The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55 km, where temperatures and pressures are benign (∼25°C and ∼0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.

Planetary balloons have a long history already. A small super-pressure balloon was flown in the atmosphere of Venus in the eighties by the Russian-French VEGA mission. For this mission, CNES developed and fully tested a 9 m diameter super-pressure balloon, but finally replaced it by a smaller one due to mass constraints (when it was decided to send Vega to Halley’s Comet). Furthermore, several kinds of balloons have been proposed for planetary exploration [Blamont, J., in: Maran, S.P. (Ed.), The Astronomy and Astrophysics Encyclopedia. Cambridge University Press, p. 494, 1991]. A Mars balloon has been studied for the Mars-94 Russian-French mission, which was finally cancelled. Mars and Venus balloons have also been studied and ground tested at JPL, and a low atmosphere Venus balloon is presently under development at JAXA (the Japanese Space Agency). Balloons have been identified as a key element in an ongoing Flagship class mission study at NASA, with an assumed launch date between 2020 and 2025. Recently, it was proposed by a group of scientists, under European leadership, to use a balloon to characterize – by in-situ measurements – the evolution, composition and dynamics of the Venus atmosphere. This balloon is part of a mission called EVE (European Venus Explorer), which has been proposed in response to the ESA AO for the first slice of the Cosmic Vision program by a wide international consortium including Europe, Russia, Japan and USA. The EVE architecture consists of one balloon platform floating at an altitude of 50–60 km, one short lived probe provided by Russia, and an orbiter with a polar orbit to relay data from the balloon and probe, and to perform remote sensing science observations. The balloon type preferred for scientific goals is one, which would oscillate in altitude through the cloud deck. To achieve this flight profile, the balloon envelope would contain a phase change fluid. While this proposal was not selected for the first slice of Cosmic Vision missions, it was ranked first among the remaining concepts within the field of solar system science.

Abstract

The European Venus Explorer (EVE) mission was proposed to the European Space Agency in 2007, as an M-class mission under the Cosmic Vision Programme. Although it has not been chosen in the 2007 selection round for programmatic reasons, the EVE mission may serve as a useful reference point for future missions, so it is described here. It consists of one balloon platform floating at an altitude of 50–60 km, one descent probe provided by Russia, and an orbiter with a polar orbit which will relay data from the balloon and descent probe, and perform science observations. The balloon type preferred for scientific goals is one which oscillates in altitude through the cloud deck. To achieve this flight profile, the balloon envelope contains a phase change fluid, which results in a flight profile which oscillates in height. The nominal balloon lifetime is 7 days—enough for one full circumnavigation of the planet. The descent probe’s fall through the atmosphere takes 60 min, followed by 30 min of operation on the surface. The key measurement objectives of EVE are: (1) in situ measurement from the balloon of noble gas abundances and stable isotope ratios, to study the record of the evolution of Venus; (2) in situ balloon-borne measurement of cloud particle and gas composition, and their spatial variation, to understand the complex cloud-level chemistry; (3) in situ measurements of environmental parameters and winds (from tracking of the balloon) for one rotation around the planet, to understand atmospheric dynamics and radiative balance in this crucial region. The portfolio of key measurements is complemented by the Russian descent probe, which enables the investigation of the deep atmosphere and surface.

Up to now, the only means to land payloads on Mars have involved a heavy, complicated, expensive retro-rocket landing system. Another method to land payloads is to use a novel solar heated hot-air balloon, or Solar Montgolfiere, which looks promising to replace the retro-rocket landing system, while increasing usable landed payload.The Solar Montgolfiere is a simple device that deploys like a parachute, fills with ambient atmosphere, and then quickly heats up by solar heating. to achieve significant buoyancy within 1-2 min. The combined effect of the balloon buoyancy and the parachuting effect of the balloon results in landing speeds of < 5 m/s for the payload, as opposed to Pathfinder retro-rocket landing speeds of about 20 m/s.Using this balloon landing system enables a number of payloads with entry masses as large as 15 kg to be carried inexpensively to Mars on the Ariane 5 Piggyback launches. (C) 2002 Elsevier Science Ltd. All rights reserved.

