Spirit is tiptoeing ever so carefully down the north edge of the elevated volcanic plateau known as "Home Plate." Having completed a 4-centimeter (1.6-inch) drive on sol 1463 (Feb. 13, 2008), the rover's current northerly tilt is 27.1 degrees. Spirit's handlers plan to have the rover drive another 4 centimeters on sol 1464 (Feb. 14, 2008).

They expect Spirit to be at the rover's final winter perch by the end of next week, following a few more 4-centimeter drives. Given recent progress, Spirit may achieve a northerly, Sun-facing tilt of 30 degrees, higher than originally anticipated. Spirit remains healthy.

Sol-by-sol summary

In addition to measurements of atmospheric dust levels with the panoramic camera and daily communications activities, which include morning direct-from-Earth uplinks over the rover's high-gain antenna and evening relays to Earth via the UHF antenna on the Mars Odyssey orbiter, Spirit completed the following activities:

Sol 1457 (Feb. 7, 2008): Spirit surveyed the Martian sky and ground using the miniature thermal emission spectrometer, stowed the rover's robotic arm, and drove downslope.

Sol 1458: Spirit acquired images of the dune field known as "El Dorado," surveyed the Martian horizon, and took spot images of the sky for calibration purposes using the rover's panoramic camera. Spirit acquired movie frames in search of dust devils using the navigation camera.

Sol 1459: Spirit drove 12 centimeters (4.7 inches) further down the north edge of Home Plate. The rover surveyed the sky and ground with the miniature thermal emission spectrometer.

Sol 1460: Spirit surveyed the sky and ground using the miniature thermal emission spectrometer and took thumbnail images of the sky with the panoramic camera.

Sol 1461: Spirit acquired mid-field and far-field images using the panoramic camera.

Sol 1462: Spirit acquired a second set of mid-field images as well as thumbnail images of the sky using the panoramic camera.

Sol 1463 (Feb. 13, 2008): Spirit drove another 4 centimeters (1.6 inches) downslope. Using the panoramic camera, Spirit surveyed the horizon, monitored changes in the El Dorado dune field, and took spot images of the sky.

Odometry

As of sol 1463 (Feb. 13, 2008), Spirit's total odometry was 7,528.01 meters (4.68 miles).

Spirit is responding to declining winter power levels by dipping into the batteries on one sol (Martian day), then recharging them on the next. In addition, one of the first things the rover did to conserve energy was eliminate afternoon data transfers to the Odyssey orbiter on days when the batteries were being recharged. Though this approach meant the rover wouldn't be able to send data on those sols, it saved enough energy to be worth the sacrifice. As Martian winter deepens, it's possible that, as the Opportunity rover did during the depth of the dust storm a few months ago, Spirit will use only every third Odyssey pass or adopt even more stringent power-conserving measures.

Meanwhile, Spirit remains healthy. For the most part, all subsystems are performing as expected. Energy has been holding steady in the range of 260 watt-hours. Tau (atmospheric opacity) continues to decline slightly, but the dust factor (the ability of sunlight to penetrate dust on the solar panels) also continues to decline slightly.

The winter Sun continues to dip lower in the sky. As of sol 1450 (Jan. 31, 2008), the noontime Sun was 64.6 degrees above the northern horizon, indicating that the optimal northern tilt of the rover deck would be 25.4 degrees from vertical (90 - 64.6 = 25.4). Spirit's current northward tilt is 22 degrees, relatively close to optimal. By mid-February, the optimal tilt will increase to about 28 degrees, the maximum Spirit is anticipated to be able to achieve. That's about the time when rover drivers plan to have the rover complete one more short drive to get into position for the winter.

The orbiting Odyssey spacecraft continues to provide a relay from the rovers to Earth. Each rover sends data to Odyssey via a UHF link. (UHF is "Ultra High Frequency" and represents the same band of frequencies used by the UHF channels on a TV set, channels 14-83, which transmit signals at roughly 400-500 megahertz.) Odyssey stores the data until it can transmit it to Earth using the X-band link. X-band is a microwave frequency that operates at roughly 7-8 gigahertz (megahertz and gigahertz refer, respectively, to millions of cycles per second and billions of cycles per second).

