The Kepler team continues with its very busy operations and data analysis activities. Monthly science data downloads were successfully completed in July and August 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 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 working well. Since the flight software and star tracker patches that were made in April, 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 anomaly 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, 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 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, 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 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.
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