07 December 2018

Purple Pigeon: Mars Multi-Rover Mission (1977)

Image credit: JPL/NASA
Planetary scientist Bruce Murray became director of the Jet Propulsion Laboratory (JPL) in April 1976, just three months before Viking 1 was due to land on the northern plains of Mars. Though NASA's Langley Research Center managed Project Viking, JPL included Viking Mission Control. When Viking 1 landed, JPL could expect to play host to hundreds of journalists from all over the Earth.

According to his 1989 memoir Journey into Space: The First Thirty Years of Space Exploration, Murray saw this as an opportunity. He quickly assembled a group of six engineers to propose planetary missions that he could pitch to the journalists and, through them, to U.S. taxpayers.

The missions, which Murray dubbed "Purple Pigeons," were intended to include both "high science content" and "excitement and drama [that would] garner public support." They were called Purple Pigeons to differentiate them from "Gray Mice," unexciting and timid missions which Murray felt would help to ensure that JPL had no future in the space exploration business. By August 1976, the Purple Pigeons flock included a solar sail mission to Halley's Comet, a Mars Surface Sample Return (MSSR), a Venus radar mapper, a Saturn/Titan orbiter/lander, a Ganymede lander, an asteroid tour, and an automated lunar base.

Bruce Murray, JPL director from April 1976 until June 1982. Image creditI JPL/Caltech
The Purple Pigeons effort continued even after Viking 2 landed (3 September 1976) and all the journalists went home. In a February 1977 JPL report, for example, JPL engineers described a Purple Pigeon mission that would explore Mars with up to four rovers simultaneously. The Viking-based multi-rover mission would include a pair of identical 4800-kilogram spacecraft, each consisting of a Viking-type orbiter and a 1578-kilogram Mars lander bearing twin 222.4-kilogram rovers. The rovers would, the report stated, perform traverses to "regions difficult to reach by direct landings." This would, it added, fill the gap between "detailed information" from MSSR missions and "global information" from Mars orbiters.

The image at the top of this post shows a somewhat different (probably later) multi-rover mission design. Its four six-wheel, multi-cab rovers (two of which are operating out of view over the horizon) rely on a single Viking orbiter-type spacecraft to relay radio signals to and from Earth. In principle, however, it is identical to the early multi-rover mission design described in this post.

Most MSSR plans of the 1970s assumed a "grab" sample; that is, that the stationary MSSR lander would return to Earth a sample of whatever rocks and soil happened to be within reach of its robotic sample scoop. The report suggested that the rovers of the multi-rover mission might enhance a follow-on MSSR mission by collecting and storing samples as they roved across the planet. After the MSSR lander arrived on Mars, the rovers would rendezvous with it and hand over their samples for return to Earth. The report contended that its multi-rover/MSSR strategy would be "an enormous advance over even multiple grab samples" collected by MSSR landers at widely scattered sites.

At the time the Purple Pigeons team proposed the multi-rover mission, NASA intended to launch all payloads, including interplanetary spacecraft, on board reusable Space Shuttles. The Shuttle orbiter would be able to climb no higher than about 500 kilometers, so launching payloads to higher Earth orbits or interplanetary destinations would demand an upper stage. The powerful liquid-propellant Centaur upper stage would not be ready in time for the opening of the Mars multi-rover launch window, which spanned from 11 December 1983 to 20 January 1984, so JPL tapped a three-stage solid-propellant Interim Upper Stage (IUS) to push its Purple Pigeon out of Earth orbit toward Mars.

After an Earth-Mars cruise lasting about nine months, the twin multi-rover spacecraft would arrive at Mars a week or two apart between 16 September and 27 October 1984. They would each fire their main engines to slow down so that Mars gravity could capture them into an elliptical orbit with a periapsis (low point) of 500 kilometers, a five-day period, and an inclination of 35° relative to the martian equator.

The multi-rover landers would then separate and each fire a solid-propellant de-orbit rocket at the apoapsis (high point) of its orbit to begin descent to the surface. Landing sites between 50° north latitude and the south pole would in theory be accessible, though the need for a direct Earth-to-rover radio link would in practice prevent landings below 55° south.

The landers would each be encased within an aeroshell with a heatshield for protection during the fiery descent through the martian atmosphere. The aeroshell would have the same 3.5-meter diameter as its Viking predecessor, though its afterbody would be modified to make room for the large cooling vanes of the twin rovers' electricity-producing Radioisotope Thermal Generators (RTGs).

JPL's dual rovers packed inside their modified Viking-type aeroshell. Image credit: JPL
After the landers touched down, the orbiters would maneuver to areosynchronous orbit. In such an orbit, 17,058 kilometers above the martian equator, only minor orbital corrections would enable a spacecraft to "hover" indefinitely over one spot on the equator. Each orbiter would position itself over a spot on the equator near its lander's longitude so that it could relay radio signals between its rovers on Mars and operators on Earth.

