Kepler Mission Manager Update   
       
   August 24, 2010   
       
   The Kepler team continues with its very busy operations and data analysis   
   activities. Monthly science data downloads were successfully completed in July   
   and August 2010 on schedule. These downloads represented Quarter 6, Months 1   
   and 2, of the Kepler mission data set. Project management and engineers   
   recently gathered to assess Kepler flight system performance since operations   
   began on May 12, 2009. Spacecraft subsystem engineers presented data summaries   
   and analyses on several functional areas: power, thermal, attitude   
   determination and control, telecommunications, avionics, propulsion and   
   photometer. These engineers were able to see how their systems performed over   
   a Kepler year (371 days), and even a bit of an overlap with the beginning of   
   year 2.   
       
   During the course of a Kepler Year, several changes occur. As the spacecraft   
   orbits the sun, while maintaining the telescope pointed at the science field   
   of view, the sun position changes on the solar panels, changing the solar   
   array output. As equipment moves in and out of the sun, heaters go on and off   
   to maintain stable temperatures. The star trackers, which are used for coarse   
   attitude control, use different stars each season. The relative angles to the   
   Earth also change throughout each quarter, causing the telecom signal levels   
   to change. All these things can change the spacecraft's performance. In   
   addition, with the year 2 overlap, we are able to see how performance has   
   changed since we were at the same conditions as we were one year ago.   
       
   In general, predictions were close to actual flight performance for all   
   systems. The power systems (solar panels and battery) have performed somewhat   
   better than expected. Thermal analysis shows the spacecraft is a bit warmer   
   than expected in some areas, probably due to some initial degradation of the   
   surfaces exposed to the sun, but well within expected ranges and all heaters   
   are working well. Since the flight software and star tracker patches that were   
   made in April 2010, the ADCS system has been nominal and the fine pointing   
   control used in science observations are well below the required accuracy and   
   stability requirements.   
       
   The telecom system has been performing without problems and is matching   
   predicted performance levels. Kepler is the first mission that uses Ka-Band to   
   downlink science data and we have been very pleased with both our spacecraft   
   performance and that of the Deep Space Network operations. We have been able   
   to communicate with the spacecraft at lower elevation angles than initially   
   planned, and the data drop-out rate has been very small. Avionics have been   
   working well, with good margins on processing capacity.   
       
   The propulsion system on Kepler is used about once every 3 days to spin down   
   the reaction wheels that are used to maintain attitude control. Analysis shows   
   that we are using slightly less propellant to do this than our conservative,   
   pre-launch models. The photometer continues to work well and although one   
   module failed back in January 2010, we still exceed the requirement for field   
   of view.   
       
   Predictions for the rest of the mission are all positive, and we see nothing   
   that would cause us to change our operating plans. The only expendable   
   resource we have on Kepler is propellant, and estimates are that we have   
   sufficient propellant for another 10 years (well above the 2.5 years remaining   
   in the nominal mission). Currently our most challenging issue as we look out   
   in the long term, is the telecom margin as the spacecraft gets further from   
   the Earth. We will have to continue to drop our data rate over time, as the   
   signal strength drops due to distance. Overall, the project is quite pleased   
   with the spacecraft's performance so far.   
       
   Meanwhile, the Kepler Science Team has been quite busy analyzing all the data   
   Kepler has collected to date. There are many planetary candidates that the   
   team must assess and verify as a true planet or a false signature. As you   
   know, the Kepler Mission has a primary goal of measuring the brightnesses of   
   100,000+ stars with unprecedented precision. If an Earth-sized planet orbits   
   in front of a sun-like star, the blocking of the starlight causes the star to   
   dim over and over, allowing Kepler to detect the planet. The bigger the   
   planet, the more light it blocks, allowing the Kepler team to determine the   
   diameter of the planet.   
       
   Such a discovery is called a "planet candidate" because it has not yet been   
   verified as a true planet. If it isn't a planet, why does the star appear to   
   dim, over and over? One nagging possibility is that behind the star are two   
   additional stars that orbit each other, eclipsing themselves when they cross   
   in front of each other. Such a background "eclipsing binary star" would dim   
   once per orbit, mimicking the dimming signature of a planet. In that case, the   
   "planet candidate" would not be a planet at all. We would be fooled. With   
   hundreds of planet candidates emerging from Kepler, as announced in June 2010,   
   the challenge of weeding out the eclipsing binary stars from the bona fide   
   planets is a daunting task.   
       
