Curiosity on Mars
[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]

The Escalade of Mars Rovers
Written and edited by Bob Keller

Curiosity Rover at Work

Curiosity, The Escalade of Mars Rovers

Curiosity was named by 6th grade student Clara Ma, who entered a NASA sponsored essay contest providing kindergarten through high-school students with the opportunity to suggest names for the Mars Science Laboratory rover. Nine finalist rover names were then ranked by the general public through a poll conducted on the NASA website in 2009, selecting "Curiosity" as the winner.

The Curiosity Mars Rover represents a significant leap forward in size, complexity and capability over its predecessors Sojourner, which landed on Mars in 1997, and the twin rovers Spirit and Opportunity, which landed on Mars in 2004. Compared to Sojourner, Spirit and Opportunity, Curiosity is the Escalade of Mars rovers.

Curiosity compared to Sojourner and Spirit/Opportunity

Above, two spacecraft engineers stand with a group of vehicles providing a dramatic comparison of three generations of Mars rovers. In the foreground is the flight spare for the first Mars rover, Sojourner. On the left is a test rover that is a working sibling to Spirit and Opportunity. On the right is a Mars Science Laboratory test rover the size of Curiosity. Sojourner is 2 feet long and weighs 23 pounds. Spirit and Opportunity are 5.2 feet long and weigh 408 pounds. Curiosity is 9 feet, 10 inches long not counting its robotic arm and weighs 1,982-pounds.

The 10 science instruments aboard Curiosity have a combined mass of 165 pounds, compared with a five-instrument science payload totaling 11 pounds on each of the twin rovers Spirit and Opportunity. The mass of just one of Curiosity’s 10 instruments, 88 pounds for Sample Analysis at Mars, is nearly four times the total 23 pound mass of the Sojourner rover.

Curiosity's entry, landing and descent system consisting of its aeroshell and fueled descent stage weighed an additional 5,293 pounds. Curiosity stands 7 feet tall at the top of its mast, and is equipped with a 7 foot long robotic arm for exploring, manipulating objects of interest and delivering geologic samples to its scientific instruments for analysis.

RAD750 CPU At the heart of Curiosity's on-board computer system, called the Rover Compute Element (RCE), are twin computers utilizing BAE RAD750 radiation-hardened central processors. Curiosity's computers must utilize special, radiation hardened components, as ordinary electronics would quickly fail in interplanetary space or in the harsh conditions of the Martian surface, where the thin Martian atmosphere fails to shield rovers from high radiation levels. RAD750 CPUs are capable of absorbing a total radiation dose of up to one million rad - exposures of 500 to 1000 rad are almost invariably fatal to human beings.

Curiosity's RAD750 CPUs operate at 200 megahertz with 2 gigabytes of flash memory (10 times the speed and 8 times the memory of Spirit's and Opportunity's prior generation RAD6000 computers) and are capable of executing more than 400 million instructions per second. One of Curiosity's two on-board computers is configured as a live backup and will take over the tasks of navigation, control, and communications in the event of problems with the primary computer.

Curiosity is controlled by resident software running under VxWorks, a proprietary real-time operating system designed for use in embedded systems that features a multitasking kernel with preemptive and round-robin scheduling and fast interrupt response. Other spacecraft running VxWorks include Clementine, the Mars Reconnaissance Orbiter, the Phoenix Mars Lander, the Deep Impact probe, and the Sojourner, Spirit and Opportunity Mars rovers.

Curiosity's computer system is constantly self-monitoring and executing a main control loop to keep the rover operational. The main control loop essentially keeps Curiosity 'alive' by constantly checking itself to ensure that it is both able to communicate throughout the surface mission and that it remains within nominal operational parameters and thermally stable at all times. It does so by periodically checking temperatures, particularly in the rover body, and responding to potential overheating conditions, recording power generation and power storage data throughout the Mars sol, and scheduling and preparing for communication sessions. Curiosity's on-board computer system additionally served double duty as the vehicle flight computer while Mars Science Laboratory was traveling from Earth to Mars.

Unlike its predecessors which relied on solar cell charged batteries for electrical power, Curiosity is equipped with a radioisotope thermoelectric generator. Radioisotope power systems are generators that produce electricity from the natural decay of a non-weapons-grade form of plutonium-238 used in power systems for other NASA spacecraft. Thermocouples are employed to convert heat released by the natural decay of this isotope into electricity, providing constant power during all seasons and through the day and night.

Curiosity is powered by a latest generation radioisotope power system, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), developed by NASA and the US Department of Energy for use in a variety of space missions. The MMRTG contains a total of 10.6 pounds of plutonium dioxide in ceramic form that initially provides approximately 120 watts of electrical power from 2000 watts of thermal power. Curiosity's MMRTG generates 2.5 kilowatt hours of electrical power per day, more than 4 times the 0.6 kilowatt hours per day generated by Spirit's and Opportunity's solar panels. Additional advantages of Curiosity's radioisotope power system are that the rover's power will not be impaired and degraded by the accumulation of dust on solar panels, as were its predecessors, and the capability to explore latitudes that are too high to efficiently use solar power.

