Introduction to How the Mars Exploration Rovers Work

Mars arouses deep human curiosity for lots of different reasons:
  • There is the whole historical notion of little green men, or big scary aliens, coming to Earth from Mars.
  • There is the fact that Mars is the only other planet in the solar system that is even vaguely Earth-like, and therefore the only other planet where life might have evolved (another possibility is the ice moon Europa orbiting Jupiter, but that is not a planet).
  • Mars is the only planet in the solar system that human beings have any hope of landing on, walking on and exploring in a traditional sense any time soon.
  • Mars is the only planet that might be terraformed and turned into an Earth-like planet.
Mars Rover Image Gallery


Photo courtesy NASA
Mars, as seen from the Mars Exploration Rover Spirit: The two rovers' pictures are the most detailed images of the Mars surface ever seen. 
See more Mars Rover pictures


For these reasons and more, humans have sent over 30 different spacecraft to explore Mars. NASA's two Mars Exploration Rovers Spirit and Opportunity are the planet's most recent visitors.

In this article, you will have the chance to learn about these rovers and understand what they will be able to discover as they explore the planet Mars!



Why Not Send People?

The current mission to Mars involves a pair of robotic rovers that are known as the Mars Exploration Rovers (MER). Why are we sending robotic rovers rather than sending people like we did when we explored the Moon?


Photo courtesy NASA
An artist's rendering of a Mars Exploration Rover on the surface of Mars

We aren't really at the point yet where we can send human beings to Mars. The first and most important reason for that hesitation is our track record -- different nations have sent more than 30 probes toward Mars, but fewer than one-third of those probes have survived the trip. It's not a very good track record, and certainly not one that would encourage us to replace those robotic probes with human beings, at least until we've improved the odds of success.

The second reason is cost. As we will see in a moment, it is currently costing about a million dollars per pound to design and deliver a robot to Mars, and robots don't have to worry about complicated things like life support systems. Nor do robots have to worry about coming home -- something that adds a great deal of weight to a mission. Nor do robots require a soft landing on the surface of Mars. It would take a minimum of 100,000 pounds of vehicle, equipment, food and water to get a small team of people to Mars (each person, for example, will require 900 pounds (408 kg) or more of dehydrated food). At a million dollars a pound, that's $100 billion right there. And chances are that a manned mission would cost more per pound than a robotic mission because of the significant safety margins needed for human passengers.


Photo courtesy NASA
The Mars Exploration Rover Spirit, at NASA

The third reason is the engineering challenges. For example, to make a manned mission possible, one likely scenario is to produce fuel for the return flight from the Martian atmosphere. However, nothing like this has ever been attempted, and it would take a number of test missions to prove out the concept. Another big consideration is the cosmic radiation that astronauts would absorb during such a long mission, and how to block it. Much of this radiation is blocked on Earth by the Earth's magnetic field. Mars has no magnetic field.

So, we will not be sending humans to Mars any time soon. That leaves us with the option of sending robots instead. The MER robots are a manifestation of that philosophy.

Sticking the Landing


Photo courtesy NASA
NASA testing the Mars Rover air bags

One of the trickiest parts of the Mars Exploration mission is actually getting the rovers to Mars in working condition. Imagine trying to drop a sophisticated robot just 10 stories without breaking it (or even dropping an ordinary DVD player, for that matter). That's nothing compared to landing a rover on another planet.


Image courtesy NASA

So how did they do it? In this NASA video, the experts explain, using fantastic computer animation. The video file is big, so it will take a while to load, particularly on slower connections (but it's worth the wait).

Rover Capabilities

When NASA sent the twin Viking landers to Mars in the 1970s, they had the three basic components of any interplanetary robot:

  • They could produce the power they needed to execute their missions.
  • They could gather information with their sensors.
  • They could send the sensor information back to Earth.

The one thing the Viking landers could not do is move, although they did have robotic arms that could reach out and scoop up soil.

NASA first solved the movement problem with the Pathfinder mission in 1997. A tiny rover (just 25 pounds/11 kg) could leave the lander and travel up to 5 meters (15 feet) away from it to look at rocks.


Photo courtesy NASA
Two generations of Mars rovers

The MER robots are the largest rovers to ever successfully land on another planet. On this mission, NASA has designed the MER robots to act as robotic geologists. The instruments and equipment packed into the rovers are primarily designed to look at rocks.

Here's what each rover can do:

  • The rovers can generate power with their solar panels and store it in their batteries.
  • The rovers can take color, stereoscopic images of the landscape with a pair of high-resolution cameras mounted on the mast.
  • They can also take thermal readings with a separate thermal-emission spectrometer that uses the mast as a periscope.


Photo courtesy NASA
The rover's mast

  • Scientists can choose a point on the landscape and the rover can drive over to it. The rovers are autonomous -- they drive themselves, because the time lag for radio signals to travel between Earth and Mars is too great for the rovers to be radio controlled. Three pairs of black-and-white cameras on the front, back and mast of the rover let the robot see its surroundings and navigate around obstacles. The rovers have six wheels, with a motor in each wheel, to move around.