   The Kepler Mission assesses these false planets with its "Follow-up Observing   
   Program" (FOP) designed to distinguish true planets from the imposters. The   
   FOP consists of 15 team members, each with different expertise in different   
   methods of identifying pesky eclipsing binary stars. The first approach is to   
   obtain high quality pictures of the field of stars around the main stars. The   
   FOP takes images of the field surrounding Kepler stars using a 1-meter   
   telescope at Lick Observatory, 2-meter telescopes operated by the Las Cumbres   
   Observatory, and even the Keck telescope in Hawaii for the highest priority   
   stars. So far the FOP has obtained images of over 400 Kepler stars. To obtain   
   more detailed images, the FOP uses the adaptive optics system on the 5-meter   
   Palomar telescope and the MMT telescope on Mt. Hopkins. Adaptive optics can   
   take pictures capable of detecting any eclipsing binary located exceedingly   
   close to the star. Any star showing no eclipsing binary by adaptive optics is   
   unlikely to have one still hiding, by chance, behind the glare of the star.   
       
   Another way the FOP weeds out eclipsing binary star is by taking a   
   "reconnaissance" spectrum of the star. Using telescopes with 3-meter diameter   
   mirrors at Mt. Hopkins, McDonald, Lick and the Canary Islands observatories,   
   the light of a star can be spread out with a spectrometer into the colors   
   (i.e. wavelengths) of which the light is composed. Eclipsing binary stars   
   reveal themselves by the two distinct rainbows of colors they each produce,   
   painted one on top of the other, but displaced from each other by the Doppler   
   Effect. The Doppler effect is what allows a police officer to detect a   
   speeding car on the highway. An eclipsing binary star would exhibit two   
   different speed readings in its spectrum of colors, betraying the existence of   
   two orbiting stars whizzing around each other. The reconnaissance spectrum   
   also permits the FOP to determine how many "spectral lines" the star has and   
   how sharp those lines are. Spectral lines are light at a particular frequency,   
   just as a piano has notes of a particular frequency. The spectral lines come   
   from atoms in the star's atmosphere, and a large number of lines and their   
   sharpness offers a chance to measure the Doppler effect with extreme   
   precision, measuring the speed of the star to within human walking speed.   
   Indeed, the highest priority planet candidates (those nearly Earth-sized) are   
   then observed with the Keck telescope in Hawaii with its "HIRES" spectrometer,   
   with the goal of measuring the Doppler Effect with extreme precision of one   
   meter per second. A planet will pull gravitationally on its host star, yanking   
   it to and fro, and such motion of the star can be detected by the changing   
   Doppler effect. Thus, the planet candidate can be certified as a bona fide   
   planet by detecting the orderly "wobble" of the star as seen in the   
   continuously oscillating Doppler effect.   
       
   Moreover, the Kepler FOP measures the amount of Doppler effect of the star.   
   The more massive the planet, and greater the gravitational tug on the host   
   star. So the FOP can use the amount of Doppler effect of the star to measure   
   the mass of the planet. This is a glorious achievement, as the dimming   
   measured by Kepler gives us the planet's diameter, while the Doppler effect   
   gives us the planet's mass. The beauty of this is that we can directly   
   determine the density of the planet, which is its mass divided by its volume.   
   Planets like Earth have the high density of rock, about 5 grams per cubic   
   centimeter, while gaseous planets like Jupiter have much lower densities of   
   about 1 gram per cubic centimeter. The FOP measurement of the planet's density   
   allows the Kepler team to distinguish true rocky planets, like Earth, from   
   gaseous planets, like Jupiter.   
       
   The FOP has its work cut out for it. With hundreds of candidate planets from   
   Kepler, there are thousands of imaging and spectroscopic observations that   
   must be made. The FOP scientists are incredibly hard-working, spending   
   hundreds of long nights at telescopes around the world. The two goals of the   
   FOP are crucial to the Kepler mission, namely to weed out the eclipsing binary   
   stars that mimic planets and to measure the masses of the credentialed   
   planets. So far, many hundreds of images and over 700 spectra have already   
   been taken of the Kepler planet candidates. A dozen eclipsing binaries have   
   indeed been found, cleansing them from the planet candidates that Kepler   
   continues to pursue. In the end, Kepler plus the FOP will provide the hard   
   data that secure the discovery of Earth-sized planets around other stars.   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.54   
    * Origin: NCS BBS (1:3828/7)