The multi-rover lander, which would serve no purpose beyond rover delivery, would constitute a radical departure from the triangular Viking lander design, though it would use Viking technology where possible to save development costs. It would comprise a rectangular frame to which would be attached three uprated Viking-type terminal descent engines, two spherical propellant tanks, and three beefed-up Viking-type landing legs.

Multi-rover lander. Image credit: JPL
The 1.5-meter-long rovers would be mounted on the lander frame with their four 0.5-meter-diameter wire wheels compressed. Releasing a latching mechanism would permit the wheels to expand, raising the rover off four stabilizing "taper pins." The pins and one terminal descent engine would then swing out of the way, ramps would deploy, and the first rover would roll onto the rocky martian surface. The second rover would then ride a motor-driven "dolly" to the first rover's initial position before unlatching and joining its twin on the ground.

JPL envisioned that its four-wheeled rovers would each deploy a one-meter-tall boom holding a still-image camera, a floodlight, a strobe light, a weather station, and a pointable horn-shaped radio antenna. The camera/antenna boom, the tallest part of the rover, would stand about two meters above the surface. Controllers on Earth would then put the rovers through an initial checkout lasting at least two weeks. The checkout would culminate in slow "manual" (Earth-controlled) and faster semi-autonomous (Earth-directed but rover-controlled) traverses.

JPL's nuclear-powered rover viewed from above (top) and from the side. Image credit: JPL 
In semi-autonomous mode, operators would plan traverse routes and science targets using stereo images from the rover camera taken from terrain "high points," then would command the rover to proceed. The rovers might assist each other in traverse planning; for example, "high point" pictures from one might fill in blind spots in the other's field of view. "After the first few kilometers of traverse," the JPL engineers assumed, operators on Earth would "begin to build an intuitive feeling for the Martian geography and its impact on the rover capabilities, allowing them to plan better paths." The rovers would also photograph each other to enhance the mission's "general public appeal."

The rover mobility system would include one electric drive motor per wheel, eight proximity sensors for obstacle detection, inclinometers to monitor rover tilt, motor temperature sensors to judge wheel traction, a gyrocompass/odometer, a laser rangefinder with a 30-meter range, and an "8-bit word, 16k active, 64k bulk, floating point arithmetic and 16-bit accuracy" computer. The JPL engineers judged that their rovers would be capable of moving at up to 50 meters per hour over terrain similar to that seen at the Viking 1 landing site.

Sunset at the Viking 1 landing site in Chryse Planitia. Image credit: NASA
Alpha-scattering X-ray fluorescence and gamma-ray spectrometers would collect data while the rovers were in motion, but all other science, including imaging and sample collection, would occur only while they were parked. Each rover would gather samples using an "articulated arm" with an "electromechanical hand."

In order to avoid "an overabundance of data from a single track," the rovers would travel slightly different routes and rendezvous at the end of each leg of their traverse. They would, however, travel close enough together that each could aid the other in the event of trouble. If one rover became stuck in loose dirt, for example, its companion could use its articulated arm to place rocks under its wheels to improve traction. If one rover of a pair failed, the report maintained, the other would continue to yield "good, solid science."

The rovers would be designed to operate for at least one martian year (about two Earth years) to help ensure that at least one of the four could successfully rendezvous with the follow-on MSSR mission, which would leave Earth in 1986. Estimates of rover traverse distances in 1970s and 1980s studies were typically highly optimistic, and the multi-rover mission was no exception: each of the mission's four rovers was expected to travel up to 1000 kilometers. The JPL engineers concluded their report by calling for new technology development to ensure that adequate power and mobility systems would become available by the time their Purple Pigeon was due to fly.

Sources

Journey into Space: The First Thirty Years of Space Exploration, Bruce Murray, W. W. Norton & Co., 1989

Feasibility of a Mars Multi-Rover Mission, JPL 760-160, Jet Propulsion Laboratory, 28 February 1977

More Information

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980s (1967)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Making Propellants from Martian Air (1978)

01 December 2018

Exploring Mars from Pole to Pole: MESUR Network (1991)

Pioneer Venus 2 releases its three small Venus atmosphere entry probes. Through artist license, the large probe is visible against the clouds of Venus; it would not in fact have been visible at the time the small probes were released. Image credit: NASA
On 8 August 1978, NASA launched Pioneer Venus 2 (PV2) on an Atlas-Centaur rocket. The 904-kilogram spacecraft, known also as Pioneer Venus Multiprobe, released a 1.5-meter-diameter battery-powered atmosphere entry probe on 16 November and three 76-centimeter-diameter probes on 20 November.

