v1.0.2 / 01 jul 04 / greg goebel / public domain
* While work has been done since the 1960s on robot "rovers" to explore the Moon and other worlds in our Solar System, such machines have limitations. They tend to be expensive and have limited range, and due to the communications timelag over interplanetary distances, they have to be smart enough to navigate without running into obstacles or falling off cliffs.
For planets with atmospheres of any substance, however, there is an alternative: the balloon. Flying above obstructions and carried by the winds, a balloon could inspect large regions of a planet in great detail for relatively low cost. This document provides a short overview of current and past work in planetary robot balloons, or "aerobots".
* A balloon is conceptually the simplest of all flying machines, consisting of nothing more than a cloth "envelope" filled with a "lifting gas" that is lighter than the surrounding atmosphere. As the gas-filled balloon is less dense than its surroundings, it rises, taking along with it a "gondola" attached underneath that carries passengers or payload.
The first balloon, launched in 1783 by two Parisian brothers named Montgolfiere, used hot air as the lifting gas. Balloons using the light gas hydrogen for buoyancy were also developed at the same time. Although a balloon has no propulsion system, as balloonists became more experienced they learned a degree of directional control through the measure of rising or sinking in altitude to find favorable winds. Both the hot-air, or "Montgolfiere", balloon and the light gas balloon are still in common use for Earthly activities. Montgolfiere balloons are relatively cheap as they do not require high-grade materials for their envelopes nor bottles of light gas, and they are often bought and flown by hobbyists.
Light gas balloons are predominant in Earth-based scientific applications, since they are capable of reaching much higher altitudes for much longer periods of time. They are generally filled with helium. Although hydrogen has more lifting power, it is explosive in an atmosphere full of oxygen. Modern scientific balloon missions are unmanned.
There are two types of light-gas balloons: "zero-pressure" and "superpressure". Zero-pressure balloons are the traditional form of light-gas balloon. They are partially inflated with the light gas before launch, with the gas pressure the same both inside and outside the balloon. As the zero-pressure balloon rises, its gas expands to maintain the zero pressure difference, and the balloon's envelope swells. At night, the gas in a zero-pressure balloon cools and contracts, causing the balloon to sink. A zero-pressure balloon can only maintain altitude by releasing gas when it rises too high, where the expanding gas can threaten to rupture the envelope; or by releasing ballast when it sinks too low. Loss of gas and ballast limits the endurance of zero-pressure balloons to a few days.
A superpressure balloon, in contrast, has a tough and inelastic envelope that is filled with light gas to a pressure higher than that of the external atmosphere, and then sealed. The superpressure balloon cannot change size very much, and so maintains a nearly constant volume. The superpressure balloon maintains an altitude of constant density in the atmosphere, and can maintain flight until gas leakage gradually brings it down. Superpressure balloons offer flight endurance of months instead of days. In fact, in typical operation a Earth-based superpressure balloon mission is ended by a command from ground control to open the envelope, not by the natural leakage of gas.
* While the idea of sending a balloon to another planet sounds strange at first, balloons have a number of advantages for planetary exploration. They can be made light in weight and are potentially inexpensive. They can cover a great deal of ground, and their view from a height gives them the ability to examine wide swathes of terrain with far more detail than would be available from an orbiting satellite. For exploratory missions, their relative lack of directional control is not a major obstacle, since there is generally no need to direct them to a specific location.
Balloon designs for possible planetary missions have involved a few unusual concepts. One is the solar, or infrared (IR) Montgolfiere. This is a hot-air balloon where the envelope is made from a material that traps heat from sunlight, or from heat radiated from a planetary surface. Black is the best color for absorbing heat, but other factors are involved and the material may not necessarily be black. Solar Montgolfieres have several advantages for planetary exploration, as they can be easier to deploy than a light gas balloon; do not necessarily require a tank of light gas for inflation; and are relatively forgiving of small leaks. They will tend to drop to the ground when the sun goes down, but as is explained below, that isn't completely a disadvantage.
