NASA - Kepler Mission Manager Update: Pointing Test

The team has continued exploratory recovery testing of Reaction Wheel 4 (RW4). On Thursday, July 25, 2013, the wheel spun in both directions in response to commands.

While both RW4 and RW2 have spun bi-directionally, friction levels remain higher than would be considered good for an operational wheel. However, it will be important to characterize the stability of the friction over time. A constant friction level may be correctable in the spacecraft’s attitude control system, whereas a variable friction level will likely render the wheels unusable.

 

High-precision pointing of the Kepler spacecraft is controlled by reaction wheels, which are small electric motors mounted on the spacecraft that control the three axes of motion: up/down, forward/back and left/right.

Image Credit:  Ball Aerospace

With the demonstration that both wheels will still move, and the measurement of their friction levels, the functional testing of the reaction wheels is now complete. The next step will be a system-level performance test to see if the wheels can adequately control spacecraft pointing.

The team is preparing for the next test using RW2. Friction levels on RW4, the wheel that failed in May, are higher and no additional testing is planned at this time. The pointing test involves determining the performance of the wheel as part of the spacecraft system. The test will be conducted in three stages.

The first stage of the pointing test will determine if the spacecraft can sustain coarse-point mode using RW1, 2 and 3. Coarse-point mode is regularly used during normal operations, but has insufficient pointing accuracy to deliver the high-precision photometry necessary for exoplanet detection. During coarse-point the star trackers measure the pointing accuracy of the spacecraft. When using wheels to control the spacecraft, pointing is typically controlled to within an arcsecond, with a fault declared if the pointing error exceeds a quarter of a degree. This degree of pointing accuracy would be equivalent to keeping an imaginary Kepler telescope pointed at a theatre-size movie screen in New York City's Central Park from San Francisco.

 

The spacecraft provides the power, pointing and telemetry for the photometer. Other than the four reaction wheels used to maintain the precision pointing and an ejectable cover, there are no other moving or deployable parts on the spacecraft.

Image Credit:

NASA Ames/Ball Aerospace

 

In the first stage, testing will demonstrate whether or not operation with RW2 can keep the spacecraft from entering safe mode. A safe mode is a self-protective measure that the spacecraft takes when an unexpected event occurs, such as elevated friction levels in the wheels.

In the second stage, testing will investigate RW2's ability to help control the spacecraft pointing with enough accuracy to transmit science data to the ground using NASA's Deep Space Network. If RW2 can sustain coarse-point in stage 1, the second stage of the test will be to point the high-gain antenna to Earth and downlink the data currently stored aboard. This requires that the pointing be controlled more tightly than simply avoiding safe mode, yet does not require the very fine control needed to return to science data collection.

The final stage of the test will determine if RW2 can achieve and maintain fine-point, the operating mode for collecting science data. During fine-point the fine-guidance sensors measure the spacecraft pointing. When using wheels to control the spacecraft, pointing is controlled to within a few milliarcseconds. Using our imaginary Kepler telescope example, this degree of pointing accuracy would be equivalent to pointing at a soccer ball in New York City's Central Park from San Francisco.

The team anticipates beginning the pointing performance testing on Thursday, August 8, 2013 and will continue into the following week if all goes well. A determination of whether Kepler can return to exoplanet data collection is expected a couple weeks after these pointing tests are complete.

As engineers explore recovery of the spacecraft, scientists continue to analyze the existing data. Earlier this week the team delivered their findings for 1,236 new Kepler Objects of Interest (KOIs) to the NASA Exoplanet Archive. The new KOIs were found by searching the observational data from Quarters 1 to Quarter 12. Of the 1,236 new KOIs, 274 were judged to be planet candidates, while many others were determined to be false positives. These newly announced Kepler planet candidates bring the current count to 3,548. Some of these new planet candidates are small and some reside in the habitable zone of their stars, but much work remains to be done to verify these results.

Also announced this week is the Kepler Science Conference II Nov. 4-8, 2013 at NASA Ames Research Center at Moffett Field, Calif. Registration is now open.

Regards,
Roger

NASA

Kepler Overview

Kepler: NASA's first mission capable of finding Earth-size and smaller planets around other stars

The centuries-old quest for other worlds like our Earth has been rejuvenated by the intense excitement and popular interest surrounding the discovery of hundreds of planets orbiting other stars. There is now clear evidence for substantial numbers of three types of exoplanets; gas giants, hot-super-Earths in short period orbits, and ice giants. The challenge now is to find terrestrial planets (i.e., those one half to twice the size of the Earth), especially those in the habitable zone→ of their stars where liquid water might exist on the surface of the planet.

 

The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone→ and determine the fraction of the hundreds of billions of stars in our galaxy that might have such planets.

The Transit Method of Detecting Extrasolar Planets
When a planet passes in front of a star as viewed from Earth, the event is called a “transit”. On Earth, we can observe an occasional Venus or Mercury transit. These events are seen as a small black dot creeping across the Sun—Venus or Mercury blocks sunlight as the planet moves between the Sun and us. Kepler finds planets by looking for tiny dips in the brightness of a star when a planet crosses in front of it—we say the planet transits the star.

Once detected, the planet's orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler's Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. From this the question of whether or not the planet is habitable (not necessarily inhabited) can be answered.
Kepler instrument
The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope — 105 square degrees, which is comparable to the area of your hand held at arm's length. It needs that large a field in order to observe the necessary large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for the life of the mission—3.5 or more years.

The photometer must be spacebased to obtain the photometric precision needed to reliably see an Earth-like transit and to avoid interruptions caused by day-night cycles, seasonal cycles and atmospheric perturbations, such as, extinction associated with ground-based observing.

Results from the Kepler mission will allow us to place our solar system within the context of planetary systems in the Galaxy.

Additional Links
› About Kepler→
› JPL's New Worlds Atlas→

Kepler Science

The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to:

• Determine the percentage of terrestrial and larger planets that are in or near the habitable zone of a wide variety of stars
• Determine the distribution of sizes and shapes of the orbits of these planets
• Estimate how many planets there are in multiple-star systems
• Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
• Identify additional members of each discovered planetary system using other techniques
• Determine the properties of those stars that harbor planetary systems.

Target Field of View

Since transits only last a fraction of a day, all the stars must be monitored continuously, that is, their brightnesses must be measured at least once every few hours. The ability to continuously view the stars being monitored dictates that the field of view (FOV) must never be blocked at any time during the year. Therefore, to avoid the Sun the FOV must be out of the ecliptic plane. The secondary requirement is that the FOV have the largest possible number of stars. This leads to the selection of a region in the Cygnus and Lyra constellations of our Galaxy as shown.
Additional Links
› Kepler Science Basics→
› Kepler Discoveries→

Guillermo Gonzalo Sánchez Achutegui
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