by Donald Georgi
June is the month at which the travel distance to Mars is the shortest, allowing visitors to minimize launch fuel costs, maximize payloads and minimize transit time. Taking advantage of that feature, three separate missions, two U.S. and one European, are in the process of visiting Mars to determine the geology, specifically to verify the presence or past presence of water on the fourth planet from the sun. Since October of 2001, the Mars Oddesy has been orbiting this planet about twice the size of Earth’s moon. In May 2002, its gamma ray spectrometer sensed hydrogen less than 3 feet below the Martian surface. The projected thinking is that the planet is in the form of ice which could have supported life in the past or in future Earth missions.
Each interplanetary launch is a historical event, but these three missions usher in a new era in battery chemistries, for all are relying on Lithium-ion chemistry for power to complement that produced by photovoltaic panels. Although the European Space Agency has already placed Lithium-ion in satellites and the U.S. Air Force has powered its Micro-satellite, XSS-10 with Lithium-ion polymer batteries, these new Mars missions are pioneering steps in the progress of higher energy densities, cycle performance, environmental performance and calendar life for energy storage in space.
The photo at the right (From the U.S. Air Force Archives,) shows the successful launch of the Rover A mission, named ‘Spirit,’ aboard a Delta II vehicle from Cape Canaveral Air Force Station on June 10, 2003. Batteries are included. As of June 29th, the launch of Rover B, named ‘Opportunity,’ has been delayed due to problems of insulation materials. The launch window of Opportunity is from June 25 through July 15th. Despite variations in launch times for both vehicles, arrivals are scheduled for January 4th and January 25th, 2004.
The photo below, is from the NASA Jet Propulsion Laboratory archives, and shows the Rover which will travel from the Lander to various sites to carry out geological experiments. This photo may give the impression of a giant vehicle, but in reality, Rover is 5.2 feet long and weighs 384 pounds on earth. Since the Martian gravity is only 38% that of Earth’s, the Rover weighs only 146 pounds on Mars. Carrying the thinkinga bit farther, Rover batteries are made up of two 28 Volt, 10Ah Lithium-ion batteries with energy density which we can assume for a moment to be in the ballpark of 150Wh/kg. That is the earthly spec, but due to the smaller gravity on Mars, the energy density would rise to a huge 395 Wh/kg. (This interesting but useless comparison uses force -weight, not mass, as the mass energy density would be the same on both planets.)
The last successful landing on Mars was in 1997 when the tremedously successful Pathfinder Lander and Sojourner Rover provided earthlings with astounding pictures of the surface of Mars from close-ups to panoramic horizons. As covered in the August 1997 issue of BD, the batteries of the Lander were rechargeable Silver-zinc chemistry built by BST Systems Inc. Silver-zinc has limited recharge capabilities of about 40 cycles. The Rover used 300 Wh of Lithium-thyonol chloride batteries which are not rechargeable. At the time of the design, had rechargables been used, the chemistry would have been Nickel-cadmium, which would have required charging during the cruise stage and necessitated a hard wire connection from Lander to Rover; thus Nickel-cadmium was not chosen. Despite the lack of rechargeability, Lithium thyonol chloride batteries did provide the Lander and Rover with operational time exceeding the 30 day requirements by operating from July 6th to September 27th.
Now with better information on the weather and sunlight conditions on Mars, the new look in batteries for interplanetary systems is Lithium-ion because of its high gravimetric energy density, limited self discharge and high cycle life. In the application for the Mars expeditions, the limiting case may no longer be shelf and cycle life but rather the ability of the photovoltaics (PV) to provide power. On the Martian surface, continuous 80 mile per hour blowing dust deposits on the PV surfaces to absorb light energy before energizing the PV junctions.
The Lander uses an 8 cell, 25 Ahr Lithium-ion battery which has undergone testing to confirm the ability to maintain charge during a 272 day cruise condition. Other testing showed the follow-on ability of the Lander battery to provide the entry, descent and landing power.
At 70 minutes before entering the Martian atmosphere, the cruise stage housing separates so the Lander Lithium-ionatteries are the primary source of power for the final sequences of entry and landing. Piecing together bits of information, it appears to BD that this Lander consists of 5 eight cell Lithium-ion batteries each delivering 25 Ah at a nominal 28 Volts. It would seem that, the use of Lithium-ion, without cycling would be a waste of one of its major capabilities, so topping-off after a seven month cruise seems appropriate. All these factors leads BD to believe that the Lander batteries were connected to the cruise stage PV which would allow greater peak power for radio transmissions and subsequent recharge availability just before Martian entry. Neither the battery manufacturer nor NASA could give us insight into this.
