News - 26-Mar-2021

OASIS: a Far-Infrared Space Mission

(The same article was published at SRONieuws, March 2021)

OASIS is een missieconcept dat een droom waar kan maken voor submm/ver-infrarood sterrenkunde: hoge-resolutie imaging van bijvoorbeeld water op te sporen op asteroïden of in bewoonbare zones van exoplanetenstelsels. Terwijl in het verleden geprobeerd werd dit te verwezenlijken door interferometrie vanuit de ruimte zo als bij de Westerbork Radiotelescoop vanuit de aarde en dit als te duur werd bevonden, is het revolutionaire nu om een opblaasbare spiegel de ruimte in te brengen met een diameter van maar liefst 20 meter. Vergelijk dit met de James Webb ruimtelescoop die een diameter van ‘maar’ 6 meter heeft. OASIS zal later dit jaar voorgesteld worden als een MIDEX-missie van NASA. SRON is door OASIS-PI Chris Walker (Universiteit van Arizona) gevraagd om tegen betaling 3 HEB mixer detectoren te bouwen.

OASIS sounds like a truly dream space mission for the sub-millimeter (Submm) and far-Infrared (FIR) space instrumentation community because it can offer a huge telescope of 20 meters in diameter, and because it is at a fraction of the cost of a traditional telescope. The large size of a telescope is in high demand for Submm and FIR wavelengths since we would like to resolve the observed objects spatially. The so-called spatial resolution is fundamentally limited by the size of the telescope. For the larger wavelengths of the radiation, a larger aperture of the telescope is a must, which is imposed by the law of optical diffraction. Additionally, a larger telescope can also collect more photons, increasing the signal to noise ratio. The Herschel Space Telescope, launched by ESA in 2009, has a diameter of 3.5 meters, which is limited by the launching rocket. To go beyond the Herschel telescope size, one needs a different configuration of a telescope than the James Webb Space Telescope (JWST), which has an effective diameter of 6 meters, but cost 9-10 billion US dollars, which is truly an astronomic number.

OASIS stands for “Orbiting Astronomical Satellite for Investigating Stellar Systems”, which is a mission concept, to be proposed to NASA at the end of 2021 in response to the Astrophysics Medium Explorers (MIDEX) Announcement of Opportunity (AO). The cost cap will be 290 M$. If all is going well, the launch date will be around 2028. It will be operated in a Sun-Earth L2 halo orbit. The targeted mission life time is not less than 2 years.

OASIS is originally going to employ a 20-meter inflatable aperture with heritage from the Inflatable Aperture Experiment (IAE) mission, which demonstrated in-orbit deployment of a 14-meter aperture system and is shown in Figure 1. OASIS plans to utilize a Hencky reflector geometry together with proven adaptive optics techniques to yield a wide-field-of-view inflatable Mylar mirror operating. The key structure of the telescope consists of three struts with filled tubes, and an inflated support ring. The mirror (or antenna) consists of an inflatable Mylar based disk, where one side is a clear Mylar layer, while the other side is metalized Mylar that acts as the mirror.

A new idea for OASIS is to apply a flight-proven deployable mesh reflector with an aperture size of 20 meters to be developed by a US company (Northrop Grumman). Their designs are scalable, inherently stiff, incredibly light with unmatched surface accuracy, and free from the so-called Passive Inter-Modulation. Figure 2 shows the reflector structure, a drum like structure, fully deployed and latched. This family of deployable mesh reflectors have extensive flight heritage with 100% on-orbit success. Such a reflector surface structure has been good to detect radiation up to 50 GHz. However, for submm and FIR radiation targeted by OASIS, the mesh surface is too coarse to meet the science goals. So, as the latest development, OASIS will apply a Northrop Grumman’s deployable reflector structure as shown in Figure 2, but the Hencky reflector will apply the inflatable disk with metalized Kapton, similar to IAE (shown in Figure 1). The reflector looks really huge, but it is supposed to be small/compact before and during the launch since all the unfold structures of supporting struts and truss rings are stowed.

The instrument will consist of heterodyne receivers, which are similar to the receivers in HIFI of the Herschel Space Telescope, to cover four frequency bands from roughly 500 GHz to 5 THz. Band 1, which is frequency tunable, is to cover a few lines of HDO, H2O, 13CO. Band 2 is to detect H2O line at 1.67 THz. Band 3 is for the HD line at 2.68 THz, and Band 4 for H2O at 5 THz. The baseline of the detectors for Bands 2-4 will be the NbN hot electron bolometer (HEB) mixers, the same as used for the GUSTO project, for which SRON together with TU Delft has delivered three flight HEB arrays. The University of Arizona (PI institute) and NASA consider the Heritage of the GUSTO technology to be essential for OASIS. Prof. Chris Walker (PI) has invited SRON to contribute the HEB mixers. To get the utmost out of the OASIS mission, it will be really interesting if one could take advantage of a so-called dual polarization mixer by introducing a novel antenna configuration on chip although with the same detector technology. Such a mixer acts as two conventional mixers. Thus, OASIS will effectively collect a double amount of science data or effectively double OASIS mission life time.

OASIS will observe the universe using wavelengths that would allow it to detect the presence of water in deep space. It can help locate water-rich asteroids within our solar system, or help detect water in the habitable zones of other solar systems. There is also the prospect to detect gaseous water near the stars in protoplanetary systems, which might explain the mystery of how Earth came to be covered with so much water. The OASIS science team is preparing more science topics, which OASIS might be able to address. Frank Helmich from SRON and Prof. Xander Tielens from Leiden University, representing the Dutch astronomers, are part of the team.

After Herschel, people believed that having a space interferometry mission is the only way to go, but it is too expensive and also too challenging technically to realize. OASIS opens a new avenue towards FIR space science and is game-changing. OASIS offers a dream project for SRON and also for those who have contributed heterodyne technology development in the past many years. The University of Arizona plans to cover the cost of building the mixers at SRON, in the same way as for the GUSTO project.

Jian-Rong Gao