Curiosity's Multi-Mission Radioisotope Thermoelectric Generator Multi-Mission Radioisotope Thermoelectric Generator Cutaway Model

Shown above left is Curiosity's Multi-Mission Radioisotope Thermoelectric Generator. Above right is a cutaway model of the MMRTG. The vertical red blocks in the center represent individual heat source modules and the white fins on either side are heat radiators.

The MMRTG is also crucial for the rover's thermal stability. Waste heat from the unit is circulated via pipes throughout the rover system to keep instruments, computers, mechanical devices and communications systems within their operating temperature ranges. This system-wide thermal control does not draw on the rover's electrical power, and precludes the need for radioisotope heater units for spot heating. The MMRTG is capable of powering Curiosity at optimum levels for a minimum of 14 years as its plutonium fuel decays. At 14 years the MMRTG's electrical power output will be reduced approximately 15% to 100 watts. Preceding generations of radioisotope generators have been used in over 25 NASA missions. Voyager 1 and 2, launched in 1977, employ radioisotope generators, and are still returning data at distances much farther away from Earth and the Sun than Pluto.

Curiosity is equipped with triple telecommunication redundancy with several means of communication including an X band transmitter and receiver that can communicate directly with Earth at speeds up to 32 kbit/s, and a redundant pair of UHF Electra-Lite radios for communicating with the Mars Odyssey, Mars Reconnaissance and Mars Express orbiters at speeds up to 2 mbit/s. Communication with orbiters is expected to be the main path for data return to Earth, since the orbiters have both more power and larger antennas than the lander, and the Mars Odyssey orbiter is capable of relaying UHF telemetry back to Earth in real time. However, each orbiter is only able to communicate with Curiosity for about 8 minutes per day. The exact radio signal propagation time varies with the distance between Earth and Mars and required 13:46 minutes at the time of landing.

Curiosity Rover Undergoing Mobility Tests

Tread Pattern in Curiosity's Wheels Curiosity will roll about on six independently motored, 20 inch diameter wheels while providing mobility for its robotic arm and 165 pound payload of scientific instruments.

The tread pattern of each tire contains groups of openings which function as visual odometry markers. Rear view imagery of the repeating pattern these markers create in the Martian soil serve to help the rover and its human operators determine how far the rover has actually moved, the amount of slippage it is experiencing, and the softness and cohesion of the traversed terrain.

It would appear the JPL engineers responsible for the design of Curiosity's tire tread are not without a sense of whimsy - or beyond making their mark on Mars in a literal way. A close inspection of the odometry marker openings in the tread reveals the sequential patterns followed by followed by . Which is Morse code for "J", "P", "L"...

Curiosity is capable of rolling over obstacles approaching 30 inches in height and has a maximum terrain-traverse speed of approximately 300 feet per hour by automatic navigation. Average traverse speeds will likely be about 100 feet per hour, based on variables including power levels, terrain difficulty, slippage, and visibility. The rover is expected to traverse a minimum of 12 miles during its two-year primary mission.

Curiosity's Camera Locations

Curiosity is equipped with a plethora of cameras - 17 in all. The above illustration shows the locations of the cameras on Curiosity. The rover's mast features seven cameras: the Remote Micro Imager, part of the Chemistry and Camera suite; four black-and-white Navigation Cameras (two on the left and two on the right) and two color Mast Cameras (Mastcams).

There is one camera on the end of a robotic arm that is stowed in this illustration; it is called the Mars Hand Lens Imager (MAHLI). There are nine cameras hard-mounted to the rover: two pairs of black-and-white Hazard Avoidance Cameras in the front, another two pair mounted to the rear of the rover, and the color Mars Descent Imager (MARDI).

The Mastcams will take color images, three-dimensional stereo images, and color video footage of the Martian terrain and have a powerful zoom lens. Like the cameras on the Mars Exploration Rovers that landed on the Red Planet in 2004, the MastCam design consists of two duplicate camera systems mounted on a mast extending upward from the Mars Science Laboratory rover deck.

The Laser-Induced Remote Sensing for Chemistry and Micro-Imaging (ChemCam) will fire a laser and analyze the elemental composition of vaporized materials from areas smaller than 1 millimeter on the surface of Martian rocks and soils. An on-board spectrograph will provide unprecedented detail about minerals and microstructures in rocks by measuring the composition of the resulting plasma - an extremely hot gas made of free-floating ions and electrons.

The Mars Hand Lens Imager is the equivalent of a geologist's hand lens and will provide close-up views of the minerals, textures and structures in Martian rocks and the surface layer of rocky debris and dust. With this new device, earthbound geologists will be able to see Martian features smaller than the diameter of a human hair.

[The Landing: Another Small Step for Man - A Giant Leap for Robot Kind]    [The Landing Site: Geological Jackpot at Gale Crater]   
[Curiosity: The Escalade of Mars Rovers]    [The Science Payload]    [References and Further Reading]

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Bob Keller