A minute before impact, the Lander will transmit conditions to the Mars Oddesy Spacecraft, circling above in an orbit since July of 2001. (The Oddesy generates power from solar panels and charges Nickel-hydrogen batteries built by EaglePicher, Inc which supply Oddesy when circling behind Mars in the eclipse of the sun.) Oddesy then relays this information to Earth. Once on the surface of Mars, Lander’s battery supplies power to deploy its solar panels and allows separation of the Rover.
Whatever the actual connectivity, the Lander batteries are on their own from the time the heat shields point forward to the Martian atmosphere. During the rest of this sequence, the Lander batteries must provide power for: navigation, sequential operations of separating the cruise stage hardware, deploying the parachute, deploying the heat shield, lowering the Lander on a 66 foot long tether, inflating the airbags, firing the retro rockets, cutting the tether and bouncing in the airbags on the surface. Batteries in both the Lander and Rover have withstood 50 g. impact testing to survive this bounce. Lander’s batteries must still be called on to power the motors to retract the airbags, a process intended to take an hour, then open the four petal shaped sides of the Lander, exposing Rover to the Martian atmosphere and a sky which will provide sunlight for Rover’s power needs.
With the petals open, the Rover is now open to the Martian atmosphere and takes center stage. The Lander has completed its mission; this is unlike the 1997 Pathfinder Lander which had to relay communications from its Rover to Earth. Now, Rover is a self contained system first deploying its solar panels to begin powering its setup tasks which will require several sols or Martian days, each of which lasts 24 hours, 39 minutes and 35 seconds. Of that time, only about 6 hours provide enough brightness for PV output.
With surface temperatures ranging from - 199 0F to +80 0F and averaging -64 0F, the Lithium-ion batteries need some thermal protection. During charge, the battery temperature must stay above +32 0F, and above -4 0F when discharging. Heat from the electronic components, electrical heaters, and eight radioisotope heaters keeps the batteries at or above minimum temperatures. The radioisotope heaters, consisting of plutonium dioxide and housed within ruggedized containers are not anticipated to create earthly radiation hazards in the event of a launch failure. Other radioactive sources used for instrument calibrations have relatively low melting temperatures and are anticipated to be widely dispersed in low concentrations if the launch fails. Despite such possibilities, radiological monitoring teams stand ready to evaluate, identify and safeguard any radiologic events.
This program which cost approximately $800 million dollars will ask each Rover to investigate Mars for 90 days or more. The combination of PV and Lithium-ion batteries will be responsible for providing power to the following:
The Panoramic Camera and two high-resolution stereo cameras
The Mini-Thermal Emission Spectrometer for mineral deposit composition
The Microscope imager for extreme close-ups of rocks
The Mossbauer Spectrometer to determine the composition of iron-bearing minerals
The Alpha particle X-Ray Spectrometer to determine the content of the rocks
The rock abrasion tool to break rocks
The magnet arrays to collect airborne dust
On June 10th the first successful launch in the Martian convoy was the Mars Express developed by the European Space Agency. It’s mission is to orbit Mars for two years carrying out a number of scientific experiments, relaying communications to Earth to linking its lander to Earth . Hitching a ride on the Mars Express, successfully launched by a Russian Soyuz-Fregat vehicle, is the British lander, Beagle 2, which is to separate from the Mars Express and land on the surface of Mars on December 26th which is Boxer Day in the U.K. Beagle 2’s purpose is to advance the knowledge about any past presence of life.
Both the Express and Beagle 2 have pioneering interplanetary Lithium-ion batteries for stored power. The Mars Express has a combination of 11.42 square meters of photovoltaic panels which are sized to receive lower intensity sunlight than do Earth satellites because of the greater distance from the Sun to Mars. During the Cruise and Approach stage, the PV panels will be deployed to provide power to the Express and top off the batteries. A drive mechanism maneuvers the PV panels to maximize sunlight capture. With an orbital mission requirement of 500 Watts, the panels are capable of producing 650 Watts when in orbit around Mars. These panels will also charge the Lithium-ion battery consisting of three 22.5 Ah packs, because in each orbit, the Express will require battery power when eclipsed by Mars over 1,400 times in its two year life.