The other is a "reversible fluid" balloon. This type of balloon consists of an envelope connected to a reservoir, with the reservoir containing a fluid that is easily vaporized. The balloon can be made to rise by vaporizing the fluid into gas, and can be made to sink by condensing the gas back into fluid. There are a number of different ways of implementing this scheme, but basic principle remains the same.
A balloon designed for planetary exploration will carry a small gondola containing an instrument payload. The gondola will also carry power, control, and communications subsystems. Due to weight and power supply constraints, the communications subsystem will generally be small and low power, and interplanetary communications will be performed through an orbiting planetary probe acting as a relay.
A solar Montgolfiere will sink at night, and will have a guide rope attached to the bottom of the gondola that will curl up on the ground and anchor the balloon during the darkness hours. The guide rope will be made of low friction materials to keep it from catching or tangling on ground features. Alternatively, a balloon may carry a thicker instrumented "snake" in place of the gondola and guiderope, combining the functions of the two. This is a convenient scheme for making direct surface measurements.
A balloon could also be anchored to stay in one place to make atmospheric observations. Such a static balloon is known as an "aerostat".
One of the trickier aspects of planetary balloon operations is inserting them into operation. Typically, the balloon enters the planetary atmosphere in an "aeroshell", a heat shield in the shape of a flattened cone. After atmospheric entry, a parachute will extract the balloon assembly from the aeroshell, which falls away. The balloon assembly then deploys and inflates.
Once operational, the aerobot will be largely on its own and will have to conduct its mission autonomously, accepting only general commands over its long communications link to Earth. The aerobot will have to determine its position in three dimensions; acquire and store science data; perform flight control by varying its altitude; and possibly make landings at specific sites to perform close-up investigations.
* The first, and so far only, planetary balloon mission was performed by the Russian space agency IKI in cooperation with the French space agency CNES in 1985. A small balloon, similar in appearance to Earthly weather balloons, was carried on each of the two Soviet VEGA Venus probes, launched in 1984. The first balloon was inserted into the atmosphere of Venus on 11 June 1985, followed by the second balloon on 15 June 1985. Each balloon operated for a little under two Earth days, until the batteries ran down.
The Venus VEGA balloons were the idea of Jacques Blamont, then chief scientist for CNES and the father of planetary balloon exploration. He energetically promoted the concept and enlisted international support for the small project.
The balloons were spherical superpressure types with a diameter of 3.54 meters (11 feet 7 inches) and filled with helium. A gondola assembly weighing 6.9 kilograms (15.2 pounds) and 1.3 meters (4 feet 3 inches) long was connected to the balloon envelope by a tether 13 meters (42 feet 7 inches) long. Total mass of the entire assembly was 21 kilograms (46.31 pounds).
The top section of the gondola assembly was capped by a conical antenna 37 centimeters (14.6 inches) tall and 13 centimeters (5.12 inches) wide at the base. Beneath the antenna was a module containing the radio transmitter and system control electronics. The lower section of the gondola assembly carried the instrument payload and batteries. The instruments consisted of:
The small low-power transmitter only allowed a data transmission rate of 2,048 bits per second, though the system performed data compression to squeeze more information through the narrow bandwidth. The bandwidth was adequate, since the sampling rate for most of the instruments was only once every 75 seconds. The balloons were tracked by an international network of 20 radio telescopes back on Earth.
The balloons were dropped onto the planet's darkside and deployed at an altitude of about 50 kilometers (31 miles). They then floated upward a few kilometers to their equilibrium altitude. At this altitude, pressure and temperature conditions of Venus are similar to those of Earth, though the planet's winds move at hurricane velocity and the carbon-dioxide atmosphere is laced with sulfuric acid, along with smaller concentrations of hydrochloric and hydrofluoric acid. The balloons moved swiftly across the night side of the planet into the light side, where their batteries finally ran down and contact was lost. Tracking indicated that the motion of the balloons included a surprising vertical component, revealing vertical motions of air masses that had not been detected by earlier probe missions.
