Sunday, November 18, 2007

Why a Rover? Aren't They Already on Mars?

Indeed, NASA has so far successfully operated three rovers on the surface of Mars. The first one, Sojourner was part of the Pathfinder mission. Sojourner, the rover, operated on the surface of Mars for 83 Sols (Martian days) and returned 2.6 billion bits of information, including more than 16,000 images from the lander and 550 images from the rover, as well as more than 15 chemical analysis of rocks and extensive data on winds and other weather factors. Even though science data ranged from geology and geomorphology through mineralogy, geochemistry, orbital dynamics, atmospheric science, astronomy and orbital dynamics - the mission was most importantly set to test novel approaches to the exploration of Mars. Most notably it used successfully a parachute and an array of airbags to accomplish a smooth landing. This approach coupled with a wealth of engineering data opened the path for the next rover mission to Mars.

Scientific highlights of the Mars Pathfinder mission:

* Martian dust includes magnetic, composite particles, with a mean size of one micron.
* Rock chemistry at the landing site may be different from Martian meteorites found on Earth, and could be of basaltic andesite composition.
* The soil chemistry of Ares Vallis appears to be similar to that of the Viking 1 and 2 landing sites.
* The observed atmospheric clarity is higher than was expected from Earth-based microwave measurements and Hubble Space Telescope observations.
* Dust is confirmed as the dominant absorber of solar radiation in Mars' atmosphere, which has important consequences for the transport of energy in the atmosphere and its circulation.
* Frequent "dust devils" were found with an unmistakable temperature, wind and pressure signature, and morning turbulence; at least one may have contained dust (on Sol 62), suggesting that these gusts are a mechanism for mixing dust into the atmosphere.
* Evidence of wind abrasion of rocks and dune-shaped deposits was found, indicating the presence of sand.
* Morning atmospheric obscurations are due to clouds, not ground fog; Viking could not distinguish between these two possibilities.
* The weather was similar to the weather encountered by Viking 1; there were rapid pressure and temperature variations, downslope winds at night and light winds in general. Temperatures were about 10 degrees warmer than those measured by Viking 1.
* Diversity of albedos, or variations in the brightness of the Martian surface, was similar to other observations, but there was no evidence for the types of crystalline hematite or pyroxene absorption features detected in other locations on Mars.
* The atmospheric experiment package recorded a temperature profile different than expected from microwave measurements and Hubble observations.
* Rock size distribution was consistent with a flood-related deposit.
* The moment of inertia of Mars was refined to a corresponding core radius of between 1,300 kilometers and 2,000 kilometers (807 miles and 1,242 miles).
* The possible identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water, during a warmer past in which liquid water was stable.

Following the path of the Pathfinder mission was the Mars Exploration Rovers mission which landed two rovers (Spirit and Opportunity) in January 2004 on opposing sides of the planet. Although designed to operate for 90 Sols, both rovers continue to operate today almost 4 years into the mission. These rovers were designed as robotic geologists, with the primary mission being the search for water or signs of past water on the surface of Mars. The big science question for the Mars Exploration Rovers is how past water activity on Mars has influenced the red planet's environment over time. While there is no liquid water on the surface of Mars today, the record of past water activity on Mars can be found in the rocks, minerals, and geologic landforms, particularly in those that can only form in the presence of water. That's why the rovers are specially equipped with tools to study a diverse collection of rocks and soils that may hold clues to past water activity on Mars.

A third roving mission to Mars, the Mars Science Laboratory, is scheduled for launch in 2009 and arrive on Mars in 2010. This mission will conduct more complex science and it will have a range of operation of up to 20km from the landing site.

