Ahead of the encounter the Deep Space 1 mission experiments will continue to assess our they operate over longer periods in space.

One such test on Friday gathered data on the ion propulsion system. This exotic system has now operated for over 74 days and, as all loyal readers know, has shown itself to be a very very efficient and reliable means of propulsion.

To measure whether the extensive use of this system has caused any small changes in its performance, a special experiment was executed in which the spacecraft turned to point the ion engine at Earth. The engine was operated at 5 different throttle levels while the Deep Space Network measured the Doppler shift of Deep Space 1's radio signal.

This phenomenon, familiar to anyone who has heard the pitch of a siren change as it approaches or recedes from the listener, allows a measurement of the change in speed as the thruster gently pushes on the spacecraft.

This will be used to search for subtle changes in the thrust of the engine since the beginning of the mission. All the sensors in DS1's suite of devices to measure ions and electrons as well as magnetic and electric fields were tuned and alert during the test so that comprehensive information on the ion engine could be collected.

The ion engine is mounted on a pointing system that allows fine tuning the direction of thrust. This system was commanded to move through a prescribed pattern before the engine was powered on then again at the lowest and highest throttle levels used in the test.

This allows for still more measurements of the effects on the ion propulsion system on the spacecraft's environment. It takes several days for the large volume of data to be radioed back to Earth.

One of the technologies that has been in use since the first day of the mission is the advanced solar array. The two wings contain 720 lenses to focus sunlight onto 3600 cells, each converting the light into electricity to power the ion propulsion system and the rest of the spacecraft.

A test performed this week measured the electrical characteristics of some specially instrumented groups of cells to contribute to ongoing studies to determine the extent to which radiation or other hazards encountered in space cause the array to degrade.

Additional tests were performed this week with other technologies that legions of Deep Space 1 mission log groupies are well acquainted with. The miniature integrated camera spectrometer, the plasma experiment for planetary exploration, the low power electronics, the sophisticated on/off switches, and the multifunctional structure all were exercised.

But the primary tests on these and the other technologies are mostly complete, so although the systems were considered high-risk when they were selected for flight on Deep Space 1, the advanced capabilities they offer are now ready for inclusion in future missions.

With most of the technology testing behind it, the operations team's attention is now turning to preparations for the July 29 encounter with an asteroid with the richly suggestive, yet startlingly simple, name 1992 KD. The primary objective of the event will be to provide the final test for the autonomous navigation system, known to its close friends as AutoNav.

Since February, AutoNav has reliably been determining DS1's location. It does so by commanding the spacecraft to turn to point its camera at asteroids and stars and taking images of them.

The apparent position of an asteroid relative to the much more distant stars allows AutoNav to calculate where it is in the vast solar system. This is based on parallax and is the same phenomenon you observe if you hold a finger in front of your face and view it through each eye separately.

The apparent position of your finger shifts as you switch from one eye to the other. As an example of how this is applied, suppose that distant trees are visible through a window in your house.

If I took a picture from inside your house and showed it to you, you could find exactly where I had been standing when I took the picture by lining up the edge of the window with the distant trees.

Similarly, because AutoNav knows where the asteroids are and where the more distant stars are, it can determine where it is in the solar system when the picture is taken.

AutoNav has also controlled the ion propulsion system to put the spacecraft on course for this summer's asteroid encounter. The last 5% of the testing of AutoNav will come with its attempt to guide the spacecraft to the closest encounter ever attempted with a solar system body without actually landing on it.

The encounter will be very challenging indeed, but whatever the outcome, it is certain to provide an excellent test of AutoNav. The encounter also offers the additional opportunity to conduct exciting science. The instruments Deep Space 1 carries for testing will be used to study the asteroid and its environment. Details of the plans for the encounter will be provided in future logs.

In the meantime, however, new software to complete AutoNav's capabilities to conduct the encounter is in final testing here on Earth now. It's part of an overall upgrade to the extremely complex software that controls DS1.

The spacecraft has received in-flight improvements before, and those successes are forming the basis for the upcoming installation of software. As the next step, the process of radioing the files containing the software to the spacecraft will begin next week.

Much of the rest of June will be devoted to certifying the new software on the spacecraft and conducting other preparations for the encounter.

Deep Space 1 is now nearly 90% as far as the Sun and almost 350 times farther than the moon. At this distance of over 133 million kilometers, or about 83 million miles, radio signals, traveling at the universal limit of the speed of light, take almost 15 minutes to make the round trip.