SPHEREx: An All-sky Infrared Spectral Survey Explorer Satellite
James J. Bock
Marvin L. Goldberger Professor of Physics
Jet Propulsion Laboratory Senior Research Scientist
Caltech
Sponsored by PSW Science Member Martin Harwit
About the Lecture
The Spectro-Photometer for the History of the universe, Epoch of Reionization, and Ices Explorer (“SPHEREx”) is an all-sky survey satellite launching in early 2025. A mission in NASA’s Medium Explorer (MIDEX) program, it will probe the physics of cosmic inflation, chart the origin and history of galaxy formation, and study the origin of water and biogenic molecules in ices in interstellar space. As a new member of a tradition of all sky missions, SPHEREx will be the first near-infrared survey. It will produce four complete all-sky maps that will serve as a rich archive for the astronomy community, containing spectra of over a billion galaxies, hundreds of millions of high-quality stellar and galactic spectra, and over a million ice absorption spectra. The archive will enable a wide range of investigations that will inform and expand many aspects of astronomical and cosmological knowledge and understanding.
Selected Reading & Media References
A list of pertinent publications is available at https://spherex.caltech.edu/
About the Speaker
James (Jamie) Bock is an experimental cosmologist and the Principal Investigator of the SPHEREx mission (Spectro-Photometer for the History of the universe, Epoch of Reionization, and Ices Explorer).
His research focuses on pioneering new technologies and instruments designed for fundamental measurements of the state of the universe.
Prior to SPHEREx, Jamie developed an instrument architecture with wide-field telescopes and superconducting bolometer arrays to search for a telltale polarization signal in the Cosmic Microwave Background (CMB) associated with a background of gravitational waves from cosmic inflation. It is central to the BICEP-Keck experiment based at the South Pole observations from which have led to constraints on inflation. He also led the development of an earlier generation of focal plane detectors for the Planck and Herschel satellites. These devices arose from detectors he pioneered as a graduate student for the BOOMERANG CMB project that provided initial evidence that the universe has a flat geometry. Finally, he developed a novel sounding rocket project to study the near-infrared extragalactic background light with small, specialized imaging and spectroscopic telescopes. Many of these methods were later incorporated into the design of SPHEREx.
Jamie is an author on over 800 scientific papers, mainly focused on technology, astronomy, and cosmology.
Among other honors and awards, he received the Gruber Cosmology Prize as a member of the Planck team, the AAS Weber Award for Astronomical Instrumentation, and the SPIE Goddard Award.
Jamie earned a BS in Physics and Mathematics at Duke and a PhD in Physics at UC Berkeley.
Social Media
https://pma.caltech.edu/people/james-j-jamie-bock
https://cosmology.caltech.edu/
https://spherex.caltech.edu/
Minutes
On April 25, 2025, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,514th meeting of the Society to order at 8:03 p.m. ET. He began by welcoming attendees, thanking sponsors for their support, announcing new members, and inviting guests to join the society. Scott Mathews then read the minutes of the previous meeting which included the lecture by Joshua Semeter, titled “Unidentified Anomalous Phenomena- From Sensationalism to Science”. The minutes were approved, pending a minor correction.
President Millstein then introduced the speaker for the evening, James Bock, of the Jet Propulsion Laboratory at Caltech. His lecture was titled “SPHEREx: An All-sky Infrared Spectral Survey Explorer Satellite”.
The speaker began by saying that SPHEREx was designed to explore fundamental questions about astrophysics and cosmology. These include: the origin of the Universe, the history of galaxies, and the origin of water in planetary systems. He said that SPHEREx will perform the first all-sky, spectral survey, using a single observing mode, with no moving parts.
The speaker showed an animation of SPHEREx, describing many of its unique features, including: the large cones (which act as infrared heat shields), the passive radiative cooling (which uses no cryogens or mechanical coolers), the 20 cm aperture telescope, the linear variable filters, the mosaic focal plane arrays, and the LEO spacecraft supporting and aiming the instrument. He said that the telescope is currently operating at below 80 K and the focal plane array is below 55 K.
