The MOST Project at a Glance
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Scientific Goals
(in astro jargon)
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Detection and characterisation of: (1) acoustic oscillations in
Sun-like stars, including very old stars (metal-poor subdwarfs) and
magnetic stars (roAp), to probe seismically their structures and
ages;
(2) reflected light from giant exoplanets closely orbiting
Sun-like stars, to reveal their sizes and atmospheric compositions;
and (3) turbulent variations in massive evolved (Wolf-Rayet) stars
to understand how they add gas to the interstellar medium.
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Scientific Goals
(in plain English)
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Can we understand our Sun in the context of other stars? By putting
a birthdate on the oldest stars in the solar neighbourhood, can we
set a limit on the age of the Universe? How do strong magnetic
fields affect the physics of other stars and our own Sun? What are
mysterious planets around other stars really like? How did the
atoms which make up our planet and our very bodies escape from
stars in the first place?
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The Experiment
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To perform ultra-high-precision photometry (i.e., measurement of
brightness variations to a level of 1 part per million) of stars down
to the naked-eye limit of visibility (magnitude 6) for up to two
months without major interruptions. (Note: To put the sensitivity
of MOST in perspective, look at a streetlamp 1 km away and then
move your eye 0.5 mm closer to it. The streetlamp is now about 1
ppm brighter to your eye.)
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The Instrument
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An optical Telescope with a collecting mirror only 15 cm across,
feeding a CCD (Charge Coupled Device) camera with twin Marconi
47-20 frame-transfer devices (1024 by 1024 pixels) side-by-side.
One CCD is used for science measurements; the other is read out
every second to track guide stars for satellite attitude control.
The Instrument contains a single broadband filter which selects
light in the wavelength range 350 - 700 nm.
The camera is equipped with an array of Fabry microlenses which
project a large stable image of the Telescope pupil illuminated
by target starlight, which is key to the photometric precision
of MOST. For low cost and high reliability, the Instrument has
no moving parts - the structure automatically maintains the same
focus across a wide range of temperatures, and exposure times
are controlled by rapid frame transfer of the CCDs. The CCDs
are cooled by a passive radiator system.
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The Spacecraft
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The Instrument is housed in a suitcase-sized microsatellite
(65 x 65 x 30 cm; mass ~ 60 kg) powered by solar panels and
oriented by a system of miniature reaction wheels and magneto-
torquers. The attitude control system should keep the
Telescope pointing within 10 arcseconds of the desired target
99% of the time. This is an improvement of two orders of
magnitude over previous micro-satellite pointing capability.
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Launch & Orbit
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MOST was carried aloft aboard a Russian three-stage Rockot
(a former Soviet ICBM now being put to peaceful service) on June 30, 2003, from
a launch site in northern Russia (Plesetsk). MOST was
injected into a low-Earth polar orbit (approx. 820 km altitude;
period ~ 100 min) in a Sun-synchronous mode remaining over the
terminator of the Earth. From that vantage point, it will have
a Continuous Viewing Zone (CVZ) spanning declinations from about
-19 to +36 degrees, in which a selected target star will remain
observable for up to 60 days without interruption.
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Communications
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Three S-band stations with 2.5-m dishes are located in Toronto,
Vancouver, and Vienna to allow the MOST team to send commands and
receive data from the microsat. We will be in direct contact with
MOST for up to 40 minutes per day per ground station, during which
commands will be uploaded at 9,600 kBs and data downlinked at 38,400 kBs.
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