The James Webb Space Telescope’s first year in orbit featured a six-month calibration process, mirror-shattering micrometeoroid strike, and infrared imaging of one of the earliest galaxies ever captured, GLASS-z12, an estimated 350 million years after the universe began. The previous record for such an observation was 400 million years after the Big Bang for GN-z11, captured by the Hubble Space Telescope in 2016.
Webb began full scientific operations July 11, following six months of instrumentation cool-down, calibration, and mirror-aligning among other commissioning activities. National Aeronautics and Space Administration (NASA) officials on July 12 performed a public unveiling of JWST’s first image, “Webb’s First Deep Field,” showing the galaxy cluster SMACS 0723 as it appeared 4.6 billion years ago.
“Webb’s First Deep Field” is a tiny slice of the expanding universe, about the size of a grain of sand held at arm’s length, according to NASA. It is a compilation of images taken at different infrared wavelengths over the course of 12.5 hours with JWST’s Near-Infrared Camera (NIRCam). In comparison, the Hubble Space Telescope’s iconic 1995 deep field image required a total exposure time of more than 100 hours spread across 10 days of in-orbit exposure time.
The team of more than 20,000 engineers that developed Webb originally envisioned a 5-10 year mission duration. However, after a flawless detachment from Ariane 5, NASA has confirmed Webb has enough jet propellant onboard to last 20 years in-orbit.
Cooling Down and Calibrating
Why did it take Webb more than six months from its December 25, 2021 launch to begin official science operations in July? The deployment of JWST to the second Lagrange point (L2) — 1.5 million kilometers (km) from Earth — lasted one month with an on-orbit deployment process that included 344 potential single point failures.
Most of those failures were overcome within the first two weeks of deployment, where the 18 individual hexagonal segments of Webb’s gold-plated Primary Mirror were unfolded. Using the 155 motors attached to the backs of those segments, the mirror was aligned to peer outward into the earliest regions of the universe, separated by Webb’s sunshield from the side of the telescope that faces the light and heat generated by the Sun, Moon, Earth and other planets in the Solar System.
Following the unfolding and mirror alignment process, between January and April, the Space Telescope Space Institute (STScI) team operating JWST focused on cooling the infrared instruments to their operational temperatures. This time period was also used to calibrate and confirm the collective optical image quality and performance of Webb’s four infrared instruments: the NIRCam, the Near-Infrared Spectrograph (NIRSpec), the Mid-Infrared Instrument (MIRI), and the Fine Guidance Sensor/Near-Infrared Imager and Slitless Spectrograph (FGS/NIRISS).
MIRI requires the coldest operating temperature of the four instruments because it is tasked with observing the darkest and most distant portions of the universe in the mid-infrared 5000 to 28000 nm wavelength range. The mid-infrared camera/spectrograph reached its final operating temperature of 7K (~ -447°F, -230°C) on April 7, according to NASA.
The near-infrared instruments — NIRCam, NIRSpec, FGS/NIRISS — operate at a warmer temperature of about 39 K (~ -389°F, -234°C) enabled by a passive cooling system. Each of the other three instruments reached their target temperatures a few weeks prior to MIRI reaching 7K.
Webb has a total of 17 instrument modes. Upon completing the entire commissioning process in July, the JWST team reviewed performance data collected from all 17 modes against their mode-specific readiness criteria for science instrument performance.
This process was completed on July 10, however, the STScI team continues to make updates to calibration, performance and resolution data collected to JDox — the online JWST user documentation system that tracks every user related update for every system component and application on the telescope. Based on final commissioning data featured in the JWST Science Performance report, JWST’s mirrors are cleaner than their requirements, overall optics are better aligned, and the fine guidance system points the observatory “several times more accurately and precisely than required,” according to the report.
Lee Feinberg, optical telescope manager at NASA’s Goddard Space Flight Center, was part of the Optical Telescope Element (OTE) commissioning team. In a published paper, “Commissioning the James Webb Space Telescope Optical Telescope Element,” presented by Feinberg during a 2022 SPIE conference in Montreal, Feinberg and a team of researchers evaluate the wavefront error of each channel and found the telescope to be diffraction limited at 1.1um in NIRCAM, well below the Webb’s mission readiness requirement of being diffraction limited at 2um.
“It varies by instrument channel and a factor of 3 in the MIRI channel,” Feinberg writes in an emailed statement, “but roughly a factor of 2 in the NIRCam short wavelength channel which is the key driver for the Level 1 image quality requirement (79nm RMS vs. 150nm requirement).”
In June, NASA released an update about a micrometeoroid strike that caused significant damage to C3, one of the Primary Mirror’s 18 individual hexagonal segments. The strike occurred sometime between May 23-25.
While JWST was engineered and developed to withstand strikes from micrometeoroids and other dust-sized particles moving at extreme velocities near its orbit, that May incident was larger than anything they predicted or simulated testing for.
Wavefront sensing performed shortly after the strike confirmed that the telescope was still performing above its Level 1 imaging requirements.
Further analysis of the strike by a working group of optics and micrometeoroid experts from NASA Goddard‘s Webb team, the telescope’s mirror manufacturer, the STScI, and the NASA Meteoroid Environment Office concluded the higher-energy impact observed in May was a rare statistical event both in terms of energy and in hitting a particularly sensitive location on Webb’s primary mirror.
Based on that analysis, the team developed a new approach to mitigating such strikes moving forward, outlined in an update about the strike released by NASA in November.
