Spacecraft

SPHEREx Observatory

The assembled SPHEREx observatory (including its outer photon shield but excluding its solar panel) stands on one end in the BAE Systems spacecraft assembly facility in Boulder, Colorado. Credit: NASA/JPL-Caltech/BAE Systems | Full Image

The SPHEREx observatory is built to survey the entire sky in infrared light. Standing 8.5 feet (2.6 meters) tall, its most prominent feature is a set of cone-shaped photon shields that measures 5.6 feet (1.7 meters) tall and 10.5 feet (3.2 meters) in diameter. The three concentric shields surround the telescope, which uses three mirrors to collect light from cosmic sources and feed it to the detectors. The shields protect this hardware from light and heat from the Sun and Earth. The cones, telescope, and detectors sit atop a trio of structures called v-groove radiators. Each radiator consists of three conical mirrors that resemble an upside-down umbrella and are stacked atop one another. Composed of a series of wedges that redirect infrared light so it bounces through the gaps between the photon shields and out into space, they help to keep the telescope cool.

Underneath the photon shields and v-groove radiators is the part of the observatory known as the spacecraft bus. It includes the main computer, solar panel, and equipment for communicating with Earth and controlling the spacecraft’s orientation.

Telescope and Detectors

The telescope for NASA’s SPHEREx mission undergoes testing at NASA’s Jet Propulsion Laboratory in Southern California. Credit: NASA/JPL-Caltech | Full Image

The heart of SPHEREx is its telescope, which collects infrared light from distant sources using three mirrors. The primary mirror has an effective diameter of 7.9 inches (20 centimeters) and an 11.3-degree by 3.5-degree field of view. Its images will each contain over 24 million pixels.

The light from the telescope is split into two beams that are directed to sets of detectors and filters, collectively called the focal plane arrays. The observatory features six detector arrays, each 1.6 inches (41 millimeters) square and containing 4.2 million pixels. On top of the detectors are color filters that enable SPHEREx to conduct spectroscopy, separating light from a source into its component wavelengths.

Three of the filters on SPHEREx’s detectors that will allow it to image the sky in 102 separate infrared colors (distinct wavelengths). Credit: NASA/JPL-Caltech | Full Image

Most astrophysics telescopes that do spectroscopy use detectors that collect light from one source at a time. With its unique design, SPHEREx will conduct spectroscopy on hundreds of thousands of objects simultaneously. Combined with the telescope’s wide field of view, this capability will enable SPHEREx to complete a spectroscopic map of the entire sky every six months.

SPHEREx has only six filters, but they detect 102 wavelengths of infrared light in total. Each filter has 17 color bands, distinct regions that look like stripes. The wavelengths vary continuously over the filter in a gradient, similar to the gradual transition between the colors of a rainbow. Each point of the sky is observed in one of the 17 color bands at a time. A similar approach is used in planetary science to conduct spectroscopy up close across the surface of planets or moons.

Pointing

For the SPHEREx telescope to complete its overlapping images of each section of the sky, the entire observatory shifts position — the mirrors and detectors don’t move as they do on other telescopes. And rather than using thrusters or chemical propellant to alter the direction it is pointing, the spacecraft relies on a system of gyroscopes, reaction wheels, and magnetic torque rods.

Cooling System

The SPHEREx telescope and detectors must be cooled down once the observatory reaches its orbit. The detectors need to be kept around minus 350 degrees Fahrenheit (about minus 210 degrees Celsius). Otherwise, SPHEREx will generate an infrared glow that might overwhelm the faint light from cosmic sources. For comparison, temperatures in Antarctica dip as low as minus 120 F (minus 85 C), while parts of the surface of the Moon at night reach minus 280 F (minus 173 C).

The spacecraft’s innovative design enables the cooling system to be entirely passive, requiring no electricity or coolant during normal operations, simplifying operational needs. Before the observatory begins normal operations, a set of electric decontamination heaters will be used for a few days to reduce the risk of ice forming on critical components during the initial instrument cooldown.

Cooling Structures

Two key structures contribute to the passive cooling system: the set of three cone-shaped photon shields and the v-groove radiators.

