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many advantages are present when going in space .
the most exploited one has been the access to wavelengths not transmitted through the atmosphere . for photometry near the visible part of the spectrum
, there are many benefiting aspects : no atmosphere means no absorption bands of @xmath0 and @xmath1 , no scintillation , no emission or diffusion from the sky ( dark sky ) , no variable extinction . furthermore depending on the satellite orbit , it may be possible to make long continuous monitorings ( however eclipses by the moon or the earth may occur ) or to make a global sky coverage .    as a comparative example , the corot team ( see section  6 ) studied fictive earth projects ( cf .
baglin 1997a ) and showed that even an immoderate project of 8 8m - class telescopes disseminated around the earth would not surpass their small space mission .
the drawbacks of space projects are their complexity , cost , aging , limited operation time , and many technical constraints .
space missions are fragile on technical and also political point of views .
the global budgets of esa and nasa in local currencies have been relatively flat for about 8 years , but they may be subjected to high volatility according to changes in the policy .
three main types of missions will be presented : one performing astrometry , one oriented towards asteroseismology and one aiming at detecting telluric planets .
many of these projects lead to massive photometric surveys and have thus very valuable scientific returns as discussed by paczyski ( 1997 ) . then , an overview of the distinctive features of some specific satellites will be given .
after the pioneering mission hipparcos , projects performing astrometric measurements are flourishing .
thanks to the advance of technology ( ccd , interferometry ) , the jump in astrometric precision is of 2 - 3 orders of magnitude , thus making a new mission very attractive .
the accuracies reached by these projects are now labeled in @xmath2as ( micro arcsec ) , cf .
table  1 .
the interest for variable stars is threefold .
first , several proposed missions are scanning the whole sky and performing photometry .
they will thus give , as hipparcos did , huge amounts of photometric measurements , achieving magnitude completeness .
as the scanning law is optimised for the astrometric purpose , the time series have a semi - regular sampling which produce aliasing .
second , the information of parallax is a stringent constraint for testing stellar models when used in asteroseismology ( cf .
baglin 1997b , favata 1999 ) . in the case of hipparcos for instance ,
matthews et al . ( 1999 ) showed a comparison of asteroseismologic parallax versus trigonometric parallax . on an other hand ,
absolute luminosities or masses derived from parallaxes can be used as starting point of the seismologic models .
third , parallaxes are also of primordial importance for calibrating distance indicators as cepheids , rr lyrae and miras .
all the presented missions will literally pin down many of these objects .
.summary of several experiments performing astrometry .
mag is the magnitude range or limiting magnitude , n the number of measured stars in million , @xmath3 the astrometric precision in @xmath2as at @xmath4 and @xmath5 , @xmath6 is the date for realisation decision ( month / year ) , @xmath7 is the year of the launch . [
cols="<,>,>,>,>,>,>",options="header " , ]      several unsuccessful attempts have been made to measure solar - like oscillations from earth , for such measurements the precision in luminosity determination to be reached is as high as few ppm , furthermore high frequency resolution is required which imposes very long continuous monitoring . as previously remarked ,
the scintillation is a limiting factor which is magnitude independent as long as photon noise is not dominating . with space missions
these requirements can be reached .      among several methods to detect planets , the most famous is the measurement of the periodic variation in radial velocity that a planet induces on its host star .
another promising technique , shown to be more appropriate for the discovery of telluric planets , is the detection of the luminosity decrease during an eclipsing transit of the planet . in this case
, the system planet - star should be in a favorable configuration for the observer .
thus large numbers of stars must be observed to achieve valuable statistics .
the goal of the gaia ( `` http://astro.estec.esa.nl/sa-general/projects/gaia/ ) satellite is to describe our galaxy and its history by making an inventory of 1% of its stars ( down to magnitude v=20 ) in terms of their kinematical and physical properties .
however , the impact of gaia will go well beyond gaining knowledge of the galaxy , it will infiltrate many domains of astronomy such as stellar astronomy , general relativity and cosmology .
gaia should not be seen merely as a super - hipparcos ; it has several instruments among them one is performing astrometry reaching the precision of 4 @xmath2as for magnitude @xmath8 and 10 @xmath2as for magnitude @xmath5 .
these measurements lead to parallaxes and proper motions . on board
, gaia has a spectrometer which measures radial velocities ( with precision of 3 - 10 km / s for @xmath9 ) to complete the 3-dim velocity field .
a provisional 11 intermediate band photometric system was specially designed in order to determine the temperature , gravity and metallicity of the observed stars .
gaia will produce photometric time series of about 100 measurements spread over 5 years .
the precision is magnitude dependent ( cf .
grenon et al .
1999 ) and for a single transit is of the order of 0.05 mag at @xmath10 in the band f51 ( centered at 570 nm and of 90 nm width ) .
the time sampling is semi - regular as a result of the scanning law of the satellite .
eyer & cuypers ( these proceedings ) are studying the expectation for variable star numbers .
although the estimations are at preliminary stages , gaia might detect a few thousand cepheids and about 90000 rr lyrae .
gaia will permit to study star pulsations in terms of the physical parameters , especially the metallicity .
fame ( `` http://aa.usno.navy.mil/fame/ ) stands for full - sky astrometric mapping explorer .
fame will observe 40 million stars and will perform a galactic survey magnitude complete up to @xmath5 .
the astrometric precision will be @xmath11 50 @xmath2as for mag @xmath12 and @xmath11 500 @xmath2as for mag @xmath13 .
it will collect photometric measurements in four bands of the sloan system ( fukugita et al .
the announced precision of the photometry is 1.6  mmag for mag 9 and 25  mmag for mag 15 in the astrometric filter .
