Given the recent difficulties with the robotic arm, the rover hasn't been using the batteries as much as usual. Energy has averaged about 380 watt-hours (almost enough energy to light four 100-watt bulbs for one hour).

Tau, a measure of direct sunlight (and thus of dust in the atmosphere) has been fairly steady at 0.62, meaning that about half the sunlight streaming through the atmosphere of Mars reaches the ground. The rest is either absorbed or scattered. Like direct sunlight, scattered light generates power. Absorbed sunlight does not.

Opportunity's dust factor has been fairly steady at 0.7, meaning that about 70 percent of the sunlight hitting the solar arrays penetrates the dust layer to make electricity.

In mid-may (May 12, 2008), Mars will reach aphelion, its farthest point from the Sun. At that time it will be 249 million kilometers (155 million miles) from the Sun, about 1.6 times farther from the Sun than Earth ever gets.

On June 25, 2008, Opportunity will enter the winter solstice. This is the date when the Sun is lowest in the sky that marks the official start of the six-Earth-month Martian winter.

Ever since sol 654 (Nov. 25, 2005), Opportunity has experienced occasional motor stalls in the shoulder joint of the robotic arm known as Joint 1. Joint 1 is the shoulder azimuth joint, the one that swings the arm out from the rover, and left or right in front of the rover. The motor has worked long past its expected lifetime.

Apparently random in occurrence, the stalls have been accompanied by step increases in electrical resistance, which is consistent with a broken winding within the motor. (The rover's motors have bifilar coils, consisting of two parallel windings. If one wire breaks, a second coiled wire provides some torque to turn the motor. Torque is a force that causes rotation about an axis. With one wire broken, there is less torque during part of each rotation.)

Each motor has magnetic detents -- permanent magnets that pull the rotor into a fixed position to prevent it from rotating when necessary. The detents can also prevent the motor from turning when engineers want it to turn. At that point, it takes a bit more torque to start the motor turning. If a broken coil happens to align with a detent, the remaining, unbroken coil has a hard time starting to rotate. This can result in a stall.

Opportunity's handlers have been living with this for nearly 900 Martian days and until now, they have overcome every stall simply by trying the motion again.

To minimize the chance of getting "stuck" in an unfavorable position where Joint 1 is permanently stalled, the rover's handlers keep the robotic arm deployed (that is, unfolded and suspended in front of the rover) except when driving. This approach is known as the "Stow/Go/Unstow" strategy.

When rover drivers want to drive, they stow the arm (that is, fold it up with the elbow in horizontal position and the fist against the chest). After the drive, they promptly unstow the arm (move the elbow down and the turret, or fist, up).

On Sol 1502 (April 15, 2008), during a routine post-drive unstow, Opportunity experienced a Joint 1 stall that was quantitatively different from prior stalls. Tests since then have continued to result in stalls with significantly higher electrical resistance five to 10 times greater than previously measured values. Motor currents have been very low, consistent with higher electrical resistance.

Engineers are conducting diagnostic tests of Joint 1 and other components. So far, every attempt to move Joint 1 has failed, producing at most a single motor revolution. The joint seems to stall in the same spot each time. The rover's handlers are pursuing a slow and steady approach of fully understanding the problem and possibly devising strategies for living with or working around the issue.

They already know that even if the joint is permanently stalled, Opportunity can still do some science observations with instruments on the robotic arm.

Sol-by-sol summary:

In addition to receiving morning instructions directly from Earth via the rover's high-gain antenna, relaying data back to Earth via the UHF antenna on the Mars Odyssey orbiter, and measuring atmospheric dust with the panoramic camera, Opportunity completed the following activities:

Sol 1505 (April 18, 2008): Opportunity ran diagnostic tests of the robotic arm.

Sol 1506: Opportunity acquired six time-lapse movie frames in search of morning clouds with the navigation camera and surveyed the horizon with the panoramic camera. Later, the rover acquired a 3-by-1 panel of images with the navigation camera.

Sol 1507: Opportunity took morning spot images of the sky for calibration purposes with the panoramic camera. The rover ran more diagnostic tests of the robotic arm.

Sol 1508: Opportunity began work on a 360-degree, two-tier panorama of lossless-compression (highly detailed and precise) images with the navigation camera.

Sol 1509: Opportunity spent most of the day characterizing dust in the atmosphere.

Sol 1510 (April 23, 2008): Opportunity acquired full-color images, using all 13 filters of the panoramic camera, of one of the two trenches made by the rover's wheels, dubbed "Williams." At 11:05 a.m. local Mars time, Opportunity acquired the left-hand view of a mosaic of panoramic-camera images.

The rover acquired part 2 of the lossless-compression, 360-degree panorama with the navigation camera and surveyed the sky at high Sun with the panoramic camera. The following morning, Opportunity was to acquire full-color images, using all 13 filters of the panoramic camera, of the other wheel trench, dubbed "Harland."

Odometry: As of sol 1510 (April 10, 2008), Opportunity's total odometry was 11,689.53 meters (7.26 miles).

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