The scientific results of the Venus VEGA probes were modest. More importantly, the clever and simple experiment demonstrated the validity of using balloons for planetary exploration.
* After the success of the Venus VEGA balloons, Blamont focused on a more ambitious balloon mission to Mars, to be carried on a Soviet space probe.
The atmospheric pressure on Mars is about 150 times less than that of Earth. In such a thin atmosphere, a balloon with a volume of 5,000 to 10,000 cubic meters (178,500 to 357,000 cubic feet) could carry a payload of 20 kilograms (44 pounds), while a balloon with a volume of 100,000 cubic meters (3,570,000 cubic feet) could carry 200 kilograms (440 pounds).
The French had already conducted extensive experiments on Earth with solar Montgolfieres, performing over 30 flights from the late 1970s into the early 1990s. The Montgolfieres flew at an altitude of 35 kilometers, where the atmosphere was as thin and cold as it would be on Mars, and one spent 69 days aloft, circling the Earth twice.
Early concepts for the Mars balloon featured a "dual balloon" system, with a sealed hydrogen or helium-filled balloon tethered to a solar Montgolfiere. The light-gas balloon was designed to keep the Montgolfiere off the ground at night. During the day, the Sun would heat up the Montgolfiere, causing the balloon assembly to rise. Eventually, the group decided on a cylindrical sealed helium balloon with an envelope made of Mylar, and with a volume of 5,500 cubic meters (196,000 cubic feet). The balloon would rise when heated during the day and sink as it cooled at night.
Total mass of the balloon assembly was 65 kilograms (143 pounds), with a 15 kilogram (33 pound) gondola and a 13.5 kilogram (30 pound) instrumented guiderope. The balloon was expected to operate for ten days. Unfortunately, although considerable development work was performed on the balloon and its subsystems, Russian financial difficulties pushed the Mars probe out from 1992; then to 1994; and then to 1996. The Mars balloon was dropped from the project due to cost constraints, and the probe was lost on launch in 1996 anyway.
* By this time, the Jet Propulsion Laboratory (JPL) of the US National Aeronautics & Space Administration (NASA) had become interested in the idea of planetary aerobots, and in fact a team under Jim Cutts of JPL had been working on concepts for planetary aerobots for several years, as well as performing experiments to validate aerobot technology.
The first such experiments focused on a series of reversible-fluid balloons, under the project name of "Altitude Control Experiment (ALICE)". The first such balloon, ALICE 1, flew in 1993, with other flights through ALICE 8 in 1997. Related work included the characterization of materials for a Venus balloon envelope, and two balloon flights in 1996 to test instrument payloads under the name "Balloon Assisted Radiation Budget Equipment (BARBE)".
By 1996, JPL was working on a full-fledged aerobot experiment named the "Planetary Aerobot Testbed (PAT)", which was intended to demonstrate a complete planetary aerobot through flights into Earth's atmosphere. PAT concepts envisioned a reversible-fluid balloon with a 10-kilogram payload that would include navigation and camera systems, and eventually would operate under autonomous control. The project turned out to be too ambitious, and was cancelled in 1997.
JPL continued to work on a more focused, low-cost experiments to lead to a Mars aerobot, under the name "Mars Aerobot Validation Program (MABVAP)". MABVAP experiments included drops of balloon systems from hot-air balloons and helicopters to validate the tricky deployment phase of a planetary aerobot mission, and development of envelopes for superpressure balloons with materials and structures suited to a long-duration Mars mission.
JPL also provided a set of atmospheric and navigation sensors for the "Solo Spirit" round-the-world manned balloon flights, both to support the balloon missions and to validate technologies for planetary aerobots.
* While these tests and experiments were going on, JPL performed a number of speculative studies for planetary aerobot missions to Mars, Venus, Saturn's moon Titan, and the Outer Planets.
