I gave an update on the Vera Rubin Observatory. Undeniably, VRO is (will be…) a game changer. Also noteworthy but less newsworthy: the SPHEREx spacecraft is built:
SPHEREx, for Spectro-PHotometric ices and Epoch of Reionization Explorer, is a NASA mission to map the sky- whole sky- including some key infrared wavelengths. A telescope and its spectrometer instrument, pointing away from Earth, will then sweep a band of sky as the craft wheels about its orbit. Over six months, as Earth itself wheels about the Sun (carrying SPHEREx along), these bands can be joined into a full sphere- a full sweep of detectable space. If this sounds familiar, it’s the sky survey technique WISE (Wide-field Infrared Survey Explorer) used to do its imaging mission, and IRAS (InfraRed Astronomy Satellite) before that. The difference? WISE tried to cover much of the infrared range with just four bands, identified as strategic, because there were certain targets in those bands. IRAS, the first infrared space telescope of any real power, covered more wavelengths but with 1970s technology. SPHEREx, then, is the inherent follow-on mission after WISE, giving finer color detail for targets identified previously. SPHEREx even looks like WISE, except for a multilayer thermal shield.
The spectral range of SPHEREx goes from near-infrared, into thermal infrared somewhat. Not including the serious thermal range (around 10 microns) reduces the mission requirements and costs, but still includes some very interesting science. The “ices” in SPHEREx refers to water, ammonia, and light organics, all of which have a spectral feature detectable in this range. For example: WISE detected several brown dwarf stars, including some in our neighborhood of the galaxy. SPHEREx, then, has a list of brown dwarfs to check for ammonia and/or organic clouds in their atmospheres. More esoterically, the disks around young stars, where planets are being born and a new star-planet system is arising, will contain some ice content. SPHEREx will take infrared data, helping us understand what’s going on when systems form- an understanding that then applies to our Solar System’s early history.
Let’s take a deeper look into those ‘disks around stars.’ At some point, the gas and dust around a young star condenses into dust clumps and frosty multi-crystals. These in turn condense into pebbles and fractal structures (basically, dust bunnies in space), which in turn form cobbles and snowballs, which in turn so forth and so forth, until we get the bodies we know today. All macroscopic things- asteroids, comets, planets, and satellites- were once microscopic things that stuck together. When we look at these young star systems and their disks, we are, in effect, seeing various mixtures of gas, dust, and larger things: pebbles, snowballs, and at some point in time asteroids and comets.
SPHEREx, then, applies to small-body research both pedagogically and chronologically (exocomets) and directly- some number of asteroids and comets in our Solar System will by chance cross SPHEREx’s field of view. That field, after all, covers the entire sky (just not at once). This fact is not lost on scientists, in general and on the SPHEREx team. A rough estimate gave many thousands of asteroids, visible in the data set, and therefore having their infrared spectra taken. And of course, an active comet, entering the inner Solar System and growing a tail, presents a swath of dust, ices (water and otherwise), and organics (covering a range of weights and structures). Some desirable wavelengths will be missed (silicates at ~10 micron, carbon in the visible), but on the whole these objects will be examined, including in wavelengths not usable by ground telescopes… and multiple times. As a body traverses the sky, SPHEREx will take multiple passes on it.
Vera Rubin will sweep the southern sky, from its location in Chile. Some amount above the Equator will be taken, but only at its longer wavelengths due to reduced atmospheric transparencies. (That is, the slant-range, going through more air, means more scattering, fewer photons.) Vera Rubin is built with standard silicon detectors, extending from the near-UV (~330 nanometers), through visible and into the near-IR (~1070 nm). (Actual sensitivity at either end of that range will be low- most work will be done in the middle, where silicon detectors have high response.) This range, by itself, is partly but not satisfyingly covering spectral features that will tell us asteroid types. Extending out to just ~2000 nm gets us the pyroxene band, and in this way a good number of asteroid type identifications. SPHEREx, then, after its launch next year, will join VRO for a survey of the sky, adding northern coverage and some very interesting longer wavelengths. As Earth makes another lap around the Sun, VRO and SPHEREx will re-detect many of these objects. This includes objects clouded out from the ground, or just lost to telescope downtime, operator fatigue, etc.
Planetary Defense Use of the SPHEREx Solar System Object Catalog
SPHEREx work will continue, to testing and final launch prep. It will launch next year, with PUNCH (a solar monitoring mission) as a secondary payload. In not that many months, our view of the skies will become very rounded out!