Although some clever engineering solutions are proposed, no advanced technology is assumed in the basic design of the occulting vehicle.
The intent of the UMBRAS team has been to show that UMBRAS could be built yesterday. There are many clever ideas for searching for extraterrestrial planets, such as the Terrestrial Planet Finder, and the Space Interferrometry Mission, which the UMBRAS team strongly supports. There are even other proposed occulter missions such as BOSS. However, UMBRAS represents the most doable space mission which would allow extrasolar planets to be imaged in this decade.
Several different classes of UMBRAS missions are suggested, given that the occulting spacecraft has a scalable architecture.
The smallest scientifically interesting mission that we believe
UMBRAS could perform would involve an occulting vehicle under 1.4
metric tonnes with a screen some 5-8 metres across. We envision CORVET (
01-10-23) as launchable from an Atlas IIAS
(shown at right) with a small 1-metre diameter space telescope which could
be placed into an orbit about the sun far enough away from Earth's gravity
that science could be performed. CORVET would out-perform
any existing classic coronagraph, but it can directly observe
only the easiest of the currently known exoplanets.
CORVET is only a small UMBRAS mission, and even though it could do interesting science, it is really a technology demonstrator. We would expect a CORVET mission to show that the technique actually works as advertised. CORVET is only a precursor to much larger, more ambitious occulter missions.
CORVET uses the small space telescope to look for large Jovian-type planets around the nearest stars in conjunction with the occulter. From project start to launch, we estimate that less than 4 years are necessary until the first images are returned from CORVET.
Approximately 80% of the telescope time would not be used by the occulter mission. Because of the slow task of moving the occulter around at the tiny accelerations possible with the ion engines, most of the telescope time would be available for use by other programs or for use with the proposed on-board coronograph.
To ensure the highest optical quality of the images, and allow the best science to be returned, we propose that CORVET use an off-axis primary mirror with no occlusions by a secondary support in the optical path. This removes some diffraction spikes which complicate image analysis due to the light scattered in the field through to the focal plane.
Larger UMBRAS missions could be launched, including ones for
2-metre space telescopes, and even one for NGST. In fact, UMBRAS was
initially proposed as a 16-metre screen for use with NGST, however the small
screen size relative to the aperture of NGST made it of limited utility.
Since 1999, a scheme for packaging a larger 45-metre screen into
a launcher has been developed (shown at right, deployed in the
observing configuration).
The smallest UMBRAS mission conceived to date involves taking an already planned L2 spacecraft and modifying it to become an occulter. This type of minimal occulter would not be "activated" until the modified spacecraft's prime mission was complete. Several candidates for such a "piggyback-extension" to use with NGST have been identified and are under study by the UMBRAS team. For information, see the June 2001 UMBRAS newsletter.