JPL's MABVAP technology experiments were the first step towards design of operational Mars aerobots. JPL researchers envisioned three phases for such operational probes.
The first phase was a modest probe that would validate technologies and still perform useful science, at low cost. This probe, named the "Mars Aerobot Technology Experiment (MABTEX)", was envisioned as a small superpressure balloon, carried to Mars on a "microprobe" weighing no more than 40 kilograms (88 pounds). Once inserted, the operational balloon would have a total mass of no more than 10 kilograms (22 pounds) and would remain operational for a week. The little balloon gondola would have navigation and control electronics, along with a stereo imaging system, as well as a spectrometer and magnetometer.
MABTEX would be followed by a more sophisticated phase-two Mars aerobot, named the "Mars Geoscience Aerobot (MGA)". Design concepts for MGA envisioned a superpressure balloon system very much like that of MABTEX, but much bigger. MGA would carry a payload ten times larger than that of MABTEX, and would remain aloft for up to three months, circling Mars more than 25 times and covering over 500,000 kilometers (310,000 miles) of ground. The payload would include sophisticated equipment, such as an ultrahigh resolution stereo imager, along with oblique imaging capabilities; a radar sounder to search for subsurface water; an infrared spectroscopy system to search for important minerals; a magnetometer; and weather and atmospheric instruments.
MABTEX was to be followed in turn by phase-three Mars aerobot, a small solar-powered blimp named the "Mars Solar Electric Propelled Aerobot (MASEPA)".
* JPL's studies of Venus aerobots followed similar lines. A simple demonstrator designated the "Venus Aerobot Technology Experiment (VEBTEX)" was seen as a stepping stone to more capable missions.
One advanced mission concept, known as "Venus Aerobot Multisonde (VAMS)", envisioned an aerobot operating at altitudes above 50 kilometers (31 miles) that would drop surface probes, or "sondes", onto specific surface targets. The balloon would then relay information from the sondes directly to Earth, and would also collect planetary magnetic field data and other information. VAMS would require no fundamentally new technology, and could be appropriate for a NASA low-cost Discovery planetary science mission.
Significant work was performed on a more ambitious concept, designated the "Venus Geoscience Aerobot (VGA)". Designs for the VGA envisioned a relatively large reversible-fluid balloon, filled with helium and water, that could descend to the surface of Venus to sample surface sites, and then rise again to high altitudes and cool off.
Developing an aerobot that can withstand the high pressures and temperatures (up to 480 degrees Celsius, or almost 900 degrees Fahrenheit) on the surface of Venus, as well as passage through sulfuric acid clouds, would require new technologies. Prototype balloon envelopes were fabricated from polybenzoxazole (PBO), a polymer that features high strength, resistance to heat, and low leakage for light gases. A gold film was applied to allow the polymer film to resist corrosion from acid clouds.
Work was also done on a VGA gondola weighing about 30 kilograms (66 pounds). In this design, most instruments were contained in a spherical pressure vessel with an outer shell of titanium and an inner shell of stainless steel. The vessel contained a solid-state camera and other instruments, as well as communications and flight control systems. The vessel was designed to tolerate pressures of up to a hundred atmospheres and maintain internal temperatures below 30 degrees Celsius (86 degrees Fahrenheit) even on the surface of Venus.
The vessel was set at the bottom of a hexagonal "basket" of solar panels that in turn provided tether connections to the balloon system above, and was surrounded by a ring of pipes acting as a heat exchanger. An S-band communications antenna was mounted on the rim of the basket, and a radar antenna for surface studies was fitted on a mast mounted on the vessel.
* Titan, the largest moon of Saturn, is an attractive target for aerobot exploration, as it has a nitrogen atmosphere twice as dense as that of Earth's that contains a smog of organic photochemicals, hiding the moon's surface from view by visual sensors.
An aerobot would be able to penetrate this haze to study the moon's mysterious surface and search for complex organic molecules. NASA has outlined a number of different aerobot mission concepts for Titan, under the general name of "Titan Biologic Explorer".
