[meteorite-list] Deep Space 1 Mission Log - May 13, 2001

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:46:24 2004
Message-ID: <200105161502.IAA24718_at_zagami.jpl.nasa.gov>


          Dr. Marc Rayman's Deep Space 1 Mission Log

          Mission Update:

          Thank you for visiting the Deep Space 1 mission status
          information site, for more than 2.5 years the most
          frequently visited site among inhabitants of spiral or
          irregular galaxies for information on this solar system
          exploration mission. This message was logged in at 11:00 pm
          Pacific Time on Sunday, May 13.

          As Deep Space 1 continues is cosmic voyage, it is preparing
          for a very brief and extremely daring assignment later this
          year. If all goes well for the next 4 months, on September
          22 DS1 will greet comet Borrelly as the icy body and the
          spacecraft flash past each other at 16.5 kilometers/second
          (more than 10 miles/second, or 36,900 miles/hour). While
          this is a great bonus opportunity to try to gather some
          unique and wondrous information about comets, it is also a
          very, very challenging and risky undertaking. But with a
          marvelously successful primary mission to its credit as
          well as a remarkably exciting and rewarding extension, the
          bold challenge of the comet encounter is a worthwhile
          adventure. Comets are believed to be remnants from the
          formation of the solar system, and studying them may shed
          light on the origin and evolution of our solar system and
          perhaps even on the evolution of Earth. With its motto of
          "If it isn't impossible, it isn't worth doing" always in
          mind, the very small Deep Space 1 team has been preparing
          for the event.

          The measurements DS1 will attempt at the comet will be
          described in detail in future logs. In brief, however, the
          probe will attempt to fly through the coma, the cloud of
          gas and dust surrounding the nucleus, and measure its
          composition. Then as it closes in to near the center of the
          coma, it will be faced with its greatest challenge -- to
          obtain pictures and infrared spectra of the diminutive
          nucleus, invisible from Earth because of its size and the
          obscuration by the coma. The craft will have to locate the
          nucleus on its own and point the camera at it as it streaks
          by. That would be difficult enough, given that we can't
          tell DS1 exactly where the nucleus is nor what it will look
          like. But the little robot's assignment will be still more
          challenging because in the absence of its star tracker,
          which failed in November 1999, it normally has to stay
          locked to a reference star to remain stable. It can't point
          its camera at a star while it is trying to find and
          photograph the nucleus, so it will have to rely on its
          gyros, which provide approximate measurements of the
          spacecraft's turns. These gyros, however, were not meant
          for such a job, and they are not accurate enough to provide
          a stable platform throughout the encounter period.

          To get an inkling of just one facet of the problem, suppose
          someone were holding a pair of high-power binoculars for
          you while you tried to look through them. Her hands would
          not be perfectly steady, and you would have a hard time
          seeing what you wanted. In fact, unless you told her how to
          position the binoculars, she might even move them around
          enough that the object of interest would completely leave
          your field of view. DS1 is faced with a similar situation,
          with the binoculars being like the camera, and the gyros
          being the assistant's hands. But now if you could tell your
          friend how to move the binoculars ("a little to the right,
          now lower them -- no, that's too much") you might be able
          to guide her well enough for you to get a good view. Some
          of the new software that was installed in DS1 in March is
          designed to analyze the pictures, look for what might be
          the nucleus, and decide how to move the spacecraft to keep
          it in the camera's sights.

          During the spacecraft's encounter with the comet, it will
          rely on the software and an extremely complex set of
          carefully timed commands to execute the myriad steps
          necessary to collect its measurements. But how do we test
          all of this? Of course, we have ground-based simulators of
          the spacecraft, but they are of only limited fidelity. So
          to make sure we are on the right track in developing the
          commands that will give the probe its best chance to point
          its camera at the comet as it closes in on it, the DS1
          control team conducted some clever experiments with the
          spacecraft on May 1 and May 8. Such tests involve some risk
          and a great deal of work to prepare and execute. The very
          long hours of hard (but, frankly, incredibly cool!) work by
          the team keep paying off however. In addition, because the
          Deep Space 1 project's resources are quite limited, the
          team's careful decisions in how it deals with risky
          undertakings have been an important ingredient in the
          success of such difficult operations.

