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Morgan O’Neill
Physics Department
UNH, Durham, NH
Maciej Bzowski
Space Research Centre
PAN, Warsaw, Poland
Eberhard Möbius
Space Science Center
& Physics Department
UNH, Durham, NH
Marek Hłond
Space Research Centre
PAN, Warsaw, Poland
Precision Pointing in the Sky
for IBEX Interstellar Flow Observations
Introduction
The Sun ejects a hot, fast wind that speeds through
the solar system, before it is slowed down by the gas
and dust in the space between the Sun and other stars
(the interstellar medium). The solar wind creates a
protective bubble, or ‘heliosphere’ around the solar
system, as we travel through the Milky Way; however,
a wind of the interstellar medium blows through the
heliosphere, and we can detect the gas from a satellite
in Earth orbit.
Motivation
Results Cont’d
Current IBEX software only recognizes the moon’s gravitational center, and we need to correct the data for
changes in phase and distance. The ability to precisely determine the moon’s position with respect to the
IBEX satellite will improve the accuracy of the star sensor.
The most important variation of the moon signal is that due to the varying phase.
• For a full moon, the center of the illuminated disk is also the center of gravity.
• For a quarter moon, the lit portion does not represent the true center of the moon.
→ An angular correction is required so that we can correctly identify the moon’s position.
A full moon signal reflects the moon’s
barycenter. We can quantify the growing
difference in peak separation as the
phase approaches a crescent. The
changing peak separation corresponds to
a different apparent right ascension for
the moon’s position. The measured
spin phase can also change depending
on the center between the two peaks.
The Sun is moving through a local interstellar cloud, and
some gas and dust from this cloud streams through the
heliosphere into our solar system.
IBEX Mission
The Moon
The Interstellar Boundary Explorer (IBEX) is the latest satellite of NASA’s Small Explorer
Mission Program. IBEX was launched in October 2008 and orbits the Earth to measure the
nature and amount of particle flow that originated in the interstellar medium.
The plot shows an IBEX orbit during which the moon was visible. Note the drop in resolution in the center to
accommodate the bright moon. The moon provides a redundant check of position for the IBEX Hi and Lo sensors.
The moon is much
brighter than other
objects in the sky so when
it comes into view, the
bias voltage of the star
sensor is greatly reduced.
At a lower voltage the
moon signal does not
saturate the star sensor.
Angular Peak Separation Varies With Moon Phase
16.4
16.3
16.27
16.15
16.2
Angular Peak Separation (in degrees)
Harald Kucharek
Space Science Center
UNH, Durham, NH
16
15.8
15.66
15.6
15.4
15.2
crescent
quarter
gibbous
full
Moon Phase
15.66°
Angular Separation
of Peaks
16.15°
16.27°
16.30°
Mission Design & Star Sensor
Moon Simulation Program & Results
To image the ISM flow and the
heliospheric boundary, IBEX instruments
will be oriented perpendicular to a sunpointing spin axis. The satellite will scan
the entire sky every six months.
A program was created that partially simulates the function of the star sensor. The
main layer is a pixelized image of the star sensor aperture. A second layer is that of the
object of interest – in this case, the moon in varying phases. This moon image is
passed over the star sensor aperture (example at left) and the amount of visible bright
pixels is counted at each position.
To locate this flow in the sky, a star sensor
is co-aligned with one of the primary
IBEX flow instruments.
The IBEX will orbit the Earth with a sun-pointing spin axis,
and will view ribbons of the sky as it rotates.
One Full Sweep of Light Source by Star Sensor
140
120
100
60
40
20
14
13.5
13.1
12.7
12.2
11.8
11.3
10.9
10.5
10
9.6
9.16
8.73
8.29
7.85
7.42
6.98
6.54
6.11
5.67
5.24
4.8
4.36
3.93
3.49
3.05
2.62
2.18
1.75
1.31
0.87
0.44
0
0
Voltage
80
-20
Degrees
The split-V aperture allows the star sensor to determine both
brightness and right ascension of an object. One isolated star
will create a signal like this one.
This star sensor senses light, and it will
create a star map as it spins with the
satellite. The influx of particles will be
overlaid onto the generated map in order
to determine the source location of the
flow precisely. However, due to zodiacal
light (background brightness) from
galactic gas and dust, most of the stars
are too dim to be distinguished. We turn
to the Moon and the superior planets as
very bright objects whose orbits are welldefined.
Planets appear as bright stars with varying positions, but the Moon is more difficult to
model. The satellite has a highly elliptical Earth orbit so the angular size of the moon
constantly changes. Also, the phase varies from a slight gibbous to a slight crescent.
On average the IBEX star sensor will see the Moon once every two weeks, and the phase
will always be very close to 1st or 3rd quarter.
This program allows us to answer a number of questions about how the moon’s phase,
distance and uneven brightness affect the star sensor signal.
•What is the difference
between 1st and 3rd
quarter phase?
As the star sensor
aperture
passes
over an image, the
image is visible
through each slit.
The time of
separation
between each slit
varies with the
right
ascension
(horizontal
position) of the
image.
Conclusions & Goals
• There is a noticeable and measurable difference between 1st and 3rd quarter phases.
• The dark mare on the moon’s surface alter the star sensor moon signal and require a
correction for measured elevation.
• As the moon’s phase approaches a crescent, the necessary angular correction grows
Upon full completion of phase characterization, a moon phase function will be
incorporated into the main IBEX star sensor. The input will be the current time, the
satellite position and the moon position. Depending on phase and distance, the function
will pass back an angular correction for the true barycenter of the moon. Since both the
spin phase and the elevation angles can be affected, so any moon sighting will require
two different angle corrections.
A 1st quarter moon is visible longer than a 3rd quarter moon due to the geometry of the
star sensor’s aperture. This is an important difference that must be modeled.
•Do the lunar maria
(dark areas) affect
the moon signal?
Yes, and the elevation angle may be more strongly affected than the spin phase angle.
Furthermore, this is also a phase-dependent effect.
References
1.
Acton, C.H.; "Ancillary Data Services of NASA's Navigation and
Ancillary Information Facility;" Planetary and Space Science, Vol. 44, No. 1,
pp. 65-70, 1996.
3.
The Interstellar Boundary Explorer (IBEX) Authors: McComas,
D. J.; The Ibex Science Team Journal: 35th COSPAR Scientific
Assembly. Held 18 - 25 July 2004, in Paris, France., p.163.
2.
McComas et al., Physics of the Outer Heliosphere, AIP Conference
Proceedings,Vol. 719, held 8-13 February, 2004 in Riverside, California.
Edited by Vladimir Florinski, Nikolai V. Pogorelov, and Gary P. Zank.
Melville, NY: American Institute of Physics, 2004., p.162-181.
4.
IBEX Web-page: http://www.ibex.swri.edu/mission/index.shtml
5.
NAIF SPICE Web-page: http://naif.jpl.nasa.gov/naif/