Originally, the Mars rover mission was intended to be an X-band mission, with the rovers using their X-band transmitters to return data directly to Earth. The UHF radio was intended to serve as a backup. But because relay operations via Odyssey proved wildly successful, Odyssey became the preferred avenue for returning data. For one thing, the UHF link can accommodate data at either 128 kilobits per second or 256 kilobits per second (in these cases, "kilo" actually means 1,024 rather than the usual 1,000). The highest X-band data rate for the rovers is only 28.4 kilobits per second, and most of the time, even that rate is unachievable. Odysssey, however, can return data at up to 124.4 kilobits per second using the X-band link. In fact, the lowest rate of data transmission from Odyssey is the same as the highest rate for the rovers.

During the past week, Spirit twice conducted a "grind scan" procedure with the rock abrasion tool. After the tool's encoder failed some time ago, engineers redesigned how the tool is used. Part of that redesign includes doing a "scan" before the rover grinds or brushes any surface (except, of course, that Spirit is no longer grinding into rock surfaces -- after performing more than 10 times longer than expected and contributing a great deal of science data to the mission, the grind bit wore out.)

During the "scan" procedure, the rover pushes the brush and bit into a targeted surface until it measures an increase in electrical current, which implies contact with the rock surface. Detecting the exact location of a possibly irregular surface is important for safely operating the rock abrasion tool.

The only reason Spirit is using the rock abrasion tool at this time is that the rover has moved to a new location as of sol 1440 (Jan. 21, 2008). As Spirit inches down the north-facing slope of "Home Plate," the rover will sample each new location for insight into how the rocks change and how Home Plate was formed.

As Spirit is stationed on a fairly steep slope, engineers have been concerned that moving the robotic arm might cause the rover to slip. The arm is very light and represents only a percent or two of Spirit's weight, but on a slope, it's always possible that even a slight shift could cause a slide. On top of that, the "grind scan" function requires the rover to press down on ("preload") the target. The pressure isn't much, only 15 newtons (not quite 3.5 pounds), but that, too, could conceivably cause a slip. The rover's handlers have created a plan that includes some safety checks.

The first time around, based on receipt of erroneous data, they had to double-check that the slope beneath the rover didn't exceed 20 degrees, the maximum slope previously allowed. Because Spirit was already tilted 22.4 degrees, the safety check tripped immediately and the rover did not conduct any of the planned activities with the rock abrasion tool. Rover handlers re-planned the activities and successfully completed the "scan" procedure on sol 1448 (Jan. 29, 2008).

The next step will be to actually brush the surface. But that's a story for another week.

Sol-by-sol summary

In addition to receiving morning instructions directly from Earth via the high-gain antenna, measuring atmospheric dust levels with the panoramic camera, and checking for drift (changes in time) in the miniature thermal emission spectrometer, Spirit completed the following activities:

Sol 1445 (Jan. 26, 2008): Spirit conducted a grind scan procedure at a target called "Freeman." Spirit took high-resolution images with the navigation camera to verify on-board software measurements of rover slippage based on comparison of before-and-after stereo images of the terrain (the software is known as the visual odometry system).

Sol 1446: Spirit re-took part 12 of a mosaic of images of the rover deck and took thumbnail images of the sky with the panoramic camera. The rover acquired data from rock targets known as "Fuzzy_Smith_2" and "Winston_Gaskins_2" using the miniature thermal emission spectrometer.

Sol 1447: Spirit monitored dust accumulation on the rover mast, completed a survey of rock clasts with the panoramic camera, and surveyed the external calibration target, the sky, and the ground with the miniature thermal emission spectrometer.

Sol 1448: Spirit took spot images of the sky and surveyed the horizon with the panoramic camera and scanned the sky for clouds with the navigation camera. The rover monitored changes in the distant dune field known as "El Dorado" with the panoramic camera. Spirit conducted a grind scan at Freeman with the rock abrasion tool. Spirit took high-resolution images with the navigation camera to verify on-board software measurements of rover slippage based on comparison of before-and-after stereo images of the terrain.

Sol 1449: (Jan. 30, 2008): Spirit surveyed the sky and ground with the miniature thermal emission spectrometer and acquired movies in search of dust devils using the navigation camera. The following day, plans called for the rover to survey the external calibration target using the miniature thermal emission spectrometer.

Odometry

As of sol 1448 (Jan. 29, 2008), Spirit’s total odometry remained at 7,527.83 meters (4.68 miles).

With only about a month remaining before Opportunity's fourth anniversary (in Earth years) of Mars exploration, NASA's robotic geologist is still grinding into the surface of rocks to unlock the secrets of their interior chemistry. Meanwhile, fall arrived in the southern hemisphere of Mars on Dec. 9, 2007, Opportunity's 1,378th Martian day, or sol, of exploration of the Red Planet. Ten days later, Earth made its closest approach to Mars, coming within 88 million kilometers (54.8 million miles).

