For months, NASA’s Cosmic Origins Program Analysis Group (COPAG) has been discussing the prospects for a large ultraviolet-visible-wavelength telescope.

A visible-wavelength telescope that would fly sometime after 2020 and explore the architecture of galaxies and the details of their formation. Then the Exoplanet Exploration Program Analysis Group (ExoPAG) got wind of the plan. Astronomer Kenneth Sembach of the Space Telescope Science Institute in Baltimore, says that he was originally expecting a small group at a joint COPAG and ExoPAG meeting recently. In the end, 49 astronomers attended. Exoplanet hunters are keen for a new instrument. Results are pouring in from Kepler, NASA’s first orbiting telescope dedicated to finding other earths. Although Kepler can detect planets between about 184 and 920 parsecs away that are transiting in front of their host stars, it is generally unable to determine their masses or compositions. Researchers hope for a follow-up mission that would image earth-like planets around the nearest 100 sun-like stars at sufficient resolution to detect traces of oxygen or water in their atmospheres.

The Terrestrial Planet Finder – a NASA project – was indefinitely postponed in 2007, but the agency has continued to put $6 million a year into developing technology for exoplanet searches. Planet hunters think that joining forces with COPAG will be the winning strategy. Jim Kasting, a planetary scientist at Pennsylvania State University in University Park who is chair of the exoplanet group, says the two communities would both like to see a 4-8-meter telescope in space that would cost in excess of $5 billion.
A cosmic-origins mission would need to collect as much ultraviolet and visible light as possible to image intergalactic gas, star formation and sun-like stars in nearby galaxies.

A planet-hunting probe would need a coronagraph to block direct light from host stars, and would have to be sensitive to the visible and near-infrared wavelengths that earth-like planets primarily emit. The infrared 6.5-meter James Webb Space Telescope (JWST), scheduled for launch in 2014, will be able to see larger planets, but not earth-sized ones. The key will be to develop a reflective coating for the telescope’s mirror that works from the ultraviolet to the infrared ranges and does not distort the incoming light waves in a way that would make it difficult to tease out the image of an exo-earth. Protocol ExchangeSara Seager, a planet hunter at the Massachusetts Institute of Technology in Cambridge, is not convinced.

Although smaller-scale planet-hunting missions set to launch in the next decade will not be sensitive enough to find other earths, she adds, they may help increase momentum to build a dedicated space telescope. But Natalie Batalha of San Jose State University in California, deputy team leader for Kepler, says a joint mission may be the logical response to NASA’s money troubles.