Photo courtesy NASA
The rover's wheels

  • The rovers can use a drill, mounted on a small arm, to bore into a rock. This drill is officially known as the Rock Abrasion Tool (RAT).


Image courtesy NASA
Artist's rendering of one of the rovers using the RAT on Mars


Image courtesy NASA

Photo courtesy NASA

  • The rovers have a magnifying camera, mounted on the same arm as the drill, that scientists can use to carefully look at the fine structure of a rock.
  • The rovers have a mass spectrometer that is able to determine the composition of iron-bearing minerals in rocks. This spectrometer is mounted on the arm, as well.
  • Also on the arm is an alpha-particle X-ray spectrometer that can detect alpha particles and X-rays given off by soil and rocks. These properties also help to determine the composition of the rocks.
  • There are magnets mounted at three different points on the rover. Iron-bearing sand particles will stick to the magnets so that scientists can look at them with the cameras or analyze them with the spectrometers.
  • The rovers can send all of this data back to Earth using one of three different radio antennas.

Rover Specs

To pack in all of this instrumentation, motorization and power generation, the rovers are pretty big -- perhaps the size of a small riding lawn mower. Here are the stats [ref]:


Photo courtesy NASA
Size comparison: the new Mars rover, some of the assembly team, and a rover predecessor, a duplicate of the Sojourner rover from the Pathfinder mission

  • 1.5 meters (4.9 feet) high (with the mast up)
  • 2.3 meters (7.5 feet) wide
  • 1.6 meters (5.2 feet) long
  • 174 kilograms (384 pounds)
  • Maximum speed: Perhaps 30 meters (about 100 feet) per hour, and 100 meters at most per day
  • Panoramic cameras: According to this page:

      Pancam is a multispectral, stereoscopic, panoramic imaging system consisting of two digital cameras mounted on a mast 1.5 m above the Martian surface. The mast allows Pancam to image the full 360° in azimuth and ±90° in elevation. Each Pancam camera utilizes a 1024 × 1024 active imaging area frame transfer CCD detector array. The Pancam optics have ... a field of view of 16° × 16°.

  • Cost: Approximately $820 million total (for both rovers)
    $645 million for design/development + $100 million for the Delta launch vehicle and the launch + $75 million for mission operations

Inside the Rovers

The body of a rover is an enclosed box called the Warm Electronics Box (WEB). This box is essential because temperatures at night can fall to -100 degrees C (-150 F). The batteries would stop functioning, as would many of the electronic components, if some warmth were not provided to get the temperature up toward 0 degrees C (32 F).


Image courtesy NASA

The WEB is an insulated box that contains:

  • The rover's computer brain
  • The lithium-ion batteries
  • The radios and amplifiers for the radios
  • Various pieces of control electronics for the different spectrometers, etc.


Image courtesy NASA
Rover components

Basically, anything that cannot survive at -100 degrees C is inside the box.

The box keeps warm through three different mechanisms:

  • When they are on, the different electronics modules produce their own heat. The computer, for example, can consume 7 watts and therefore produces heat like a 7-watt night-light bulb would.
  • There are small, 1-watt resistance heaters that the computer can turn on to raise the temperature.
  • Eight radioactive pellets (plutonium dioxide) give off heat as the plutonium atoms decay. The pellets are tiny -- the size of a pea. They are wrapped in a protective alloy and then in a carbon-fiber case. If the Delta launch rocket had blown up on the pad, or the spacecraft had re-entered earth atmosphere, these cases would protect the pellets.

The Onboard Computer
The rovers use a RAD6000 computer produced by BAE systems. This processor is nearly identical in architecture to an old PowerPC processor used in early Macintosh computers. By today's standards, these processors are slow. They run at 20 megahertz, about 1/100th the speed of a typical desktop computer today. They have 128 kilobytes (KB) of RAM, 256 KB of flash memory and some ROM to hold the boot code and operating system. There are no disk drives.

Although they are slow and incredibly expensive ($200K to $300K per computer), they have two big advantages:

  1. They are radiation-hardened so they are immune to the cosmic radiation falling on Mars.
  2. They run the ultra-reliable VxWorks (PDF) real-time operating system from Wind River Systems.

This computer makes the rover that much more reliable than a typical desktop computer because it is never crashing or corrupting data.

The computer helps with power management, image processing, motor control, and instrument management. It also handles navigation. The rover has six navigation cameras arranged in three pairs. The computer processes stereo images from the camera pairs. It uses binocular vision algorithms, and it can identify the distance to and size of the different rocks in the field of view. Using this information, the computer can build a map of all the nearby obstacles and then maneuver the rover to avoid them when it is moving.