Opportunity used the rock abrasion tool to bore a shallow hole into a rock target known as ''Lyell_1'' and then spent about 70 hours integrating data about iron minerals inside the rock using the Mossbauer spectrometer.

During integration with the Mossbauer spectrometer, the rover adds measurements in a running total, sort of like exposing film. A longer exposure builds up the light areas, improves contrast, and results in a clearer, more distinct image, whereas a shorter exposure produces an image that is somewhat underexposed, darker, and less well defined. Similarly, longer integrations with the Mossbauer spectrometer yield more distinct signatures of iron content and the chemical state of the iron.

At the end of the Mossbauer campaign, Opportunity re-positioned the robotic arm to take images of the grind hole. The rover's handlers postponed acquiring images until after the holidays. On sol 1395 (Dec. 27, 2007), Opportunity acquired a mosaic of microscopic images of the ground rock surface before placing the alpha-particle X-ray spectrometer on a new rock target known as ''Lyell_2.'' The resulting 2-by-2-by-14 mosaic was a collection of microscopic images arranged side-by-side like the four windowpanes in a square window. Within each of the four panes, Opportunity took 14 microscopic images at various distances from the rock surface. Because the microscopic imager is a fixed-focus camera, this process of acquiring images at different heights enables the rover to obtain images with different focal points. Because engineers don't always know where the best focus point will be, they start high, move closer, and finish low. Ideally, the middle pictures will be perfectly focused and higher and lower images will be slightly fuzzy.

Usually, the rover takes a stack of five microscopic images. This time, however, Opportunity took one image up high, one image down low, and four images at each of the three intervening heights. The multiple images will allow image processing experts to determine a digital average and cancel out unwanted data, known as ''noise'' to engineers.

In addition to studies using the Mossbauer and alpha-particle X-ray spectrometers, Opportunity conducted routine atmospheric tests, acquiring so-called Tau measurements of atmospheric dust with the panoramic camera. The rover took additional panoramic camera images of the immediate area using multiple filters. By combining images taken with different filters, engineers can create both true- and false-color views.

Following is a typical sol in the life of the Opportunity rover: Each Martian day is divided into blocks of activities separated by naps. The first block, known as the ''engineering block,'' begins when sunlight is strongest and temperatures are warmest. This is when the rover performs the bulk of the day's activities, including drives and housekeeping activities such as arm movements. After this, Opportunity takes a ''nap'' with no activities to allow the early afternoon sun to recharge the rover batteries.

In the late afternoon, the rover wakes up for a communication session with the orbiting Odyssey spacecraft. This period is known as the ''Odyssey block'' and involves ''pre-Odyssey,'' ''Odyssey,'' and ''post-Odyssey'' activities. Afterward, the rover naps or goes into a deep sleep. During deep sleep, the rover shuts off power to almost everything on board. The following morning, the rover may wake up autonomously if there is enough solar power -- this time period is called ''solar array wakeup.'' During this block, engineers usually schedule one or two small activities, followed by another nap to recharge the batteries. If there isn't enough solar power, the rover omits the solar array wakeup block.

Finally, the rover wakes up for the daily X-band communication session with Earth. This is known as the ''AM block.'' At this time, the rover generally does imaging activities in parallel with communications. This block ends with a so-called ''handover'' from the previous sol's plan to the new sol's plan.

Sol-by-sol summary

In addition to morning uplinks directly from Earth via the rover's high-gain antenna, evening downlinks to Earth via the Odyssey orbiter at UHF frequencies, and panoramic camera measurements of atmospheric opacity caused by dust, Opportunity completed the following activities:

Sol 1389 (Dec. 20, 2007): Opportunity acquired panoramic camera images of the turret, placed the Mossbauer spectrometer on Lyell_1, and acquired approximately 12 hours worth of data with the instrument.

Sol 1390: In the morning, Opportunity surveyed the horizon with the panoramic camera. The rover restarted the Mossbauer spectrometer and spent approximately 12 hours integrating data with the instrument. Opportunity acquired a 13-by-1 mosaic of panoramic camera images of Lyell_1 and used the navigation camera to pinpoint the Sun's location in support of the upcoming Mars Science Laboratory mission. The rover went into a mini-deep sleep.