So why more rover science? While we have learned from these missions their ultimate purpose is to open the possibility of a future human mission to Mars, and surely one day to the inevitable colonization of the planet and further exploration through the Solar system and beyond. A human mission to Mars will no doubt rely on robotic vehicles to accomplish science and mission goals, and will in this interaction minimize some of the risks associated with crew's extravehicular activities (EVAs). Having humans and robotic vehicles interact on the surface of Mars changes many of the aspects of rover operation. Normally in purely robotic missions the planners need to factor in the communication delay which can range from 6.5 to 44 minutes for a round trip of commands and responses from the rover. When the crew is on Mars they will have the advantage of being able to operate the vehicles autonomously or in real time and so new scenarios for their use become possible. Moreover these type of human rover interactions will likely take place on return missions to the Moon as well. Therefore one of the primary roles for the NorCal rover operating at the MDRS alongside mission teams will be to explore science and mission operation scenarios where both are involved. The rover has enough computational power, navigational and communications capability and science instrumentation to model a good number of situations. In addition it's twin will be used by mission support teams as a sandbox model available for testing in advance of procedures and program changes suggested or requested by the MDRS crew thereby adding to the realism of the analog environment. Also, while this rover in itself will not test the particular hardware which may one day end on such a voyage it will nonetheless be able to test a good number of design and software architecture concepts which may eventually get incorporated into the actual mission.

Wednesday, November 7, 2007

Rover Selection Criteria

It took the group a while to turn the general plan of developing a tele-operated vehicle to support crews in a Mars analogue environment into a more detailed set of constraints. We agreed on a set of goals; augment the capabilities of the research crews, with a tool that is flexible and on which we can test different software architectures. The point being on providing standardized interfaces so teams with various backgrounds can develop various functional modules - i.e. a fancy word for adding different science capabilities to the same vehicle. The focus being on using the vehicle to train the future space explorers by having students and teachers devise and use the rover for scientific goals without worrying about the underpinning technical details. Not unlike a relation scientist have with supporting industries today. Rather then designing the rover from scratch we decided the interesting parts are on the interface between the the rover and the science - trading hardware development (which is interesting in itself but better done by people who already have experience in the field) for a platform rich enough to enable development of interesting software and algorithms. Nonetheless, selecting the right hardware in light of deployment in difficult terrain, but also keeping in mind that it needs tobe familiar to students and teachers we devised the following constraints to select the rover.

Top five criteria:

1. within budget
2. tele-operational, upgradeable to autonomous
3. reasonably all-terrain
4. video camera/wifi
5. flexible control unit

Additional specifications:

All terrain should include protection from fine dust and sand and somewhat rain-proof with insulated wires, robust. With one of them we are going into a serious environment, the rover needs to cope with it.

GPS with easy upgradeability. While the first cut is tele-operation, science objectives and future autonomous mode will greatly benefit from a gps unit.

Wifi unless not available, then radio control.

Video camera on a mast, start with one, write software for two, capable of mono or stereo.

Flexible control unit so rover programming is accessible to students and teachers.

Manipulator capability, arm with one degree of rotation

Laser ranger or other ranging hardware in order to keep the rover safe so we are able to give wide access to it yet avoid hazards in the field.

Build or purchase a storage house for the vehicle that can be located away from the hab and has easy open doors that will not need human intervention.

Power management that will allow vehicle to be fully charged for 2-4 hours, go out, come back, and charge itself, possibly shut down while driving to the goal

With this in the back of our minds we started the selection process. At the end, and after comparing side by side more than half a dozen manufacturers that were close to our budget we converged on Senseta's MAX series. The other two very close contenders were Pioneer P2AT and the Segway robotic platform. All of these were good contenders but after weighing primarily the cost and the other factors, we decided on Senseta. It is a good platform, and uses a regular small form factor PC capable of running widely available tools and hardware. If Apple ][ in its days was a great gift for schools, we think this rover could do the same for schools today, yet it is tested in the field and can perform a multitude of science goals at an attractive price. Being a standard PC it is easy to distribute development among teams not having direct access to it as it supports a decent simulation environment in software. Moreover, Senseta is closely affiliated with the robotics group at CMU which had some impressive record of innovation lately, and they were very enthusiastic to provide the initial support to get our group of the ground.