          After much planning, on May 1 DS1 took advantage of a
          coincidental alignment of itself with two planets to
          conduct a valuable test of the new software. On that date,
          when DS1 pointed its main antenna to distant Earth, its
          camera ended up pointing to still-more-distant Jupiter.
          With controllers thus able to monitor data (of course
          delayed by the long wait for signals to travel from the
          probe to the second floor of JPL's Space Flight Operations
          Facility on Earth), DS1 used this new software to keep
          Jupiter in the view of its camera for the duration of the
          test -- over 2 hours. This provided the spacecraft with a
          rare opportunity to try to track a target other than a
          star, which would have appeared only as a pinpoint.
          Enormous Jupiter is around 30,000 times larger than the
          nucleus of the comet (whose actual size is very poorly
          known) DS1 will meet in September. So although it was over
          820 million kilometers (510 million miles) from the craft,
          the planet, the largest in our solar system, looked to DS1
          about the same size that the comet will appear when DS1 is
          on its final approach, only about half an hour before the
          moment of closest encounter. (This also illustrates part of
          the difficulty of the encounter -- this comet nucleus is
          going to be very tiny and thus difficult to locate!) The
          software successfully detected Jupiter (appearing as just a
          little fuzzy ball) in the picture frame and correctly
          computed compensations for the gyros to hold Jupiter in
          about the right spot.

          Jupiter was so far away that its position did not vary
          during the test, but when the spacecraft gets to the
          vicinity of comet Borrelly, it will have to keep turning to
          keep its camera pointed at the moving target. In addition,
          it will execute many other commands to control its
          scientific instruments, to move and record data in its
          computer system, to set various operating modes of the
          spacecraft systems, etc. To rehearse all of that, on May 8
          DS1 executed a practice encounter with comet Spoof. This
          comet exists only in the virtual universe of software (as
          well as, of course, the hearts and minds of the mission
          operations team), but DS1 did not know the difference (and
          don't tell the impressionable probe!). It dutifully
          followed the sequence of commands, all the while recording
          its own performance for later analysis by engineers. Each
          time it took a picture, the computer file containing the
          image was intercepted by a special routine on board that
          "painted" a comet nucleus on it. The software determined
          how big Spoof should be at that point in the encounter, and
          how much of the portion visible to the spacecraft would be
          illuminated by the Sun. The image file was subsequently
          sent back on its electronic way, and nothing else on board
          knew that the nucleus in the picture was synthetic. The
          spacecraft then processed each of these pictures and
          exercised the systems that will be used to try to follow
          the nucleus during the encounter. By using the actual
          camera on the actual spacecraft, the test included such
          phenomena as unwanted stray light, camera flaws, and cosmic
          rays (which can show up in some pictures and confuse the
          software); this made the rehearsal much more realistic. The
          test proved very successful, giving the DS1 team important
          information on the detailed performance of the spacecraft
          using the software and the commands that have been
          formulated thus far. This will be important in helping
          guide our work in designing the comet encounter, as we now
          have a new comparison of the operation of the genuine
          spacecraft with that of the Earth-based simulator. An
          encore performance rehearsal will take place near the end
          of June.

          The Sun, now at the peak of its 11-year cycle of activity,
          is spewing forth much more radiation than usual. Any
          readers in the vicinity of Earth are protected from this by
          our planet's vast magnetic field, and those near the
          surface have the extra protection of the thick (and mostly
          breathable) atmosphere. Those of you on several of our
          solar system's planets (Earth being a fine example) may
          still be treated to some lovely auroras these days
          triggered by the solar activity, and observers who are very
          careful can see Sun spots, some large enough to be visible
          without magnification. But lonely DS1 does not have a
          planet's magnetic field or atmosphere to shield it from the
          buffeting of the raging storms on the Sun. Nevertheless,
          much to the relief of the busy and fatigued operations
          team, it is managing to fly smoothly and happily; solar
          radiation does not appear to be causing problems.

          As DS1 continues its flight, the thrusting with the ion
          propulsion system has passed several milestones. On March
          21, DS1 had accumulated 10,000 hours of thrusting. This
          number is not inherently special, but it certainly does
          illustrate the system's fantastic longevity. (In fact, your
          correspondent, in the nerdy language he occasionally lapses
          into when among his colleagues, described this as being
          significant only in that the mantissa of the common
          logarithm of the number of hours was identically 0. Readers
          unfamiliar with such gibberish are advised to remain that
          way.) The ion drive has more than 15 months of operating
          time now.

          On May 1 DS1 had completed enough firing of its ion engine
          to coast to the comet -- we're on target! But as several
          mission logs have described (the October 29, 2000 log has
          an explanation that made it into several popular text books
          in the halo of the Milky Way), the spacecraft is so low on
          its supply of the conventional rocket fuel known as
          hydrazine that it must keep the ion engine thrusting at a
          low throttle level to control its orientation in space. So
          it will remain at "impulse power" for most of the time
          until shortly before the spacecraft reaches Borrelly.

          DS1 is now about 157 million kilometers, or 97 million
          miles, from comet Borrelly.

          Deep Space 1 is 1.9 times as far from Earth as the Sun is,
          and more than 750 times as far as the moon. At this
          distance of 290 million kilometers, or 180 million miles,
          radio signals, traveling at the universal limit of the
          speed of light, take over 32 minutes to make the round

          Thanks again for visiting!
Received on Wed 16 May 2001 11:02:26 AM PDT

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