Science Goals and Objectives: "What's up with Water?" Water is the key component of our planet's biosphere. All life uses water as its chemical medium. Water is also the linchpin molecule in the search for habitable worlds. Nevertheless, the origin of Earth's water, and more broadly solar system water, remains uncertain. We will develop Water Hunting Advanced Terahertz Spectrometer on a Ultra-small Platform (WHATSUP) to address this key question. WHATSUP, implemented on ultra-small platforms such as CubeSat and SmallSat, will measure the D/H and 17O/18O isotope distribution of water sublimating from comets and is responsive to the Decadal Survey's goal of deciphering the record in primitive bodies of epochs and processes not obtainable elsewhere. The origin of D:H and oxygen isotope anomalies in meteorites has long been a matter of debate, and WHATSUP observations will provide unique information as to whether isotopic ratio gradients are present in the solar system. This would shed light on the origin of the mass-independent isotopic anomaly, and would moreover reliably provide new isotopic constraints on important reservoirs that might have supplied water to the Earth. Methodology: WHATSUP features a low-mass and low-power 500-600 GHz molecular spectrometer capable of remotely measuring water isotopes based on their unique rotational molecular spectra at sub-millimeter wavelengths. The D/H and 17/18O relative abundance of comets will be measured to a precision better than a few percent in a few hours of observation from <15,000 km distance by pointing the spectrometer just above the comet's limbs. This performance estimate comes from a combination of the receiver sensitivity of JPL's well-characterized 560 GHz Schottky diode-based room temperature mixer, combined with some assumptions about water column distribution of typical comets. The spectrometer includes a heterodyne sensor that is a direct descendant of the 560 GHz MIRO receivers flying on Rosetta; low mass/power silicon integrated circuits that have been developed and demonstrated as part of previous MATISSE effort (a synthesizer and a Fourier transform spectrometer); and an integrated low-loss waveguide switch for differential radiometric calibration. The novel calibration switch uses a low-loss waveguide structure with a large deflection microelectromechanical systems (MEMS) actuator to mechanically alter the waveguide dimension to create the switching function. This integrated calibration switch is a big step forward in these spectrometer instruments as it will make the use of power-hungry flip-mirror assembly unnecessary for instrument calibration. The spectrometer's ~18 cm aperture diameter achieves a ~3 mrad beam footprint for remote sensing. We have developed a spiral leaky-wave planar metasurface antenna with a circular waveguide feed that can be easily integrated to the calibration switch and the spectrometer front-end. The mass and power of the instrument are below 2.0 kg and 5 W, making it ideal for implementation on ultra-small platforms such as CubeSat and SmallSat. Relevance to MatISSE Goals: The proposed instrument is relevant to NASA's goal to "explore and observe objects in the solar system to understand how they form and evolve" and "improve our understanding of the origin and evolution of life on Earth" through an investigation of the processes involved in the formation of cometary water and the oxygen isotopic fingerprint. This work addresses several objectives of the MatISSE program element as listed in the NRA. Technology developed for WHATSUP can be applied to future submillimeter-wave radiometer/spectrometers to cometary bodies. The instrument developed under this program can also be used to missions to Mars, Europa, Enceladus, Venus, and Titan. Under this program, we will mature instrument technology to TRL 6 (currently at TRL 4) and demonstrate the working instrument in the laboratory and validate TRL 6 for most critical components.