  We describe the space observatory architecture and mission design of the
SALTUS mission, a NASA Astrophysics Probe Explorer concept. SALTUS will address
key far-infrared science using a 14-m diameter <45 K primary reflector (M1) and
will provide unprecedented levels of spectral sensitivity for planet, solar
system, and galactic evolution studies, and cosmic origins. Drawing from
Northrop Grumman's extensive NASA mission heritage, the observatory flight
system is based on the LEOStar-3 spacecraft platform to carry the SALTUS
Payload. The Payload is comprised of the inflation control system (ICS),
Sunshield Module (SM), Cold Corrector Module (CCM), Warm Instrument Electronics
Module, and Primary Reflector Module (PRM). The 14-m M1 is an off-axis
inflatable membrane radiatively cooled by a two-layer sunshield (~1,000 m2 per
layer). The CCM corrects for residual aberration from M1 and delivers a focused
beam to two instruments - High Resolution Receiver (HiRX) and SAFARI-Lite. The
CCM and PRM reside atop a truss-based composite deck which also provides a
platform for the attitude control system. The 5-year mission lifetime is driven
by a two-consumable architecture: the propellant system and the ICS. The Core
Interface Module (CIM), a multi-faceted composite truss structure, provides a
load path with high stiffness, mechanical attachment, and thermal separation
between the Payload and spacecraft. The SM attaches outside the CIM with its
aft end integrating directly to the bus. The spacecraft maintains an attitude
off M1's boresight with respect to the Sun line to facilitate the <45 K thermal
environment. SALTUS will reside in a Sun-Earth halo L2 orbit with a maximum
Earth slant range of 1.8 million km thereby reducing orbit transfer delta-v.
The instantaneous field of regard provides two continuous 20-deg viewing zones
around the ecliptic poles resulting in full sky coverage in six months.