Sol 1391: Upon solar array wakeup, Opportunity monitored atmospheric dust and then monitored dust on the rover mast assembly. The rover restarted the Mossbauer spectrometer and spent about 12 hours integrating data from Lyell_1 with the instrument. Opportunity acquired a 5-by-1 tier of navigation camera images and, after communicating with Odyssey, went into a mini-deep sleep.

Sol 1392: Opportunity restarted the Mossbauer spectrometer integration of Lyell_1 and spent about 12 hours collecting data with the instrument. Opportunity acquired a 4-by-1 mosaic of images looking downslope at a target known as ''Gilbert.'' The rover went into a mini-deep sleep.

Sol 1393: Upon solar array wakeup, Opportunity measured atmospheric dust and acquired Part 1 of a panoramic camera mosaic of foreground images. Opportunity spent another 12 hours engaged in Mossbauer spectrometer analysis of Lyell_1 and, after communicating with Odyssey, went into a mini-deep sleep.

Sol 1394: Following solar array wakeup, Opportunity monitored atmospheric dust and acquired Part 2 of the panoramic camera mosaic of foreground images. The rover conducted a survey at high Sun with the panoramic camera, restarted the Mossbauer spectrometer, and spent about 12 hours integrating data from the instrument. The rover went into a mini-deep sleep.

Sol 1395 (Dec. 27, 2007): Upon solar array wakeup, Opportunity measured atmospheric dust and scanned the sky for clouds with the navigation camera. Opportunity swung the robotic arm out of the way of the hazard avoidance camera, acquired full-color images of Lyell_1 using all 13 filters of the panoramic camera, and swung the robotic arm back into place over Lyell_1. The rover acquired stereo microscopic images of Lyell_1 and placed the alpha-particle X-ray spectrometer on Lyell_2. After communicating with Odyssey, Opportunity spent approximately 17 hours integrating Mossbauer spectrometer data from Lyell_2. The following morning, after communicating with Odyssey, Opportunity was scheduled to measure atmospheric dust and take thumbnail images of the sky with the panoramic camera.

Odometry

As of sol 1395 (Dec. 27, 2007), Opportunity's total odometry remained at 11,591.21 meters (7.2 miles).

As Martian winter deepens at Opportunity's location, solar power will slowly decrease, now that the Sun has begun to drop toward the northern horizon. The sub-solar point -- where the Sun's rays are exactly perpendicular to the rover -- passed the Opportunity site on Nov. 30, 2007 (Sol 1368). Since then, the Sun has been north of Opportunity, making the preferred tilt of the rover's solar array toward the north for maximum power production.

For the time being, Opportunity has been generating a little more than 600 watt-hours of solar power each Martian day (enough energy to light a 100-watt bulb for more than six hours). Measurements of atmospheric opacity caused by dust, known as Tau, have hovered between 7.0 and 8.0, and the solar arrays have been relatively clean, with dust factors of around 0.8 (a dust factor of 1.0 corresponds to a perfectly clean array).

Martian autumn in the southern hemisphere, where Opportunity is located, began on Dec. 9th, 2007 (Sol 1378). Martian winter officially begins June 25, 2008 (Sol 1571).

Opportunity continues to study the so-called "bathtub ring," a lighter-colored layer of rock inside "Victoria Crater." The ring is believed to be the original martian surface before it was buried beneath material thrown out when a meteor crashed into the surface and formed the crater. Most of the ejecta blanket fell nearby and created a ramp that rises from the original surface to a lip around the rim. Below the lip lies the "bathtub ring." Rover science team members have subdivided this ring of rocks into three layers and nicknamed them "Steno," "Smith," and "Lyell" -- in honor of famous geologists of the 17th through 19th centuries.

Investigation of Steno, the top layer, and Smith, the middle layer, is now complete. Opportunity then drove 6.89 meters (22.6 feet) from an exposure known as "Smith_1" to a feature on the Lyell layer called "Newell." Within Newell, Opportunity has selected a new target position nicknamed "Lyell_1." It is this target that consumed most of Opportunity's attention during the past week.

Opportunity began its investigation of "Lyell_1" by taking a stereo (three-dimensional) mosaic of microscopic images. Because the terrain is rough, the rover collected more microscopic images than usual to ensure that some would be at the best focus. Opportunity then collected data from the undisturbed rock surface using the alpha-particle X-ray spectrometer. This spectrometer provides information about the elements in the rock, from which scientists can infer the rock's chemical composition.