Power
The rovers have 1.3 square meters (14 square feet) of high-efficiency solar cells to provide power. When the panels are first unfolded by the rover, they are clean, and at noon the Sun is "strong" by Martian standards because of the season. The panels produce about 140 watts peak, or about 900 watt-hours total per day (with this much power, you could run a single 100-watt light bulb for nine hours). In other words, the Sun is bright enough to activate the solar panels for only about six hours per Martian day.

The power from the solar panels goes to the devices that need it (computer, motors, RAT, instruments, radios, etc.). Any excess power is stored in two 28-volt, 10-amp-hour, lithium-ion batteries.


Photo courtesy NASA
One of the first images from the mission, showing a rover's rear lander pedal and the Martian horizon

Robotic Exploration

Let's say scientists on Earth have chosen a rock and gotten the rover near enough to it to reach it with the rover's arm. This Cornell News article (December 19, 2003) describes what happens next:
    Billions of years of exposure to the Sun, atmosphere and extremely fine Martian dust has given Mars rocks a weathered "rind," or exterior layer. The RAT, part of the science-instrument package carried by the two Mars rovers, Spirit and Opportunity, uses a diamond-tipped robotic grinding tool to scrape away this weathered exterior, revealing a fresh surface.

    Access to the pristine rock interior is critical to understanding the history of the geology of Mars and to answering what Bartlett describes as the "big questions" to be solved by the rovers: Did water -- or even an environment suitable for life -- once exist on the red planet?


    Photo courtesy NASA

    These big questions might be answered by a very small machine: The RAT weighs only 1.5 pounds (0.68 kg) and uses less power (only 30 watts) than most light bulbs. It is about the size of a soda can.

    The RAT occupies the turret, or "hand," of the rover's robotic arm, along with other science instruments for rock analysis, a microscopic imager, and Mössbauer and alpha-particle X-ray spectrometers. The agile arm, which has shoulder, elbow and wrist joints just like a human arm, presses the RAT up against a rock's surface.

    In just two hours, the RAT's grinding wheel can shave off a disc about twice the diameter and thickness of a nickel from a hard rock surface. Two brushes sweep the resulting dust away from the hole to provide a clean surface for an up-close view.

    Once the fresh surface is exposed, the imager and the spectrometers take over, peering through the abraded opening to perform a detailed analysis of the rock's interior. In order for scientists to learn about the processes that might have weathered the rock, the rover also records temperature and current readings from the RAT's three motors while they grind away the exterior layer.

Communication

As the rovers collect data, they need to send it back to Earth. Data includes photos, spectrometer information, system-status information and so on. In addition, scientists and engineers on Earth want to send things like commands and software updates to the rover. The rover has three different radios to handle communications.


Photo courtesy NASA
You can see each of the rover's three antennas in this image: UHF, Low Gain and High Gain.

The first radio is a low-power, slow UHF radio. This link uses a low-gain, omni-directional antenna. It does not require any aiming, and it transmits back to Earth or to a satellite at a low data rate. It is an "if all else fails" way to communicate.

The second radio is a high-speed UHF radio, and it communicates with two satellites already in orbit around Mars: The Mars Odyssey satellite and the Mars Global Surveyor satellite. When a satellite appears overhead and signals the rover, the rover can dump data to the satellite at high speed for perhaps eight minutes on each pass. The rover can send data at 128 kilobits per second when the satellite is overhead, using a radio that consumes 15 watts. The satellite can then forward the information to Earth when it comes into view using its 2.5-meter (2.7-yard) antenna and 100-watt radio. This is how most image data gets back to Earth. Perhaps 10 megabytes of data per day can get back to Earth through these channels.


Photo courtesy NASA
Overhead view of Spirit on the surface of Mars

Finally, there is a 1-foot-diameter (.3 meter) directional (high-gain) antenna on the rover. When the Earth is visible to the rover, the rover's antenna tracks the Earth and can communicate directly to scientists and engineers. There is a 20-minute round-trip delay because of the 200-million-mile (322-million-km) distance between Earth and Mars. The rover uses a 40-watt radio, and it transmits at only 12 kilobits per second over this link. Because it is a direct link, NASA uses it to send commands to the rover and to get critical data back. This link is only available for about three hours per day because of the alignment of the planets and the power requirements of the radio.

A Day in the Life of a Rover

During a typical day, each rover sends image, instrument and status data to Earth. Scientists make decisions based on that data and data from the previous day. Then they send commands to the rover during the three-hour window of direct communication with the high-gain antenna. The rover is then on its own for 20 hours, carrying out commands and sending image data to the two overhead satellites. The rover's commands may tell it to move toward a new rock, grind a rock, analyze a rock, take photos or gather other data with other instruments.


Image courtesy NASA
Artist's rendering of a rover at work

The rover and the scientists repeat this pattern for perhaps 90 days. At that point, the rover's power will start diminishing. Also, Mars and Earth will be getting farther and farther apart, making communication more difficult. Eventually, the rover will not have enough power to communicate, or will be too far away, and the mission will be over.

For more information on the Mars Exploration Rovers and related topics, check out the links on the next page.

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