Bock then described how SPHEREx performs spectroscopic measurements using linear variable filters. A single point in the sky is imaged onto a single point on the focal plane array, through one of six linear filters. As the spacecraft sweeps across a portion of the sky, the point on the focal plane array moves vertically along the array, and along the variable axis of the filter. In this way, each of the vertical points on the focal plane array contains intensity information at a particular wavelength. In order to acquire a complete infrared spectrum, from below 1 μm to 5 μm, the spacecraft must sweep over the same portion of sky three times, imaging in two separate wavelength bands, and recording 51 exposures. The data is then numerically deconvolved to separate the spatial information from the spectroscopic information, so that images can be reconstructed for each wavelength.
The speaker then showed a video of the mapping strategy: keeping the telescope on the sunrise-sunset line, pointed 90° from the Sun, the telescope images a small square region of the sky, rotates through a very small angle, and images the next overlapping square region. After several small movements, the spacecraft must slew through a much larger angle in the opposite direction, to prevent the Earth’s infrared emission from entering the telescope. As the spacecraft orbits the Earth, it traverses a great circle, imaging a complete ring of sky. As the Earth orbits the Sun, the great circle precesses, thereby mapping the entire sky. Bock said that it will take six months of continuous observation to achieve full sky coverage.
Bock discussed ways SPHEREx may help answer scientific questions, including: the search for water by absorption spectroscopy, the formation and evolution of galaxies by measuring extra galactic background light, and the nature of inflation in the early universe using 3-dimensional mapping of galaxies by galactic redshift. He said that SPHEREx will provide a very rich, all-sky, spectral catalog with a complete near-infrared spectrum of every point in the sky, and therefore a large number of cosmic objects. These include: galaxies, galaxy clusters, main sequence stars, brown dwarfs, quasars, asteroids, and comets. The speaker said that the data from SPHEREx would be processed and released to the public within two months of acquisition.
Bock showed images of the assembly and transport of the SPHEREx spacecraft. He showed images of the SpaceX booster, the rocket faring, and the payload bay of the Falcon 9 that was used to launch SPHEREx from Vandenburg Space Force Base in California. He showed a video of the launch, which took place in March 2025. The speaker described the process by which the instrumentation was purged of water and cooled to operating temperature. Bock showed the “first light” images obtained by SPHEREx on March 27.
The speaker ended his talk by saying “We have a happy spacecraft. The instrument is working well, and it is a very exciting time.”
The lecture was followed by a Question and Answer session.
A member asked about galaxy formation. Does it appear that galaxies formed around pre-existing black holes, or did the black holes form inside galaxies, and could SPHEREx provide information about the process? Bock replied that SPHEREx will give information about the ionization of matter during the period of galaxy formation, but that unfortunately, SPHEREx will not be able to distinguish the source of the ionization; in other words, it cannot determine whether the ionization was caused by an accreting black hole or from the stars themselves. As a result, he said, you would need to investigate individual galaxies to get such information.
A member asked whether SPHEREx could provide better estimates of the amount of space dust than previously measured, and whether such observations could affect the estimated quantity of dark matter in the universe. Bock replied that dark matter is presumed to be non-baryonic (not conventional matter) and that space dust was almost certainly baryonic.
A member on the live stream asked if SPHEREx could measure the abundance of water in the Keiper Belt and if it could distinguish between amorphous and crystalline ice. Bock responded that the water absorption feature could be there, but SPHEREx requires a light source to “light-up that feature”. He said that this is extremely difficult to do with Keiper Belt objects, and would be much easier for comets and asteroids.
After the question and answer period, President Millstein thanked the speaker and presented him with a PSW rosette, a signed copy of the announcement of his talk, and a signed copy of Volume 17 of the PSW Bulletin. He then announced speakers of up-coming lectures and made a number of housekeeping announcements. He adjourned the 2,514th meeting of the society at 9:48 pm ET.
Temperature in Washington, DC: 21.7° Celsius
Weather: Mostly Cloudy
Audience in the Powell auditorium: 66
Viewers on the live stream: 38
For a total of 104 viewers
Views of the video in the first two weeks: 370
Respectfully submitted, Scott Mathews: Recording Secretary