On 9 December 1978, the five parts of PV2 entered the thick, hot Venusian atmosphere. The drum-shaped probe carrier burned up as planned at an altitude of 110 kilometers. Sturdy conical heat shields protected the spherical instrumented probes from aerodynamic heating. As drag slowed it, the large probe deployed a parachute.

Two of the small probes, which did not include parachutes, exceeded all expectations by surviving landing and transmitting data from the hellish Venusian surface. One, the Day Probe, transmitted for 67.5 minutes before succumbing to heat, pressure, and battery failure, setting a new world record for spacecraft endurance on Venus.

PV2 was the last U.S. planetary mission launched until 1989. NASA Ames Research Center (ARC), located near San Francisco, California, managed PV2 and its sister spacecraft, PV1 (the Pioneer Venus Orbiter).

In July 1991, ARC proposed a multiprobe system outwardly not too different from PV2, but intended to create a long-lived network of low-cost science stations on Mars. According to ARC's report on the concept, its network would reflect a design philosophy with "unique characteristics . . . derived from the Pioneer Project corporate memory."

Mars networks were first proposed in the early 1970s. Scientific advisory groups endorsed the network concept repeatedly in the following two decades as the best way to obtain global-scale weather and seismic data. In the late 1980s, at the behest of the NASA Headquarters Solar System Exploration Division (SSED), the Jet Propulsion Laboratory (JPL) Precursor Task Team included a network in its program of precursor robotic missions for paving the way for astronauts on Mars. In common with previous Mars network plans, the 1989 plan invoked spear-shaped penetrators to hard-land stations at low cost.

NASA ARC's Mars Environment Survey (MESUR - pronounced "measure"), on the other hand, invoked cheap rough-landing landers, or "stations," that would deploy protective airbags seconds before landing. MESUR would build up a "pole-to-pole" network of 16 stations during the 1999, 2001, and 2003 minimum-energy Mars launch opportunities.

Each 158.5-kilogram MESUR lander would leave Earth attached toa Mars atmosphere entry deceleration system and a simple cruise stage. Upon arrival at Mars, each would cast off its cruise stage and enter the atmosphere directly from its Earth-Mars trajectory at up to seven kilometers per second. The ARC report compared this with the Viking landers, which entered from Mars orbit at only 4.4 kilometers per second. The lander's heat shield, a two-meter-diameter flattened cone, would be designed to withstand atmosphere entry during planet-wide dust storms, when suspended dust particles might exacerbate shield erosion.

Partial cutaway of a MESUR station on the surface of Mars. Image credit: NASA Ames Research Center
The ARC report acknowledged that the disk-shaped lander might bounce to rest on Mars in either "heads" or "tails" orientation, but rejected as costly and risky a mechanical system for tipping it upright. The ARC engineers opted instead for circular ports that would enable controllers to deploy instruments from either side of the station. Instruments might include imagers, an atmospheric structure experiment, gas analyzers, a weather station, a spectrometer, and a seismometer.

The report explained that solar cells were initially ARC's preferred MESUR power system, but analysis had shown that the number of cells that could be mounted on the lander's small surface would not generate enough electricity to drive its science instruments unless landings were limited to sites within 30° of the martian equator. This limitation was deemed unacceptable by the MESUR Science Definition Team, so engineers opted for a small (nine-kilogram) General Purpose Heat Source (GPHS) Radioisotope Thermal Generator (RTG) "brick" based on Ulysses solar polar orbiter/Galileo Jupiter orbiter RTG technology.

Sixteen MESUR landers would need 16 GPHS bricks over six years. The report noted that the entire MESUR Network would need less than half as much plutonium as the Cassini Saturn orbiter, which would carry two RTGs with 18 GPHS bricks each.

Cutaway of the MESUR Network launch shroud showing four MESUR landers (one is mostly obscured behind the lander support structure) and the solid-propellant Mars transfer orbit injection stage. Image: NASA Ames Research Center
The MESUR mission would begin in 1999 with the launch of a single Delta II 7925 rocket from Cape Canaveral, Florida, with four MESUR landers mounted on a framework within its 9.5-foot-diameter streamlined launch shroud. After a solid-propellant upper stage placed them on course for Mars, the landers would separate from the framework to travel on "independent free-flyer trajectories" that would permit precise Mars landing site targeting. Three side-mounted landers would tumble after separation, but sloshing propellants in their cruise stages would gradually damp their gyrations.

The landers would discard their cruise stages 125 kilometers above Mars. Ten kilometers above the planet, each would deploy a pilot parachute, then cast off its heat shield and open its single main parachute. The landers would image the surface and collect atmospheric structure data during the final eight kilometers of descent.