One concept, known as the "Titan Aerobot Multisite (TAM)" mission, involved a reversible-fluid balloon filled with argon that could descend from high altitude to the surface of the moon, perform measurements, and then rise again to high altitude to perform measurements and move to a different site.
Another concept, known as the "Titan Aerobot Singlesite (TAS)" mission, would use a superpressure balloon that would select a single site, vent much of its gas, and then survey that site in detail.
An ingenious variation on this scheme, the "Titan Aerover", combined aerobot and rover. This vehicle featured a triangular frame that connected three balloons, each about two meters (6.6 feet) in diameter. After entry into Titan's atmosphere, the aerover would float until it found an interesting site, then vent helium to descend to the surface. The three balloons would then serve as floats or wheels as necessary. JPL built a simple prototype that looks three beachballs on a tubular frame.
No matter what form the Titan Biologic Explorer mission takes, it would likely require an atomic-powered radioisotope thermoelectric generator (RTG) module for power. Solar power would not be possible at Saturn's distance and under Titan's smog, and batteries would not provide adequate mission endurance. The aerobot would also carry a miniaturized chemical lab to search for complicated organic chemicals.
* Finally, aerobots might be used to explore the atmosphere of Jupiter and possibly the other gaseous Outer Planets. Since the atmospheres of these planets are largely composed of hydrogen, and since there is no lighter gas than hydrogen, such an aerobot would have to be a Montgolfiere; since sunlight is weak at such distances, the aerobot would obtain most of its heating from infrared energy radiated by the planet below.
A Jupiter aerobot might operate at altitudes where the air pressure ranges from one to ten atmospheres, occasionally dropping lower for detailed studies. It would make atmospheric measurements and return imagery and remote sensing of weather phenomena, such as Jupiter's Great Red Spot. A Jupiter aerobot might also drop sondes deep into the atmosphere and relay their data back to an orbiter until the sondes are destroyed by temperature and pressure.
* I became interested in writing this article after reading a comment in the yearly aerospace news review published in AEROSPACE AMERICA magazine in December 1998. The section in the review on balloon technology mentioned the JPL work on aerobots, and I looked up the website referenced.
The website proved to be extremely detailed and fascinating. I had read articles by Jacques Blamont about his Mars balloon efforts some years earlier, but that work had not resulted in a mission and I had heard nothing else since. I was fascinated to see how much effort was being exerted on the aerobot concept.
Work on aerobots seems to be at a low level right now. Another faction in NASA has been promoting sending a robot aircraft to Mars, and for some reason the two approaches have been seen as competitive, instead of complementary. An aircraft can explore targeted sites, while a balloon has much greater endurance.
There was considerable enthusiasm for a Mars airplane for a while, overshadowing Mars balloon efforts, and in fact NASA went so far as to plan the launch of a robot aircraft to Mars in 2003 to commemorate the 100th anniversary of the first flight of the Wright Brothers. NASA's Mars exploration programs then had an abrupt collision with reality when two Mars probes were lost in 1999. The embarrassing double loss, coupled with funding shortfalls, led NASA to return to the drawing board on Mars exploration, and currently neither a balloon nor airplane mission is planned. However, NASA is promoting a program of low-cost, small-scale "Scout" missions to complement larger probes, and both balloons and airplanes are being considered as candidates. CNES is collaborating with NASA on the Scouts, which may be launched as secondary payloads on French Ariane boosters, and the French remain very interested in the aerobot concept.
* This document was derived from a set of writings on the NASA JPL website at "jpl.nasa.gov". The JPL pages were very good, but sprawled over a wide area, so I ended up writing this document to get a clearer idea of what was going on.
* Revision history:
v1.0 / 15 mar 99 / gvg
v1.0.1 / 01 jul 02 / gvg / Minor cosmetic update.
v1.0.2 / 01 jul 04 / gvg / Minor cosmetic update.