K2 SPACECRAFT OPERATION
K2 inherits the on-orbit Kepler spacecraft with its 0.95-meter aperture and 100 square degree field of view, the Kepler ground systems to operate the spacecraft, and the Kepler infrastructure to process and archive the data collected. Aside from the failed reaction wheels, the spacecraft has shown little performance degradation and the remaining reaction wheels show no signs of wear. The only fundamental change to the operation of the spacecraft is attitude, pointing in its orbital plane (approximately the ecliptic) in order to minimize the impact of solar radiation pressure, and the concomitant changes to attitude control and communications strategies.
The K2 mission is driven by its ability to maintain spacecraft pointing in all three axes with only two reaction wheels. Solar pressure represents a disturbing force, which is controlled by reaction wheels about the Y & Z axes, but controlled only by thrusters about the X-axis. Sustained, stable pointing requires that the X-axis disturbing force be minimized for extended periods. This is accomplished by pointing in the orbital plane, where the apparent motion of the sun (caused by the spacecraft’s orbital motion while inertially pointed) follows the spacecraft X-Y line of symmetry, providing a balanced pressure and little X-axis disturbance. By selecting carefully the initial spacecraft roll angle and correcting for drift every 12 hours with thrusters, the spacecraft can remain stable in roll for up to 85 days with a fuel budget that allows for a 2-3 year mission duration. Meanwhile, the reaction wheels control pointing about the Y and Z axes as in the Kepler mission, absorbing the torque generated by the solar radiation pressure and dumping momentum through thruster firings every 48 hours.
K2 will observe a series of independent campaign target fields in the ecliptic plane. The duration of each observation is limited by solar illumination constraints on the spacecraft, bounded by power collection on one end, and aperture illumination on the other, with science observations limited to about 75 days per field.
K2 FIELDS OF VIEW Each campaign will start when the target field rises. A checkout period will be used to upload target tables and configure the attitude control system for each new attitude. The spacecraft is then turned to the new science attitude to collect pointing alignment data and turned back to an attitude for good communications to downlink this data. Ground Operations will then adjust the configuration as needed before turning back and beginning the science observations for the campaign. Spacecraft communications are limited during data collection as ecliptic-pointing results in unfavorable low gain antenna view angles to Earth. Spacecraft maneuvers will be required to downlink the data. In order to maximize the unbroken observation period, data is stored onboard until the end of the campaign and is downlinked before the next campaign starts. This strategy results in target/pixel limitations imposed by the finite onboard storage. To prevent excessive loss of science observing, short DSN contacts will be scheduled twice a week to check for safemode. If safemode is detected, ground controllers will determine and correct the cause of the fault and return to science observing.
The data acquired in each campaign will stand on its own. The science yield from each campaign is self-contained. Field revisits in future years are not necessary to deliver K2's science goals and therefore K2 success is robust against further hardware failure. No campaign fields have yet been finalized, but the field-selection strategy will provide for future flexibility, in e.g. identifying additional science priorities or reacting to operational constraints. A draft observing scheme through mid-2016 is provided here.
The proposed scheme is based upon community response to the two-wheel white paper call and further community feedback (e.g. Kepler Science Conference II). The proposed fields are rich in the types of targets requested by the community. One or more K2 fields will be available to ground-based observers for follow-up science on all nights of the year. Follow-up is possible from almost every observatory on the planet, bringing southern hemisphere observers into the equation. Since the ecliptic has been surveyed many times across multiple energy bands, the K2 mission can rely upon enormous resources for the community to identify potential targets of interest and characterize targets for data interpretation. The proposed sequence is anchored by Field 4, which collects observations of the Pleiades and Hyades clusters in 2015. Earlier and later fields march around the ecliptic in steps of apprximately 80 days in order to maintain solar power to the spacecraft. Field 9 is proposed to observe in the spacecraft velocity vector, instead of the anti-velocity vector direction, in order to survey Baade’s window for microlensing events while allowing for simultaneous ground-based support for microlens monitoring. Velocity vector campaigns like the proposed Field 9 are however handicapped by the bright, saturated Earth and moon migrating across the field. We will be pursuing further community feedback before selecting fields and encourage from the community at all times, but especially during Jan 2014.