The photon shields, each about 0.75 inches (19 millimeters) thick, surround the telescope and block light from the Sun and Earth from entering or warming it. Constructed with a geometry similar to cardboard, the cones feature two thin aluminum sheets separated by aluminum shaped like a honeycomb. The reflective aluminum surfaces bounce light and heat out into space, and the exterior of the outer cone is painted white to reflect sunlight and radiate heat.

One of three SPHEREx v-groove radiators — a series of mirrored wedges that redirect infrared light so it bounces out into space — is shown at the bottom of this image. Two additional v-groove radiators sit below this one, each slightly wider than the one on top of it, so they can bounce light up through the gaps between the three photon shields. Credit: NASA/JPL-Caltech | Full Image

The telescope sits on three structures called v-groove radiators, which have reflective surfaces that bounce light and heat away from the telescope out into space.

The struts that connect the telescope to the rest of the spacecraft are designed to reduce heat transfer.

Cooling Orbit and Orientation

The orbit of SPHEREx and its orientation relative to the Sun and Earth will also help keep the telescope cool. Both the Sun and Earth radiate light that can warm SPHEREx’s hardware or overwhelm its detectors, making it difficult or impossible for them to collect light from faint cosmic sources. During survey operations the telescope will therefore always be pointed fully away from Earth, looking out into space.

Because staying pointed away from the Sun is more challenging, SPHEREx will orbit from north to south over Earth’s poles along the terminator line, which separates day and night on a celestial object. From this orientation, the opening at the top of the photon shields will continuously point at least 91 degrees away from the Sun. Inside the protective shields, the telescope is slightly tipped toward the Sun so that the full sky can be observed.

Flying at the terminator line also means SPHEREx will be exposed to similar amounts of light and darkness during each orbit, which will keep the spacecraft’s temperature more stable than if its orbit passed through varying light levels.

Power

SPHEREx is powered by a single fixed solar panel that measures 8.75 feet by 3.4 feet (2.67 meters by 1.02 meters) and produces roughly 750 watts of power. The observatory’s orbit along Earth’s terminator line means its solar panel will almost always face the Sun and the power it generates won’t vary significantly in a given orbit. A battery aboard the spacecraft will provide power before SPHEREx reaches orbit and during brief periods when it falls into Earth’s shadow.

Communications and Data

SPHEREx will communicate with Earth via five microwave antennas: three S-band and two Ka-band. Two S-band antennas are located on either end of the solar panel and will be used to send telemetry (data on the health and status of the spacecraft). A higher-gain S-band antenna sits on the bottom of the spacecraft bus and will be used to receive commands. Also located on the bottom of the spacecraft are the Ka-band antennas that will send science data to Earth. These have a higher data rate than the S-band antennas; however, they are cone-shaped, while the S-band antennas are omnidirectional, so the S-band antennas can communicate with ground stations for longer periods.

Because SPHEREx has a 98-minute polar orbit at Earth’s day-night line, it will most frequently fly over areas close to the poles. To take advantage of this, the spacecraft will communicate via ground stations in Troll, Antarctica; Fairbanks, Alaska; Punta Arenas, Chile; and Svalbard, Norway. The ground stations are part of NASA’s Near Space Network, which is managed and operated by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. During each ground station pass, the S-band can transmit data for about 10 minutes and the Ka-band can transmit for about three minutes. An average of five passes will occur per day.

Near Space Network antennas at the Alaska Satellite Facility in Fairbanks, Alaska. Credit: NASA | Full Image

The observatory will take about 600 science exposures per day. Because there are six detectors, this produces about 3,600 individual images. The onboard software will take a series of steps to reduce the amount of data that needs to be sent down to Earth by a factor of about 175. This includes a compression algorithm and an algorithm that estimates the flux in each pixel and reduces it to a single value per pixel, instead of the roughly 70 values per pixel that are initially produced by the detectors.

Ground Systems

Satellite Operations

The observatory will be operated from the Earth Orbiting Mission Operations Center at NASA’s Jet Propulsion Laboratory in Southern California. Spacecraft support will be provided by BAE Systems in Boulder, Colorado. Payload instrument support will be provided by Caltech. Survey planning support will be provided by Arizona State University in Tempe.

Science Data Processing

The SPHEREx Science Data Center at Caltech’s IPAC will perform scientific data processing and analysis. Data releases to the science user community and archiving will be carried out through the NASA/IPAC Infrared Science Archive (IRSA).