit der Himmelskörper” (The Attainability of Celestial Bodies) published in 1925. Its author was Dr.-Ing. Walter Hohmann, born 18 March 1880, civil engineer for the city authorities of Essen, who had already made, during World War I, calculations as to the amount of fuel, initial mass and flight time necessary for flights from the Earth to other planets. The transfer trajectories investigated by Hohmann and today attributed with his name have a great practical significance for space flight onto the present. In the lecture a critical appreciation of Hohmann's work is given.     

Sailing with solar and planetary radiation pressure

Literature on solar sailing has thus far mostly considered solar radiation pressure (SRP) as the only contribution to sail force. However, considering a sail in a planetary mission scenario, a new contribution can be added. Since the planet itself emits radiation, this generates a radial planetary radiation pressure (PRP) that is also exerted on the sail. Hence, this work studies the combined effects of both SRP and PRP on a sail for two case studies, i.e. Earth and Venus. In proximity of the Earth, the effect of PRP can be significant under specific conditions. Around Venus, instead, PRP is by far the dominating contribution. These combined effects have been studied for single- and double-sided reflective coating and including eclipse. Results show potential increase in the net acceleration and a change in the optimal attitude to maximise the acceleration in a given direction. Moreover, an increasing semi-major axis manoeuvre is shown with and without PRP, to quantify the difference on a real-case scenario.     

Rosetta Goes to Comet Wirtanen

The International Rosetta Mission, approved by the Science Programme Committee of the European Space Agency as the Planetary Cornerstone Mission in ESA's long-term programme Horizon 2000, will rendezvous in 2011 with Comet 46P/Wirtanen close to its aphelion and will study the nucleus and the evolution of the coma for almost two years until it reaches perihelion. In addition to the investigations performed by the scientific instruments on board the orbiter, a Surface Science Package (Rosetta Lander) will be deployed onto the surface of the nucleus early during the near-nucleus study phase. On its way to Comet 46P/Wirtanen, Rosetta will fly by and study the two asteroids 4979 Otawara and 140 Siwa. This revised version was published online in June 2006 with corrections to the Cover Date.     

Concurrent image-based visual servoing with adaptive zooming for non-cooperative rendezvous maneuvers

An image-based servo controller for the guidance of a spacecraft during non-cooperative rendezvous is presented in this paper. The controller directly utilizes the visual features from image frames of a target spacecraft for computing both attitude and orbital maneuvers concurrently. The utilization of adaptive optics, such as zooming cameras, is also addressed through developing an invariant-image servo controller. The controller allows for performing rendezvous maneuvers independently from the adjustments of the camera focal length, improving the performance and versatility of maneuvers. The stability of the proposed control scheme is proven analytically in the invariant space, and its viability is explored through numerical simulations.     

A mathematical and experimental simulation of the hematological response to weightlessness

Two ground-based methods of weightlessness simulation--a computer model of erythropoiesis feedback regulation and bedrest--were used to investigate the mechanisms which lead to loss of red cell mass during spaceflight. Both methods were used to simulate the first Skylab mission of 28 days. Human bedrest subjects lose red cell mass linearly with time and in this study the loss was 6.7% at the end of four weeks (compared to 14% in Skylab). Postbedrest recovery of red cell mass was delayed for two weeks during which time a further decline in this quantity was noted. This is consistent with the first Skylab mission but not with the two longer flights of two and three months. Hemoconcentration, observed early in the study, was essentially maintained despite red cell loss because of continued loss of plasma volume. The computer model, using the time-varying hematocrit data to estimate red cell production rates, predicted dynamic behavior of plasma volume and red cell mass that was in close agreement with the measured values. The results support the hypothesis that red cell loss during supine bedrest is a normal physiological feedback process in response to hemoconcentration enhanced tissue oxygenation and suppression of red cell production. In contrast, the delayed postbedrest recovery of red cell mass was more difficult to explain, especially in the light of enhanced reticulocyte indices observed at the onset on ambulation. Model simulation suggested the possibilities, still to be experimentally demonstrated, that this period was marked by some combination of increased oxygen-hemoglobin affinity, small reductions in mean red cell life span, ineffective erythropoiesis, or abnormal reticulocytosis. The question of whether hemoconcentration is the sole contributor to spaceflight red cell losses also remains to be resolved.     

Steering the station back on course

Rising costs of the International Space Station prompted NASA to convene a panel of experts to assess the quality of ISS cost estimates and review program assumptions and requirements. The panel concluded that NASA was unable to accurately predict ISS costs or to support requests for increased funding for the ISS through 2006 and should maintain a U.S. core complete program with three-person crews with 6-month stays on the ISS. International response to the panel report was negative with space agencies from Japan, Russia, Canada, and Europe taking issue with the expected impact on ISS construction and use.     

Molecular aspects of adaptation to extreme cold environments

Here are reviewed and summarized the strategies adopted by living organisms to survive low temperatures, from a molecular and membrane point of view. The presentation is aimed at a wide variety of readers.

Two prime examples of connections between biological cold adaptation and the molecular level are (1) antifreeze proteins in fish from cold sea water, (the DNA sequence of the protein gene is now known) (2) the fluidity characteristics of cell membranes in a wide variety of organisms. In model membranes of phospholipids, stabler “s-phases” have recently been found to form at low temperatures. Antarctic endolithic organisms, living just under the surface of rocks, are exposed to long periods of low temperatures, and may develop such phases in their membranes. In the saturated phosphatidyl cholines, only lipids with a restricted range of acyl chain lengths show simultaneously s-phases and a main transition : This restricted range is about the restricted range found in natural membranes. The s-phases also form in the presence of natural cryoprotectants, and may be connected with botanical vernalization.     

Thermosphere density and wind measurements in the equatorial region using a constellation of drag balance nanospacecraft

A mission for in situ thermosphere density and winds measurement is described, based on nanospacecraft equipped with a drag balance instrument (DBI) and a GPS receiver. The mission is based on nanosatellite clusters deployed in three orbital planes. In this study, clusters of 10 nanospacecraft are considered, leading to a mission based on a total of 30 nanospacecraft. The geometry analyzed is a symmetrical one, including an equatorial orbit and two orbits with the same inclination and opposing ascending nodes. The main idea is that, by combining the accurate information on the satellite inertial position and velocity provided by the GPS receiver and the drag acceleration intensity provided by the DBI, due to the orbits’ geometrical configuration, both atmospheric drag and wind can be resolved in a region close to the orbit nodes. Exploiting the Earth oblateness effect, a complete scan of the equatorial regions can be accomplished in the short mission lifetime typical of very low Earth orbit satellites, even in high solar activity peaks, when the expected nanospacecraft lifetime is about 40 days.     

Drag balance Cubesat attitude motion effects on in-situ thermosphere density measurements

The dynamics of Cubesats carrying a drag balance instrument (DBI) for in situ atmosphere density measurements is analyzed. Atmospheric drag force is measured by the displacement of two light plates exposed to the incoming particle flow. This system is well suited for a distributed sensor network in orbit, to get simultaneous in situ local (non orbit averaged) measurements in multiple positions and orbit heights, contributing to the development and validation of global atmosphere models. The implementation of the DBI leads to orbit normal pointing spinning two body system. The use of a spin-magnetic attitude control system is suggested, based only on magnetometer readings, contributing to making the system simple, inexpensive, and reliable. It is shown, by an averaging technique, that this system provides for orbit normal spin axis pointing. The effect of the coupling between the attitude dynamics and the DBI is evaluated, analyzing its frequency content and showing that no frequency components arise, affecting the DBI performance. The analysis is confirmed by Monte Carlo numerical simulation results.