 In the early 1990s , NASA initiated the Discovery Program , which was intended to be a series of low @-@ cost scientific missions . In 1996 , the program 's study team recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine . Funding for this program remained problematic for several years , but by 2004 the Dawn vehicle had passed its critical design review . 
 In the early 1990s , NASA initiated the Discovery Program , which was intended to be a series of low @-@ cost scientific missions . In 1996 , the program <a href='null_genetive' title='1'><mark>'s</mark></a>study team recommended as a high priority a mission to explore the asteroid belt using a spacecraft with an ion engine . Funding for this program remained problematic for several years , but by 2004 the Dawn vehicle <a href='been_done' title='2'><mark>had</mark></a> passed its critical design review . 
In da early 1990s , NASA initiated tdaDiscovery Program , which was intended to be a series of low @-@ cost scientific missions . In 1996 , the program  study team recommended as a high priority a mission to explore thdasteroid belt using a spacecraft with an ion engine . Funding for this program remained problematic for several yrs , but by 2004 the Dawn vehicle done passed its critical design review .