Presentation of the new concept of maneuverable piggyback missions with possible trajectories from Geostationary Transfer Orbit to low Earth orbit, planetary flights and technology demonstrates.

Until now, the only practical balloon systems proposed to explore the martian atmosphere have been superpressure balloons, which fly at a constant altitude, or short-lived helium balloons, which precariously drag a snake through;all types of surface weather, or a day/night combination of the two. For the first time, two novel atmospheric balloon systems now appear quite viable for controlled balloon landings at selected martian surface locations. These balloons could softland payload packages, such as lightweight surface roving vehicles. The two balloon approaches and a land rover concept are described below, along with a combination of the two approaches.Solar Hot-Air Balloons: These "Montgolfiere" balloons are named after the 18th-century French brothers Joseph-Michel and Jacques-Etienne Mongolfier, who first flew hot-air balloons. Using entirely solar heat, they are ideal for landing at the martian poles during summer or for shorter flights at lower latitudes. Recent tests have already confirmed the ease of altitude deployment and filling of these solar hot-air balloons. Furthermore, actual landings and reascents of solar hot-air balloons have been recently demonstrated by JPL, using a novel, lightweight, top air vent that is radio controlled. One particularly useful application of these balloons is their use as a parachute to soft-land packages that are up to 50% of the total entry mass, which represents a fivefold improvement over present retrorocket landing-systems.Variable-Emissivity Balloons: A second atmospheric balloon system uses a variable-emissivity superpressure helium balloon that can land at night at any martian latitude. These balloons would be gold-coated, superpressure helium balloons during both night and day, They could land at prescribed targets by exposing a section of the upper white balloon surface to the radiant cooling of deep space during the night. This reduces the temperature and pressure in the balloon to create negative buoyancy, thus causing descent, while replacement of the gold top cover; causes reascent. Specific areas could be targeted for landings by using atmospheric currents at various altitudes, similar to techniques used by balloonists flying over the Earth.Inflatable Roving Vehicles: JPL has recently fabricated and tested a number of roving vehicles with large inflatable balloons that act as tires. One version, with 75-cm-diameter wheels, has already demonstrated the ability to make large traverses in JPL's simulated "Mars Yard." A full-scale version, with 1.5-m-diameter wheels, should be capable of climbing large rocks (less than or equal to 0.5 m), traveling reasonably fast (approximate to 500 m/h) and far (approximate to 10 km), and yet have very low mass (approximate to 6 kg).Low-Cost Combined Atmospheric/Surface Mission: A simple, solar hot-air balloon would act as a parachute to land a 6-kg inflatable rover. The balloon would then rise to a 3-km altitude while carrying a 2-kg camera/magnetometer/communications package for the remainder of daylight hours. The entire package would then soft-land at dusk. Total Mars entry mass would be about 20 kg, and the mission could be flown to Mars at very low cost (approximate to $5M total launch costs) via one of the CNES Ariane 5 GTO piggyback launches. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft.

It is proposed to base the scenario of the exploration of Mars on air-borne vehicles using solar power for mobility. A two-balloon option is presented. One of the balloons, closed, non superpressurized, filled with helium, provides buoyancy only for the weight of the balloons, not enough for the gondola. The other balloon is black, open, filled with martian gas. During the night, the gondola is on the ground. During the day, the black balloon, solar heated, takes the system to an altitude of 8 km and large distances can be covered during the flight. By venting the black balloon, the covered distance can be kept small, if wanted, and even, with the help of a guide rope, to “navigate” on the ground. The method can be used, not only for unique measurements on the ground, but also as a major ingredient for collecting on a global basis documented samples to be returned to Earth.

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere near Venus midnight by the VEGA spacecraft on 11 and 15 June 1985. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8,000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second; however, a much larger variation was evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature. Rapid variations in balloon velocity were often detected within a single transmission (330 seconds); however, they may represent not only atmospheric motions but also self-induced aerodynamic motions of the balloon.

