Morgan O’Neill Maciej Bzowski Physics Department Space Research Centre UNH, Durham, NH PAN, Warsaw, Poland

Eberhard Möbius Marek Hłond Space Science Center Space Research Centre & Physics Department PAN, Warsaw, Poland UNH, Durham, NH

Harald Kucharek Space Science Center Precision Pointing in the Sky UNH, Durham, NH for IBEX Interstellar Flow Observations

Introduction Motivation Results Cont’d Current IBEX software only recognizes the ’s gravitational center, and we need to correct the data for A full moon signal reflects the moon’s Angular Peak Separation Varies With Moon Phase The ejects a hot, fast wind that speeds through barycenter. We can quantify the growing 16.4 changes in phase and distance. The ability to precisely determine the moon’s position with respect to the 16.3 the , before it is slowed down by the gas 16.27 IBEX satellite will improve the accuracy of the star sensor. difference in peak separation as the 16.2 16.15 and dust in the space between the Sun and other stars The most important variation of the moon signal is that due to the varying phase. phase approaches a crescent. The (the ). The creates a 16 • For a full moon, the center of the illuminated disk is also the center of gravity. changing peak separation corresponds to protective bubble, or ‘’ around the solar 15.8 • For a quarter moon, the lit portion does not represent the true center of the moon. a different apparent right ascension for 15.66

system, as we travel through the ; however, 15.6 Separation (in degrees) (in Separation → An angular correction is required so that we can correctly identify the moon’s position. the moon’s position. The measured a wind of the interstellar medium blows through the spin phase can also change depending 15.4 heliosphere, and we can detect the gas from a satellite The Sun is moving through a local , and The moon is much Peak Angular some gas and dust from this cloud streams through the on the center between the two peaks. 15.2 in orbit. brighter than other crescent quarter gibbous full heliosphere into our solar system. objects in the sky so when Moon Phase IBEX Mission it comes into view, the The Moon bias voltage of the star sensor is greatly reduced. 15.66° Angular Separation 16.27° The Interstellar Boundary Explorer (IBEX) is the latest satellite of NASA’s Small Explorer At a lower voltage the of Peaks Mission Program. IBEX was launched in October 2008 and orbits the Earth to measure the moon signal does not 16.15° 16.30° The plot shows an IBEX orbit during which the moon was visible. Note the drop in resolution in the center to nature and amount of particle flow that originated in the interstellar medium. accommodate the bright moon. The moon provides a redundant check of position for the IBEX Hi and Lo sensors. saturate the star sensor. Mission Design & Star Sensor To image the ISM flow and the heliospheric boundary, IBEX instruments Moon Simulation Program & Results will be oriented perpendicular to a sun- A program was created that partially simulates the function of the star sensor. The Conclusions & Goals pointing spin axis. The satellite will scan main layer is a pixelized image of the star sensor aperture. A second layer is that of the the entire sky every six months. object of interest – in this case, the moon in varying phases. This moon image is • There is a noticeable and measurable difference between 1st and 3rd quarter phases. passed over the star sensor aperture (example at left) and the amount of visible bright To locate this flow in the sky, a star sensor • The dark mare on the moon’s surface alter the star sensor moon signal and require a pixels is counted at each position. is co-aligned with one of the primary correction for measured elevation. The IBEX will orbit the Earth with a sun-pointing spin axis, IBEX flow instruments. and will view ribbons of the sky as it rotates. • This program allows us to answer a number of questions about how the moon’s phase, As the moon’s phase approaches a crescent, the necessary angular correction grows One Full Sweep of Light Source by Star Sensor This star sensor senses light, and it will distance and uneven brightness affect the star sensor signal. 140 create a star map as it spins with the Upon full completion of phase characterization, a moon phase function will be 120 satellite. The influx of particles will be incorporated into the main IBEX star sensor. The input will be the current time, the 100 •What is the difference overlaid onto the generated map in order satellite position and the moon position. Depending on phase and distance, the function 80 st rd to determine the source location of the between 1 and 3 will pass back an angular correction for the true barycenter of the moon. Since both the 60

Voltage flow precisely. However, due to zodiacal quarter phase? spin phase and the elevation angles can be affected, so any moon sighting will require 40 light (background brightness) from two different angle corrections. 20 galactic gas and dust, most of the stars A 1st quarter moon is visible longer than a 3rd quarter moon due to the geometry of the

0 As the star sensor

0

10 14 9.6

4.8 References

0.87 1.31 1.75 2.18 2.62 3.05 3.49 3.93 4.36 5.24 5.67 6.11 6.54 6.98 7.42 7.85 8.29 8.73 9.16 10.5 10.9 11.3 11.8 12.2 12.7 13.1 13.5 0.44 are too dim to be distinguished. We turn star sensor’s aperture. This is an important difference that must be modeled. -20 aperture passes Degrees to the Moon and the superior as over an image, the 1. Acton, C.H.; "Ancillary Data Services of NASA's Navigation and 3. The Interstellar Boundary Explorer (IBEX) Authors: McComas, image is visible Ancillary Information Facility;" Planetary and Space Science, Vol. 44, No. 1, D. J.; The Ibex Science Team Journal: 35th COSPAR Scientific The split-V aperture allows the star sensor to determine both very bright objects whose orbits are well- pp. 65-70, 1996. Assembly. Held 18 - 25 July 2004, in Paris, France., p.163. brightness and right ascension of an object. One isolated star through each slit. • 2. McComas et al., Physics of the Outer Heliosphere, AIP Conference 4. IBEX Web-page: http://www.ibex.swri.edu/mission/index.shtml will create a signal like this one. defined. Do the lunar maria The time of Proceedings,Vol. 719, held 8-13 February, 2004 in Riverside, California. 5. NAIF SPICE Web-page: http://naif.jpl.nasa.gov/naif/ separation (dark areas) affect Edited by Vladimir Florinski, Nikolai V. Pogorelov, and Gary P. Zank. Melville, NY: American Institute of Physics, 2004., p.162-181. Planets appear as bright stars with varying positions, but the Moon is more difficult to between each slit the moon signal? model. The satellite has a highly elliptical Earth orbit so the angular size of the moon varies with the constantly changes. Also, the phase varies from a slight gibbous to a slight crescent. right ascension (horizontal Yes, and the elevation angle may be more strongly affected than the spin phase angle. position) of the On average the IBEX star sensor will see the Moon once every two weeks, and the phase image. Furthermore, this is also a phase-dependent effect. will always be very close to 1st or 3rd quarter.