Clouds and Hazes of Venus

More than three decades have passed since the publication of the last review of the Venus clouds and hazes. The paper published in 1983 in the Venus book summarized the discoveries and findings of the US Pioneer Venus and a series of Soviet Venera spacecraft (Esposito et al. in Venus, p. 484, 1983). Due to the emphasis on in-situ investigations from descent probes, those missions established the basic features of the Venus cloud system, its vertical structure, composition and microphysical properties. Since then, significant progress in understanding of the Venus clouds has been achieved due to exploitation of new observation techniques onboard Galileo and Messenger flyby spacecraft and Venus Express and Akatsuki orbiters. They included detailed investigation of the mesospheric hazes in solar and stellar occultation geometry applied in the broad spectral range from UV to thermal IR. Imaging spectroscopy in the near-IR transparency “windows” on the night side opened a new and very effective way of sounding the deep atmosphere. This technique together with near-simultaneous UV imaging enabled comprehensive study of the cloud morphology from the cloud top to its deep layers. Venus Express operated from April 2006 until December 2014 and provided a continuous data set characterizing Venus clouds and hazes over a time span of almost 14 Venus years thus enabling a detailed study of temporal and spatial variability. The polar orbit of Venus Express allowed complete latitudinal coverage. These studies are being complemented by JAXA Akatsuki orbiter that began observations in May 2016. This paper reviews the current status of our knowledge of the Venus cloud system focusing mainly on the results acquired after the Venera, Pioneer Venus and Vega missions.     

J.A. Pierce: pioneer award winner, 1961

A biography of John Alvin Pierce is given, starting with his early interest in radio. Pierce received his amateur license and operated a spark transmitter at the age of twelve. His amateur activities led to work at the Cruft Laboratory of Harvard University under the supervision of Professor H.R. Mimno. This association with the Cruft Laboratory has continued to the present day with few interruptions. Pierce's work with different groups and in various areas is sketched, focusing on his work with Loran     

The exploration of Venus

This paper is an introduction to a special issue ofSpace Science Reviews dedicated to the exploration of Venus and the role played by the Pioneer Venus program. The Pioneer Venus program consists of a Multiprobe and Orbiter mission, both to be launched and to encounter Venus in 1978. The evolution of the program is traced from its conception in 1968 as the Goddard Space Flight Center Planetary Explorer Program through its transfer to Ames Research Center in 1971 as Pioneer Venus to the present.     

Synergistic approach of asteroid exploitation and planetary protection

The asteroid and cometary impact hazard has long been recognised as an important issue requiring risk assessment and contingency planning. At the same time asteroids have also been acknowledged as possible sources of raw materials for future large-scale space engineering ventures. This paper explores possible synergies between these two apparently opposed views; planetary protection and space resource exploitation. In particular, the paper assumes a 5 tonne low-thrust spacecraft as a baseline for asteroid deflection and capture (or resource transport) missions. The system is assumed to land on the asteroid and provide a continuous thrust able to modify the orbit of the asteroid according to the mission objective. The paper analyses the capability of such a near-term system to provide both planetary protection and asteroid resources to Earth. Results show that a 5 tonne spacecraft could provide a high level of protection for modest impact hazards: airburst and local damage events (caused by 15–170 m diameter objects). At the same time, the same spacecraft could also be used to transport to bound Earth orbits significant quantities of material through judicious use of orbital dynamics and passively safe aero-capture manoeuvres or low energy ballistic capture. As will be shown, a 5 tonne low-thrust spacecraft could potentially transport between 12 and 350 times its own mass of asteroid resources by means of ballistic capture or aero-capture trajectories that pose very low dynamical pressures on the object.     

Low-energy near Earth asteroid capture using Earth flybys and aerobraking

Since the Sun-Earth libration points L₁ and L₂ are regarded as ideal locations for space science missions and candidate gateways for future crewed interplanetary missions, capturing near-Earth asteroids (NEAs) around the Sun-Earth L₁/L₂ points has generated significant interest. Therefore, this paper proposes the concept of coupling together a flyby of the Earth and then capturing small NEAs onto Sun–Earth L₁/L₂ periodic orbits. In this capture strategy, the Sun-Earth circular restricted three-body problem (CRTBP) is used to calculate target Lypaunov orbits and their invariant manifolds. A periapsis map is then employed to determine the required perigee of the Earth flyby. Moreover, depending on the perigee distance of the flyby, Earth flybys with and without aerobraking are investigated to design a transfer trajectory capturing a small NEA from its initial orbit to the stable manifolds associated with Sun-Earth L₁/L₂ periodic orbits. Finally, a global optimization is carried out, based on a detailed design procedure for NEA capture using an Earth flyby. Results show that the NEA capture strategies using an Earth flyby with and without aerobraking both have the potential to be of lower cost in terms of energy requirements than a direct NEA capture strategy without the Earth flyby. Moreover, NEA capture with an Earth flyby also has the potential for a shorter flight time compared to the NEA capture strategy without the Earth flyby.     

Economic assessment of high-thrust and solar-sail propulsion for near-earth asteroid mining

Asteroid mining has the potential to greatly reduce the cost of in-space manufacturing, production of propellant for space transportation and consumables for crewed spacecraft, compared to launching the required resources from the Earth’s deep gravity well. This paper discusses the top-level mission architecture and trajectory design for these resource-return missions, comparing high-thrust trajectories with continuous low-thrust solar-sail trajectories. The paper focuses on maximizing the economic Net Present Value, which takes the time-cost of finance into account and therefore balances the returned resource mass and mission duration. The different propulsion methods are compared in terms of maximum economic return and sets of attainable target asteroids. Results for transporting resources to geostationary orbit show that the orbital parameter hyperspace of suitable target asteroids is considerably larger for solar sails, allowing for more flexibility in selecting potential target asteroids. Also, results show that the Net Present Value that can be realized is larger when employing solar sailing instead of chemical propulsion. In addition, it is demonstrated that a higher Net Present Value can be realized when transporting volatiles to the Lunar Gateway instead of geostationary orbit. The paper provides one more step towards making commercial asteroid mining an economically viable reality by integrating trajectory design, propulsion technology and economic modelling.     

Rarefied gas effects on the aerodynamics of high area-to-mass ratio spacecraft in orbit

The aerodynamic situation of a satellite-on-a-chip operating in low Earth orbit bears some resemblance to a classical Crookes radiometer. The large area-to-mass ratio characteristic of a SpaceChip means that very small surface-dependent forces produce non-negligible accelerations that can significantly alter its orbit. When the temperature of a SpaceChip changes, the drag force can be changed: if the temperature increases, the drag increases (and vice versa). Analytical expressions available in the literature that describe the change in drag coefficient with orbit altitude and SpaceChip temperature compare well with our direct simulation Monte Carlo results presented here. It is demonstrated that modifying the temperature of a SpaceChip could be used for relative orbit control of individual SpaceChips in a swarm, with a maximum change in position per orbit of 50 m being achievable at 600 km altitude.     

Solar sail science mission applications and advancement

Solar sailing has long been envisaged as an enabling or disruptive technology. The promise of open-ended missions allows consideration of radically new trajectories and the delivery of spacecraft to previously unreachable or unsustainable observation outposts. A mission catalogue is presented of an extensive range of potential solar sail applications, allowing identification of the key features of missions which are enabled, or significantly enhance, through solar sail propulsion. Through these considerations a solar sail application-pull technology development roadmap is established, using each mission as a technology stepping-stone to the next.