The cover photo in the lower right corner is an artist’s concept of what the 66 pound Beagle 2 will look like in place, after plummeting though
the atmosphere, deploying a parachute and bouncing like a ball on airbags just as the Rovers will do. This cover photo is courtesy of the Beagle 2 project at www.beagle2.com. All rights reserved Beagle 2.
Once on the surface, the four photovoltaic panels will unfold to capture sunlight for powering the experiments. They are gallium arsenide with a germanium substrate and are covered with an 80 micron high performance cover glass. With daily additions of solar energy, the PV surface will hopefully remain sufficiently dust free to allow completion of its 180 day intended mission.
To provide broader working times and to allow activity at night, the Beagle has a 42-cell Lithium-ion battery, which if delivered successfully, will be the first Lithium-ion on the planet’s surface. These batteries, as well as the Mars Express, batteries are produced by AEA Technologies. Because AEA Technologies has been utilizing and supplying to orbital applications Lithium-ion batteries fabricated from a frozen 1995 design of a Sony hard carbon 18650 cell design, BD assumes that the Beagle’s battery uses this cell. The batteries are kept warm with a blanket made of gold-plated aluminium-tin materials.
The PV and battery power will be used for three cameras with a controllable pop-up mirror and a robotic arm to gather rock samples, strip away the outer layers and analyze the underlying materials. Two of the cameras are stereoscopic, and the third can be attached to a microscope to examine samples. Probably an interplanetary first is the inclusion of a ‘mole’ named ‘Pluto’ which can crawl up to 17 feet across the surface, then burrow to a depth of one meter. The mole capturessamples and returns them to the mother craft for analysis. Pluto includes a grinder supplied by a Hong Kong dentist to scrape away rock surface plaque.
Beagle 2 is both technologically amazing and adds another dimension, creative financing. The British government could only come up with $8 million to support the $40 million project, so Colin Pillinger, a planetary scientist at the Open University, pursued funding and obtained labor from universities, industry and research institutions. A list of supporting organizations can be found at http:/beagle2.open.ac.uk/people/people3c.htm Although the total cost is not known, Beagle 2 is on its way. Advertising on the Beagle and its devices such as the parachute could add a new dimension to the trendy look of space hardware. The Brits do not want to have it look like a Formula 1 car though. (What’s wrong with that? Isn’t everything from image, performance, sound and appearance cool in Formula 1? BD)
Nozomi and Nickel-metal hydride
Mars hospitality cannot be limited to only welcoming the Lithium-ion powered visitors because a fourth visitor with another power source is also on its way. Launched on July 4th, 1998 was the Japanese Mars orbiter, Nozomi (in Japanese, Hope). It is planned to arrive in early 2004 to study the Martian upper atmosphere with emphasis on its interaction with the solar wind.
Originally, Nozomi was to arrive on Mars on October 11th, 1999, but in swinging by Earth it did not attain the acceleration planned and subsequent engine burns left it short of fuel. At this point Nozomi’s goal was modified. The Nozomi was to remain in earth orbit for an additional four years to wait for a lower energy trip to Mars. During this time, powerful solar flares damaged the onboard communications and power systems. Hydrazine fuel froze, but later it thawed and put the craft on the correct trajectory to hopefully land on Mars in 2004.
Nozomi, which weighs only 541 kg, has 14 amazing onboard scientific instruments to measure the Martian weak magnetic field, analyze the composition of the upper atmosphere, sense temperature and plasma waves, take pictures of the Martian weather and count dust particles to determine if there is a dust ring in one of its two moons, Phobos.
With all the problems and disasters that have plagued Nozomi, it is a tribute to the power system for continuing to supply the spacecraft. Collecting solar power has been accomplished with two 3 segmented segment panels of silicon PV which supply the craft and charge its Nickel-metal hydride batteries, making this the first of that chemistry to head for Mars.
If Nozomi makes it to Mars’ orbit, the $848 million spacecraft will be operational for one Martian year which is equivalent to approximately two Earth years. If all goes well, an additional three to five years may be added to the mission.
So, as we settle into the calm of cruise stages for our four Martian visitors, heads will begin to perk as the year closes, and we all look anxiously forward to successful landings and experiments.