When the Curiosity rover landed on Mars last August, it carried a myriad of sophisticated devices to explore the planets surface and send information back to Earth. Instruments included an infrared laser, microscope, alpha-particle X-ray spectrometer, chemical analysis laboratories, sample analysis instruments, radiation detector, meteorological package, seventeen cameras and much more.
But one of the most critical components went unseen and unnoticed: namely, the grease that lubricated the wheels and articulated joints. Without a reliable grease that could withstand the severe environment of the Red Planet, Curiositys mission would not have been possible.
According to Keith Campbell, business development manager for Castrol Industrial Lubricants, a specialty grease called Castrol Braycote 601 EF helped ensure the smooth operation of Curiosity, from its wheels to its cameras. Campbell, whose team led the development of this technology, said, If its a moveable part, it most likely has Castrol Braycote 601 EF on it. In fact, the success of the Curiosity mission in part depends on the success of this grease, which has been formulated for the space program to perform in temperatures ranging from minus-80 to plus-204 degrees C.
Lubes in Space
Curiosity is the third generation of rovers NASA has sent to Mars, each larger and more instrument-packed than before. The first, 1997s Sojourner, was only about two feet long, and the next generation, Spirit and Opportunity, were about twice that size.
Curiosity was a big leap forward in terms of scale, and at 9.8 feet long and 8.9 feet wide, has a footprint closer to that of an SUV. It weighs just under a ton on Earth, and scuttles about on six independently driven 20-inch wheels that are cleated for traction on the sandy surface. After launching from Cape Canaveral on Nov. 26, 2011, it landed on Mars wind-scoured Gale Crater on Aug. 6. From here it continues to collect and transmit data on the Martian climate and geography, helping to answer the tantalizing question of whether the planet might support microbial life.
Curiositys lubricants face numerous severe challenges in space, including a wide range of environmental conditions. We start with the vacuum, which plays a big role in making a lubricant selection, said Campbell. Vacuum literally pulls the base oil out of the grease, leading to out-gassing, or the evaporative loss of the grease itself.
The issues with outgassing are twofold, he explained. First, because the oil is the most volatile part of the grease, vacuum can theoretically deplete it completely, leading to mechanical failure of the grease and the component it is protecting. Solids are not volatile, so remain within the grease structure.
Second, said Campbell, the material that outgasses can condense in other areas of the spacecraft such as the optics. These contaminants can become a major contaminant on instrumentation and can cause components to fail or operate erratically.
NASA measures outgassing according to the ASTM E 595 test, which evaluates the changes in mass of a test specimen exposed to 10-6 torr vacuum at 125 degrees C. (The torr is a unit of pressure named after Evangelista Torricelli, the 17th century inventor of the barometer.) The test also measures the mass of product that leaves the specimen and condenses on a collector at a temperature of 25 degrees C.
The simulation of the vacuum in space in this test method does not require that the pressure be as low as that encountered in interplanetary flight (which can be 10-12 Pascals or 10-14 torr). It is sufficient that the pressure be low enough that the mean free path of gas molecules is long compared to chamber dimensions.
Two parameters are measured in the test, Campbell said: total mass loss, or TML, and collected volatile condensable materials (CVCM). The amount of water vapor regained can also be measured after completing the exposures and measurements required for TML and CVCM. Typical upper limits are TML of 1 percent and CVCM of 0.10 percent.
It is not only the lubricity that this product provides that is important but also its low outgassing characteristic, said Campbell. The grease ensures that the many sensitive instruments and components on the rover can function as required, allowing Curiosity to investigate Mars, its atmosphere and its land.
Surviving Temperature and Dust
The other major environmental concern in space is operating temperature, which can be as low as minus-70 degrees C. Generally, a lubricant that works well at these cold temperatures is a low-viscosity, low-molecular weight fluid, Campbell said. However, you can have a very wide temperature range in those environments.
This means that the lubricants viscosity will be extremely low at higher temperatures, and outgassing can increase dramatically. It takes careful formulation to operate at the low temperatures, yet resist outgassing at the warmer temperatures.
Since space is so cold, it may seem that concerns about higher temperatures are unfounded. But exposure to the sun as well as equipment operation can warm the components sufficiently to reach potential outgassing temperatures. For example, on Mars, the average surface temperature is minus-20 to minus-40 degrees C, but around the planets equator during summer months, the temperature can reach up to 20 degrees C. In addition, spacecraft can be exposed to elevated temperatures during lift-off.
Curiosity can be exposed to a number of other conditions that affect operation. For instance, said Campbell, Mars experiences frequent dust storms, comprising a wide variety and size of dust particles. Even though the joints on the Mars rover generally are sealed well enough to resist dust, the lubricant still has to resist dust ingestion as a second line of defense.
Special Oil and Thickener
To get the needed combination of viscosity over a wide temperature range and low outgassing, Braycote 601 EF is formulated from a perfluorinated polyether oil thickened with polytetrafluoroethylene powder. The PFPE oil has a viscosity index of 350, which provides the wide temperature range required.
The grease, which has a paste-like consistency, is not cooked like conventional greases, Campbell said. Rather, it is manufactured by mechanical mixing to disperse the PTFE particles into the oil.
The grease also contains a corrosion inhibitor to protect components in the time before launch or during storage, where it can be exposed to high humidity. The grease is inert to acids, bases and oxidizers as well as rocket propellants. In addition, the nontoxic, nonflammable lubricant does not use any chlorofluorocarbons during manufacturing, so it is safer for Earths environment, too.
Inner and Outer Space
Campbell noted that this lubricant is frequently used in space applications, including the Space Shuttle, satellites and the International Space Station. But it is an excellent wide-temperature grease for space and vacuum manufacturing applications, such as the semiconductor industry where it is used in robots that handle wafers.
Campbell suggested that Braycote 601 EF should also be considered anywhere there are hostile chemicals or extreme environmental conditions that would preclude the use of other lubricants, such as the chemical process industry and air processing and handling.
Typical applications include ball and roller bearings, gears, and as an assembly lubricant for O-rings and elastomers. We have worked closely with NASA throughout the years to develop lubricants that meet the increasing challenges of space exploration, Campbell concluded.