After that, Opportunity prepared to bore into the target with the rock abrasion tool. Because an encoder on the tool has failed, Opportunity uses a special, two-part sequence. During the first part, known as a "grind scan," Opportunity gently extends the grinding bit onto the rock. Electrical currents and switch contacts allow a fairly accurate determination of where the rock surface actually is relative to the rover. During the second part, Opportunity applies a small preload of 10 newtons (or about 2.25 pounds of force) to slowly push the grind bit into the rock. As the surface is worn away, the bit travels deeper into the rock.

This time, however, Opportunity had a problem with the grind scan. The grind bit seemed to come into contact with the rock several millimeters (about 1/8th inch) before it was expected to do so. Measurements indicated the bit wasn't exactly perpendicular to the rock and the side tilted toward the surface had made contact a little early.

Following further analysis, the science team decided the tool's orientation was acceptable and proceeded with the grind the following sol (Martian day). As there wasn't enough time in a single day to bore as deeply as scientists had hoped, they continued the grind the next sol. At the end of the second grind, the rover moved the robotic arm out of the way to take images of the hole with the panoramic camera. Opportunity then placed the arm back over the hole, inserted the alpha-particle X-ray spectrometer in the hole, and began collecting compositional data about the interior of the rock, beneath the dust and weathered surface. Comparison of the results before and after the grind should indicate how the rock has been altered over geologic time.

Opportunity is healthy and all subsystems are operating as expected.

Sol-by-sol summary

In addition to morning uplinks directly from Earth via the rover's high-gain antenna, evening downlinks to Earth via the Odyssey orbiter at UHF frequencies, and standard measurements of atmospheric opacity caused by dust using the panoramic camera, Opportunity completed the following activities:

Sol 1382 (Dec. 13, 2007): Opportunity stowed the robotic arm, drove to Lyell, and acquired hazard avoidance camera images before and after finishing the drive. The rover used the panoramic camera to complete a quick fine attitude, in which the rover corrects for changes over time in the inertial measurement unit by checking the rover's precise position relative to the Sun. Opportunity unstowed the robotic arm and acquired a 360-degree panorama of images with the navigation camera. After relaying data during the overpass of the Odyssey orbiter, the rover went into a deep sleep.

Sol 1383: Opportunity took images of the sky for calibration purposes with the panoramic camera. After the overhead pass of the Odyssey orbiter, the rover went into a deep sleep. When the Sun's rays energized the solar array the next morning, Opportunity surveyed the horizon with the panoramic camera.

Sol 1384: Opportunity acquired stereo microscopic images of Lyell_1 and placed the alpha-particle X-ray spectrometer on the target. After communicating with Odyssey, Opportunity spent 16.5; hours collecting data with the spectrometer.

Sol 1385: Opportunity acquired a mosaic of images of Lyell_1 with the panoramic camera and, after communicating with the Odyssey orbiter as it passed overhead, spent 7.25 hours collecting data from Lyell_1 with the alpha-particle X-ray spectrometer. The rover went into a mini-deep sleep.

Sol 1386: Opportunity completed a grind scan on Lyell_1 with the rock abrasion tool and acquired a mosaic of images of the target using the panoramic camera. After sending data to Odysssey, Opportunity spent 6 hours measuring atmospheric argon with the alpha-particle X-ray spectrometer. The rover went into a mini-deep sleep. The following morning, Opportunity took thumbnail images of the sky with the panoramic camera.

Sol 1387: Opportunity ground into Lyell_1 with the rock abrasion tool. After sending data to Odyssey, Opportunity went into a deep sleep. The next morning, Opportunity completed "sunfind" activities -- a process of searching for the Sun to determine the rover's precise heading -- in support of the Mars Science Laboratory mission, scheduled for launch in 2009. The rover took spot images of the sky with the panoramic camera.

Sol 1388 (Dec. 19, 2007): Opportunity continued the grind into Lyell_1, swung the robotic arm out of view of the panoramic camera, and acquired images of Lyell_1 with the panoramic camera and the front hazard avoidance camera. The rover placed the alpha-particle X-ray spectrometer back on the target and, after sending data to Odyssey while the orbiter passed overhead, acquired 16.5; hours worth of data with the spectrometer. The next morning, Opportunity scanned the sky for clouds with the navigation camera.

Odometry

As of sol 1388 (Dec. 19, 2007), Opportunity's total odometry was 11,591.21 meters (7.2 miles).