The Vega balloons transmitted in situ measurements of pressure, temperature, vertical wind velocity relative to the balloon craft, cloud-particle backscatter coefficient, and ambient light level in the Venus middle cloud layer. Doppler tracking has yielded estimates for the velocities of atmospheric motion.

Presentation of the scientific objectives of balloon missions to Venus; description of the 1985 VEGA balloons; discussion of possible future missions as a function of technological constraints.

A compact Dopplergraph/magnetograph placed in a continuous solar-viewing orbit will allow us to make major advancements in our understanding of solar internal structure and dynamics. An international program is currently being conducted at JPL and Mt. Wilson to develop such an instrument. By combining a unique magneto-optical resonance filter with CID and CCD cameras we have been able to obtain full- and partial-disk Dopplergrams and magnetograms. Time series of the velocity images are converted into k-ω power spectra which show clear- the solar nonradial p-mode oscillations. Magnetograms suitable for studying the long-term evolution of solar active regions have also been obtained with this instrument. A flight instrument based on this concept is being studied for possible inclusion in the SOHO mission.

The discovery of large, solid particles in the clouds of Venus is one of the most significant findings of Pioneer Venus because it means that a substantial mass of the clouds is composed of a material other than sulfuric acid. The evidence which suggests that solid particles form a distinctive size mode is reexamined. The mode is defined by a discontinuity between two size ranges of the Pioneer Venus particle size spectrometer. This discontinuity could represent a real size mode. However, it could also be an artifact of the measurement technique. R. G. Knollenberg (1984) discusses several possible instrumental effects which might have caused this discontinuity. It is hypothesized herein that such effects did occur and that the large particles are really the tail of the mode 2 sulfuric acid particle size distribution and are not a separate mode of solid particles. Using such a revised size distribution, it is shown that all of the Pioneer Venus and Venera optical data from the lower clouds can be explained with sulfuric acid droplets without introducing any solid particles. As a by-product of this analysis, it is also found that the upper clouds of Venus must contain a material with a higher refractive index than sulfuric acid. A small quantity of sulfur could account for this observation.

The results of the nephelometer experiments conducted aboard the four probes of the Pioneer Venus mission are presented. The vertical structure of the clouds exhibits an upper haze region leading to three more or less clearly differentiated layers within the main cloud bank. Below the main cloud banks are variable series of strata followed by a lower atmosphere haze region with occasional larger concentrations of particulates at lower altitudes. The general structures, especially the middle cloud layers, are remarkably constant planetary features but do exhibit some local variation from one site to another. Concentrated sulfuric acid appears to be the principal constituent of most of the particulate matter in all of the cloud structure with the possible exceptions of the largest particles in the lower cloud layers, whose composition is still uncertain, and the hazes and particulate matter in the lower atmosphere. Near-UV radiation is absorbed throughout the cloud structure, primarily in the highest layers and, perhaps, below it. Absorbing species are not yet identified, but a combination of SO2 and an unspecified absorber look promising. Particles ejected from the surface of the planet on probe impact appear to be in the 10- to 100/m size range. The source of a faint glow detected by the night and north probes in the lower atmosphere is as yet uncertain.

The results of the nephelometer experiments conducted aboard the four probes of the Pioneer Venus mission are presented. The vertical structure of the clouds exhibits an upper haze region leading to three more or less clearly differentiated layers within the main cloud bank. Below the main cloud banks are variable series of strata followed by a lower atmosphere haze region with occasional larger concentrations of particulates at lower altitudes. The general structures, especially the middle cloud layers, are remarkably constant planetary features but do exhibit some local variation from one site to another. Concentrated sulfuric acid appears to be the principal constituent of most of the particulate matter in all of the cloud structure with the possible exceptions of the largest particles in the lower cloud layers, whose composition is still uncertain, and the hazes and particulate matter in the lower atmosphere. Near‐UV radiation is absorbed throughout the cloud structure, primarily in the highest layers and, perhaps, below it. Absorbing species are not yet identified, but a combination of SO2 and an unspecified absorber look promising. Particles ejected from the surface of the planet on probe impact appear to be in the 10‐ to 100‐µm size range. The source of a faint glow detected by the night and north probes in the lower atmosphere is as yet uncertain.