Just two meters above the landing site, each lander would release its main parachute and inflate its airbags. A small rocket on the parachute would ignite to prevent it from settling over the lander.

The MESUR lander design would permit landings at sites up to six kilometers above the base datum, the martian equivalent of Earth's sea level. The base datum, referenced to the minimum Mars atmospheric pressure required for liquid water to exist on the surface, was established after Mariner 9 mapped the planet from orbit in 1971-1972. (In 2001, a new system referenced to the mean radius of Mars as measured by Mars Global Surveyor's MOLA instrument replaced the base datum.)

Though all 16 MESUR landers would carry the same suite of instruments, their individual landing sites would be selected to cater to different science requirements. The report advised that weather stations should be spaced widely over the planet, while seismic stations should form closely spaced "triads." These conflicting requirements forced a "compromise network design."

MESUR Network Stations 1 and 2 would land near each other on the north rim of Valles Marineris to form a "seismic pair." Station 3, at the foot of Olympus Mons in Tharsis, would also emphasize seismic research. Station 4 would aim to extend the weather record for Chryse Planitia, where Viking 1 accumulated data from 1976 to 1983.

The Tharsis hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA
In 2001, two Delta II 7925s would launch 20 days apart bearing four more MESUR landers and a communications relay orbiter, respectively. The latter payload, based on an existing Earth-orbital comsat design, would serve as radio relay for the expanding network, enabling MESUR stations to return data from sites all over the martian surface.

It would reach Mars in 10 months on a slow "Type II" trajectory to reduce the amount of propellant it would need to slow down so that the planet's gravity could capture it. Launch of the communications orbiter would be delayed until 2001 in order to spread its cost over a longer period.

With the successful arrival of the four 2001 stations, a "minimal network" would be in place on Mars. Station 5, on the Marineris north rim, would create a "seismic triad" with Stations 1 and 2, while Station 6, northwest of Olympus Mons, would create a seismic pair with Station 3. Station 7, east of Solis Planum ("a region of known dust storm activity"), and Station 8, in western Acidalia Planum, would expand martian meteorological coverage.

The final two MESUR Delta II 7925 launches in 2003 would boost four landers each on course for Mars. Stations 9 and 10 would be located near the north and south poles, respectively, while Station 11 would report weather conditions in Aonia Terra, southwest of the great Argyre basin. Stations 12 (northwest Hellas), 13 (Elysium Planitia), and 14 (Deuteronilus Mensae) would further extend martian meteorological coverage.

Station 15 (Sirenum Terra) would form a Tharsis seismic triad with Stations 3 and 6. Station 16, in Syrtis Major on the side of Mars opposite Olympus Mons, would create a seismic pair with Station 13 and, with the Tharsis triad, enable the size of Mars's core to be determined.

The Syrtis Major hemisphere of Mars showing proposed positions of MESUR stations. See text for explanation. Image credit: NASA
The entire 16-station network and its communications orbiter would function for at least a martian year (a little more than two Earth years). This would mean that the 1999 stations would have to endure for three martian years (6.5 Earth years), while the 2001 stations and communications orbiter would need to function for two martian years (4.3 Earth years).

In its 1991 strategic plan, published the same month as ARC's MESUR report, the SSED dubbed MESUR its "baseline plan" for a Mars network mission. In November 1991, NASA elected to move MESUR Phase A development to JPL, where the project was split into two parts.

MESUR Network would be preceded by MESUR Pathfinder, a single-spacecraft mission for technology testing. Pathfinder was built larger than the the planned MESUR landers so that it could deliver to Mars a six-wheeled "microrover." JPL also opted for solar power in place of NASA ARC's RTG bricks and a petal system to permit it to flip itself upright and release the rover instead of small instrument deployment ports.

In 1994, in the wake of the Mars Observer failure, NASA funded the Mars Surveyor Program in place of MESUR Network. Work continued on Pathfinder under the auspices of NASA's low-cost Discovery Program, however, and it landed successfully on Mars on 4 July 1997.

Mars Pathfinder Lander (background) and Sojourner rover. Image credit: NASA
Sources

Mars Environmental Survey (MESUR) Science Objectives and Mission Description, NASA Ames Research Center, July 19, 1991

Solar System Exploration Division Strategic Plan: Preparing the Way to the New Frontier of the 21st Century, Special Studies Office, Space Telescope Science Institute, July 1991

More Information

Centaurs, Soviets, and Seltzer Seas: Mariner 2's Martian Adventure (1962)

"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Program

Pioneer Mars Orbiter with Penetrators (1974)

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

Touring Titan by Blimp and Buoy (1983)