Curiosity Needs to Ration Unlimited Power

Power is limited on Mars and solar source won’t cut it when continuous reliable power source is needed. That is why the Curiosity Rover is powered by a Radioisotope Thermal Generator (RTG), a nuclear powered electric generator powered by Plutonium dioxide in a ceramic form (Jet Propulsion laboratory (JPL), 2016). The decaying radioactive material creates heat which is turn into electricity using thermocouples, and the total efficiency of the system is about 6% electric power conversion (JPL, 2016). The RTG on Curiosity can generate around 110 watts of steady electric power and this resource needs to be manage and rationed throughout the day scheduling critical aspects of the mission (LaMonica, 2016). Curiosity’s RTG will last up to 14 years, but the power output at a time is limited, and available power needs to be schedule to operate the sensors, the locomotion, and the data transfer.

            One of the biggest resource hungry sensors is the Chemical and Camera or ChemCam mounted on the mast of Curiosity. The instrument utilizes a high power laser to burn rock and analyze the spectrometry to determine the material composition of the rock (Wiens, nd). The Laser Induce Breakdown Spectrometer (LIBS) needs more than 10MW/mm^2 power density resulting in a 14mJ laser impulse of 5 nanoseconds duration on a focus area of 0.3 to 0.6 mm in diameter (Wiens, nd). For a depth profile of 0.5 mm it needs to fire about 500 laser shots on the same spot this burns the material which light is capture by a demultiplexer onto a CCD light sensor which sends the data to a data processing unit communicating with the rover’s interface (Wiens, nd).
Left: Laser routine from initiation to firing. Right: Overview of logic and flow for the ChemCam. (Wiens, nd).

Curiosity is laboratory on wheels and it carries many sensors for scientific purposes and for navigation. The Mastcam is a stereo vision camera mounted on the Mast and is use for science the two camera sensors are focused at different distances and with different color filters for science while being able to retain 8 gigabytes of storage each (Malin, nd). Each camera can store more information that is possible to communicate to Earth so images a slightly compressed before downlink. The other cameras, the Navcams are black and white cameras located around the rover’s body and are used to detect objects during navigation. The Alpha Particle X-ray Spectrometer is a sensor that is deploy on the deck by the rover and gather 32 kilobytes data of elements on the atmosphere. The CheMin or Chemestry and Mineralogy instrument is a powder X-ray Diffraction instrument inside the rover’s body and is used to shoot a cobalt X-ray to a sample recovered by the rover and get data of its composition using a 2-d X-ray image which data also needs to be send back to Earth. There is the Radiation Assessment Detector (RAD) which measures radiation on Mars, the rover’s environment monitoring station (REMS) which monitors Mars weather, wind speed, direction, humidity and temperature, and the Dynamic Albedo of Neutrons (DAN) is a soil analysis used during the rover’s navigation over Mars. All the instrument on board Curiosity produce data that is processed on the rover and stored on the rover interface waiting to be communicated back to Earth.

The data collected by Curiosity is saved on board and there are two methods that could be used to relay back to Earth. The Deep Space Network (DSN) is a series of three antennas located in California’s Mojave desert, Madrid Spain, and near Canberra, Australia; these 37 yards in diameter antennas transmit on X-waves which are a lot more powerful band than FM radio waves (JPL, nd). Curiosity can speak directly with Earth from the surface of Mars sending data through the high-gain antenna (HGA) at a rate between 500 to 32000 bits per second which is half as fast a house internet modem, yet the benefit of this slower connection is the time window as Earth is visible for 16 hours to the Rover which can point the antenna to get a good connection with Earth (JPL, nd). The faster transmission rate of 2 million bits per second between the rover and the Mars Reconnaissance Orbiter, but this window is of 8 minutes a day which allows for a total data transfer between 100 to 250 megabits of data to the orbiter (JPL, nd). The Mars orbiter than can relay the data to Earth which would have taken Curiosity 20 hours to transmit directly to Earth (JPL, nd). Transmission to Earth also require a lot of power which limits the activities the rover can do while transmitting, so careful scheduling needs to be done in order to manage all the limiting factors, transmission window, data rate, available power, and importance of data.

References

Gannon, M. (2013, December 6). Zap! NASA’s Curiosity rover fires 100000th laser shot on Mars. Space. Retrieved from https://www.space.com/23851-mars-rover-curiosity-laser-100000th-shot.html
Howell, E. (2017, August 13). Mars Curiosity: Facts and Information. Space. Retrieved form https://www.space.com/17963-mars-curiosity.html
Jet Propulsion Laboratory (JPL). (nd). Mars Science Laboratory Curiosity Rover. California Institute of Technology. Retrieved from https://mars.nasa.gov/msl/mission/
Jet Propulsion Laboratory. (2016, October 13). Spacecraft ‘nuclear batteries’ could get boost from new materials. National Aeronautics and Space Administration. Retrieved from https://www.jpl.nasa.gov/news/news.php?feature=6646
LaMonica, M. (2012, August 7). Nuclear generator powers Curiosity Mars mission. MIT Technology Review. Retrieved form https://www.technologyreview.com/s/428751/nuclear-generator-powers-curiosity-mars-mission/
Malin, M. C. (nd). Mast Camera (MAstcam). Jet Propulsion Laboratory. Retrieved form https://msl-scicorner.jpl.nasa.gov/Instruments/Mastcam/
Wiens, R. C. (nd). Chemestry & Camera (ChemCam). Los Alamos National Laboratory. Retrieved from https://msl-scicorner.jpl.nasa.gov/Instruments/ChemCam/

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