PROPOSED K2 CAMPAIGN FIELDS
2014 Mar 08
2014 May 30
Near Galactic Anti-center, M35, NGC2304
2014 May 30
2014 Aug 21
North Galactic Cap
2014 Aug 23
2014 Nov 10
Near Galactic Center, M4, M80, M19, Upr Sco, rhoOph
2014 Nov 14
2015 Feb 3
South Galactic Cap, Neptune
2015 Feb 07
2015 Apr 24
M45 (Pleiades), NGC1647, Hyades
2015 Apr 26
2015 Jul 11
M44 (Beehive), M67
2015 Jul 13
2015 Oct 01
North Galactic Cap
2015 Oct 03
2015 Dec 26
Near Galactic Center, NGC6717
2015 Dec 28
2016 Apr 04
South Galactic Cap
2016 Apr 06
2016 Jun 29
Galactic Center, Baades Window, M21, M18, M25, M8
Note well that the dates and times are often preliminary do not correspond to the final operational dates and times, which can be found here. Each date listed in the table above refers to the day on which the spacecraft anti-velocity vector passes through the field-center coordinates provided. The one exception is proposed campaign 9 which points the spacecraft boresight towards the positive velocity vector. In general, campaigns can start a maximum of 52 days before each date and end no later than 30 days after each date.
K2 TARGET SELECTION K2 Operations estimate 10,000-20,000 targets per campaign collected on a 30-min cadence and an additional ~100 targets per campaign collected on a 1-min cadence. Target numbers are limited by on-board storage and compression, downlink capability, and campaign length. The precise target number will vary for each campaign and will be an optimized trade based upon target sample brightness, initial pointing knowledge, spacecraft drift and data compression efficiency. The K2 community will propose all science targets for each campaign on the draft schedule above. The K2 mission will provide a catalog of potential sources within each campaign field that is complete to a 2MASS limiting magnitude of 16. The catalog will contain celestial coordinates, proper motion, broadband magnitudes from SDSS, UCAC and 2MASS surveys, inferred Kepler bandpass magnitudes, stellar effective temperature, surface gravity, and metallicity. The catalog will be updated and archived at the MAST prior to calls to the community for target proposals. Individual proposals will nominate targets over one or more campaign fields. Final target lists will be selected by an independent, community-led peer review. There will be no restrictions on types of science proposed. Peer reviewers will develop a science program for each campaign that maximizes scientific yield for K2.
K2 DATA AND ARCHIVES Existing Kepler pipeline software will be reused for the K2 pipeline. The K2 Project deliverables are calibrated data and higher-level pipelined data product to a legacy archive. The minimum-useful set of data for all scientific investigations comprises a target catalog, calibrated focal plane pixels (targets, background, engineering and calibration), time-series target photometry and spacecraft SPICE kernels. Higher-level science products deliver progressive stages in the process of K2 exoplanet detection: i) systematic noise reduction in the target photometry ii) lists of photometric events that resemble a coherent series of exoplanet transit events, and iii) diagnostics for validating individual events as planet candidates or planet candidate false positives. There will be no exclusive use period for K2 data.
Using the science products listed, community scientists can contribute to the task of exoplanet detection and other astrophysics from several jump-off points – calibrated pixels, canned photometry, ensemble noise-reduced photometry, or transit candidate detection. Exoplanet candidate identification, validation characterization and follow-up will be the responsibility of the K2 community, using the data products provided. The K2 community will retrieve all time-series data from a legacy archive hosted at the Mikulski Archive for Space Telescopes (MAST). The K2 archive will look and feel much like the Kepler archive. Data products will be produced and archived 3 months after data collection ends, allowing the community to rapidly pursue ground-based follow-up and compete for ADAP resources as early as possible. Science products will mirror those archived by the Kepler mission. In support of community-led exoplanet candidate detection, vetting, validation, verification and follow-up, the Exoplanet Archive will host products ii) and iii) listed above and provide tools and additional data resources for their exploitation.
Questions concerning K2's science opportunities and open programs, public archive or community tools? Contact us via the