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ULTRASAT is a scientific mini-satellite carrying a telescope with an unprecedentedly large field of view (210 squared degrees) observing in the ultraviolet (220-280nm,UV). Its launch to geostationary orbit (GEO) is planned for 2023. ULTRASAT will revolutionize our understanding of the hot transient universe: the extra-Galactic volume, that will be accessible to ULTRASAT for the discovery of transient sources, is 300 times larger than that of the most sensitive UV satellite to date, GALEX, and comparable to that of the largest ground-based optical transient survey planned to begin operation in 2022, LSST.

ULTRASAT will have a broad science impact across the fields of gravitational wave sources, supernovae, variable and flare stars, active galactic nuclei, tidal disruption events, compact objects, and galaxies. It will open a new wave band (UV) and cadence access (minutes to months), with a vast space for serendipitous discovery. All ULTRASAT data will be transmitted to the ground in real time, and transient alerts will be distributed to the community in <20 min, driving a vigorous ground-based follow-up of static, variable and transient ULTRASAT sources.

One of the key science goals of ULTRASAT is the discovery of electro-magnetic emission following the detection of gravitational waves (GW) from the mergers of binaries involving neutron stars. Such detections will be the key to using these events for addressing fundamental physics questions, such as the origin of the heaviest elements and the expansion rate of the universe. ULTRASAT will be able to slew in minutes to >50% of the sky, and its wide field-of-view amply covers the angular error regions expected to be provided by GW detectors in the 2020s. It will provide continuous UV light curves as well as early alerts that will enable ground-based follow-up spectroscopy and monitoring of optical and infrared emission predicted to arise later.

The ULTRASAT spacecraft will be constructed by the Israeli Aerospace Industry (IAI), and the telescope will be constructed by Elbit/Elop. ULTRASAT is jointly funded and managed by the Israel Space Agency (ISA) and the Weizmann Institute of Science (WIS), under the scientific leadership of the WIS, and with a significant contribution of the DESY center of the Helmholtz association. ULTRASAT is planned for a 3-year operation at a GEO orbit. Its small mass and volume, 160 kg and <1m3, allows a launch to GEO as a secondary payload.

ULTRASAT is ground breaking both in terms of the science and in terms of the implementation. If successful, we will demonstrate that breakthrough science can be achieved by small satellites at an affordable cost, $100M including launch to GEO, leading the way to future similar missions. Beyond providing groundbreaking science, the high scientific profile of the mission will strengthen the world standing of the Israeli space industry as well as the international standing of Israel via the creation of collaborations with leading industries and agencies. It will also have a major educational impact drawing young talented Israeli people to science and technology.