the scanning law will determine the time sampling .
the rotation period of the satellite is 40 minutes , the length of the mission is 2 years and a half .
the satellite will furnish thousands of measurement per star , with a semi - regular sampling grouped in sequences of 9 - 31 - 9 etc minutes .
the diva ( ` http://www.aip.de/groups/diva/ ` , double interferometer for visual astrometry ) satellite is a project of the german space agency ( dlr ) .
diva will reach an astrometric precision of 150 @xmath2as at @xmath4 and 5 mas at @xmath5 .
diva will have broad - band and narrow - band systems .
the photometric precision is under study .
the scanning law is very similar to the hipparcos one , with a rotation period of 2 hours .
the corot ( ` http://www.astrsp-mrs.fr/www/pagecorot.html ` , convection and rotation ) satellite has two major goals , first it was designed for doing asteroseismology , focussing on 5 principal fields for very precise and continuous monitoring of 5 months each .
the noise level is 0.6 ppm for g type stars over 5 days .
the long time base makes possible a high accuracy on frequencies determination ( 0.1 @xmath2hz ) necessary for the detailed seismic investigation of the stellar structure proposed for corot .
a planet detection program using the transit method was added , this other goal has to survey large sample of stars ( 30  000 to 60  000 ) with a photometric precision of 0.1  % for 16 minutes of integration time in order to detect telluric planets .
this survey is achieved in dense fields down to mag 15 .
two - color information will be available for @xmath14 1/3 of the targets to establish the achromaticity of the phenomenon .
there is also an exploratory program where corot will measure 50 to 80 solar - like stars with @xmath15 at a precision better than 2.4  ppm .
mons ( ` http://www.obs.aau.dk/mons/ ` , measuring oscillation in nearby stars ) is a project of the danish small satellite program , which already launched one successful satellite ( rsted ) .
mons will observe about 20 solar - like stars over two years as a primary scientific objective ; it will also measure @xmath16 scuti , roap stars and samples of stars of all types as a secondary objective .
its eccentric orbit with high apogee ( molniya orbit ) permits access to a large part of the sky
. it will measure changes in colour ratio .
oscillations of amplitude of 1 - 10 ppm will be detected ( cf .
kjeldsen et al .
furthermore , mons will obtain science data from the star imagers designed for attitude control ; they will permit measurement of about 900 stars , 700 in with the imager si 1 , and 200 with the imager si 2
. the accuracy will be about 11 ppm ( si 1 ) and 26 ppm ( si 2 ) at magnitude @xmath17 .
most ( ` http://www.astro.ubc.ca/most/ ` , microvariability and oscillations of stars ) is a funded project which is the first satellite of the canadian space agency
. most will observe several ( 3 - 5 ) solar - like stars over 40 days .
it will also observe roap , wolf - rayet and @xmath16 scuti stars .
the photometric precision for a @xmath17 star is better than a few ppm for 10 days of integration .
although the kepler project ( ` http://www.kepler.arc.nasa.gov/ ` ) was not selected due to questions about the ability to perform the photometry in space , the scientific case was rated very highly .
the kepler team is planning to resubmit the project .
its goal is to `` explore the structure and diversity of planetary systems '' by performing differential photometry .
it will monitor 100000 main - sequence stars in the magnitude range from 9th to 14th with an accuracy of 0.002  % including instrument noise , shot noise and stellar variability for a star of @xmath8 .
the mission would point continuously at a single field in cygnus during 4 years . because only data for preselected stars are saved , all objects to be monitor must be pre - specified .
the team will probably entertain `` guest observing '' for additional interesting objects like variable stars .
a subset of a few hundred brighter stars will also be incorporated and observed with a shorter integration time for asteroseismological purposes .
many of these projects might be selected in a near future for their realisation .
the projects performing astrometry will provide deep galactic surveys and thus an inventory for galactic variable stars .
the asteroseismological projects have common goals but they use different ingenious techniques and have different and diverse by - products .    the future spatial projects will open new domains in variable star studies , therefore precise predictions are delicate . since some presented missions are already funded , there is the conviction that after the projects like hipparcos or the microlensing surveys , the domain of variable stars will still be under a strong evolution .
baglin , a. 1997a , cnes , cor - sp-0 - 83-proj baglin , a. 1997b , in the first results of hipparcos and tycho ( jd 14 of the xxiii general assembly of the iau ) , ed . c. turon , 555 grenon , m. , jordi , c. , figueras , f. , torra , j. 1999 , to be published favata , f. 1998 , in the first mons workshop : science with a small space telescope , eds . h. kjeldsen , t.r . bedding , 89 fukugita , m. , hichikawa , t. , gunn , j.e . , doi , m. , shimasaku , k. , schneider , d.p .
1996 , 111 , 1748 kjeldsen , h. , bedding , t.r . , christensen - dalsgaard , j. 1999 , document no .
mons-99/01 matthews , kurtz , d. , & martinez , p. 1999 , 511 , 422 paczyski b. 1997 , in variables stars and the astrophysical returns of the microlensing surveys ( 12th iap astrophysics colloquium ) , eds .
r. ferlet , j .-
maillard & b. raban , editions frontires , 357
esa and nasa are studying projects having a tremendous return on variable star research . 
 other national space agencies are also studying or developing projects of smaller costs but with impressive returns . 
 the projects range from global galactic surveys like the esa mission gaia which will give photometric time series for about 1 billion stars , to detailed pulsation modes studies like the cnes mission corot which could reach a photometric precision lower than 1 ppm . 
 the presentation will emphasize the future astrometric , asteroseismologic and planet detection missions .    
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