HORUS is a 2.4-meter class space telescope that will address pivotal components in the NASA Origins Roadmap. Within the Origins Science Mission (OSM) cost envelope of $670M (FY04), HORUS will provide 100 times greater imaging efficiency and >10 times greater UV spectroscopic sensitivity than currently exist on HST. We propose to study the requirements, technical implementation, management, cost, and technology roadmap of this mission for conducting critical observations of the formation of planets, stars, and galaxies.

The HORUS mission has a well-defined Origins scientific program at its heart: a statistically significant survey of local, intermediate, and high-redshift sites and indicators of star formation to investigate and understand the range of environments, feedback mechanisms, and other factors that most affect the outcome of the star and planet formation process and the path from the Big Bang to people. This program relies on focused capabilities unique to space and that no other planned NASA mission will provide: near-UV/visible (200-1100nm) wide-field, diffraction-limited imaging; and high-sensitivity, low- and high-resolution UV (100-320nm) spectroscopy.

Science investigation.

We employ a step-wise approach to our observing program in which both imaging and spectroscopy contribute essential information to our investigation. Step 1 - Conduct a census of all high-mass star formation sites within 2.5 kpc of the Sun to determine how frequently solar systems form and survive, and develop observational criteria connecting properties of the ionized gas to the underlying stellar population and distribution of protoplanetary disks. Step 2 - Survey all major star forming regions in the Magellanic Clouds, where we can still resolve important physical scales and structures, access starburst analogs, and sample star formation in an initial regime of low metallicity applicable to high-redshift galaxies. Step 3 - Extend the star formation survey to galaxies in the nearby universe in order to increase the range of galaxy interaction and metallicity environments probed. HORUS can observe entire galaxies surveyed by GALEX and Spitzer with 100 times better spatial resolution. Step 4 - Measure star formation and metal production rates in the distant universe to determine how galaxies assemble and the elements critical to life such as C and O are generated and distributed through cosmic time.

Observing efficiency.

The HORUS imager has a field of view (FOV) of ~200 square-arcminutes, uses a dichroic to create optimized UV/blue and red/near-IR channels for simultaneous observing in 2 bandpasses, and employs CMOS detectors with substantial quantum efficiency gains, especially at red wavelengths, over the CCDs used in HST's cameras. The spectrograph has far-UV and near-UV channels with Rowland mount designs optimized for high-sensitivity, small-field spectroscopy, equaling or exceeding the intended performance of the HST-COS instrument. We estimate discovery efficiency gains of factors of 100 for imaging and >10 for UV spectroscopy with HORUS relative to HST based on our design and assuming an Earth-Sun L2 orbit that provides long target visibility.

Assumptions.

We assume in designing this mission that HST Servicing Mission 4 is cancelled. If the COS and WFC3 instruments could be installed aboard HST through either astronaut or robotic servicing, we would likely reconsider the spectroscopic capability of HORUS. However, HORUS's imaging advantage over HST derives primarily from a much larger FOV, and WFC3 offers no such gain over HST-ACS and little over WFPC2. The imaging discovery efficiency advantage of HORUS therefore still applies.