Future observations using JWST will be planned to face away from the ‘Micrometeoroid Avoidance Zone’ (MAZ) developed by the working group. The newly developed MAZ is defined by STScI as a cone of a specified half angle around the orbital motion direction, also referred to as “the ram vector.” Starting in Cycle 2, — or the second planned year of science operations for Webb — the half angle will be set to 75°. Additionally, Webb has experienced 14 measurable micrometeoroid hits in total on the Primary Mirror and is averaging about one to two such strikes per month, according to NASA.
The new strategy will effectively point Webb’s mirrors, cameras and instruments away from the direction that it is moving within its orbit around L2.
While not caused by any micrometeoroid strikes, one of the science mode operations for MIRI has been impacted by in-orbit wear and tear as well. One of the instrument modes, the Medium Resolution Spectrometer (MRS) mode, for MIRI was turned off between August and November.
The JWST user’s committee reasoning for the pause was that the grating wheel on MIRI’s Medium Resolution Spectrometer experienced a change in frictional torque at some point while Webb was in orbit in early August.
What caused the change in frictional torque on the grating wheel? According to STScI’s statement describing the incident, the root cause of the observed issue is increased contact forces between the wheel central bearing assembly’s sub-components under certain conditions.
“We believe it is a combination of factors associated with the variable environmental conditions experienced by the grating wheel bearing sub-components through launch and cooldown to a temperature of about 6K,” a representative for STScI writes in an emailed statement. “Some observations that are using MIRI MRS, and were scheduled in the past 2-3 months, have been delayed until a future opportunity. Principal Investigators from those programs have been notified and we will work with them to re-schedule their observations.”
What’s Next for Webb?
What actual universal observations Webb executes while in-orbit are determined based on the cycle number Webb is operating within — which is currently the sixth month of Cycle 1.
In-orbit JWST observing programs are generally allocated over one-year cycles. STScI is still following the pre-launch program schedule for Cycle 1, Cycle 2 and Cycle 3. The JWST Users Committee-defined science and planning scheduling estimates that there are 8,760 hours available in each one-year observing cycle.
The geometry of Webb’s sunshield limits the region of the sky that the mirrors and instruments can point at, and for how long. Depending on its location in orbit, an astronomical target is usually observable during two periods of a calendar year, separated by about six months, according to STScI. This limitation is driven by the target ecliptic latitude and the orbit position of Webb, which always ensures the sunshield protects the telescope and its science instruments from the Sun’s radiation.
The observations to be performed are selected by STScI’s Time Allocation Committee, and are assigned based on as specific capabilities of individual instruments.
The committee received 1,173 total proposals for Cycle 1.
JWST science missions or operations are generally segmented into three different categories, including Guaranteed Time Observations (GTO), General Observations (GO) and Director’s Discretionary Time Programs. General Observations missions are generally open to the global scientific community, and take up the bulk of available hours in each cycle.
Proposals are submitted once a year and undergo a competitive peer review process prior to acceptance.
A total of 6,000 observational hours were selected and assigned for Cycle 1, with the majority of them falling in the General Observation category.
Another key factor for scientific investigations being selected to be performed by Webb include the proposal or investigation’s ability to address the space observatory mission’s key science themes.
Webb’s mission is divided into four key science themes, including the following areas: first light and reionization in the universe, assembly of galaxies in the early universe, the birth of stars and protoplanetary systems and the origins of life within planets.
Some of the key tenets of these science goals are already being addressed in observations being performed since July. Based on congressional testimony in November provided by Natalie M. Batalha, Professor of Astronomy & Astrophysics, UC Santa Cruz, Webb is establishing a “new epoch of exoplanet science” and understanding of how the Earth and other planets within and outside of the Solar System developed over time.
Batalha is one of the lead researchers for the Early Release Science Program (ERS-1366) that includes over 300 scientists globally. The program started as an open-science initiative in 2016, and aims to test Webb’s observing modes that are capable of performing transmission spectroscopy. The UC Santa Cruz researcher describes transmission spectroscopy as what occurs when planets eclipse their host star and some of the starlight passes through their atmosphere.
As the starlight passes through, the planet blocks different fractions of light depending on the color of certain molecules present in its atmosphere. In the past, studying planetary atmospheres with transmission spectroscopy through other observatories such as Kepler and TESS have been limited to observations of bulk planetary properties such as the planet’s total mass, radius or curvature. This limitation is the result of Kepler and TESS performing transmission spectroscopy in white light.
Webb’s infrared instruments, spectrographs and coronagraphs observe transits of exoplanets outside the Solar System in hundreds of infrared colors simultaneously. Spectrometers on each of Webb’s IR instruments measure the amount of light blocked at each infrared color observed. That allows experts like Batalha to discover what atomic elements and molecules are present within the planet’s atmosphere based on which colors are missing.
JWST has already used this method with an atmospheric spectrum created from NIRSpec PRISM of WASP-39-b, a sun-like star located “approximately 700 light-years away toward the constellation Virgo,” according to her testimony. WASP-39b’s mass is similar to Saturn’s and its dimensions are 30 percent larger than Jupiter. Using NIRSpec-powered transmission spectroscopy, the ERS-1366 team discovered the first “robust detection of CO2 in an exoplanet atmosphere.”
Evidence of several other chemical species were also discovered in the atmosphere, including four independent water (H2O) features, carbon dioxide (CO2), and carbonmonoxide (CO).
During Cycle 1 alone, more than 70 transiting exoplanets will be observed with their atmospheres studied similar to the approach used for WASP-39-b. There could be hundreds more observed over Webb’s full in-orbit life cycle, establishing the foundation for identifying habitable environments and even living worlds in the future, according to Batalha.
Another groundbreaking discovery presented during congressional testimony in November came from the Cosmic Evolution Early Release Science Survey (CEERS). Combining the capabilities of NIRCam and MIRI, CEERS targets infrared imaging of the earliest, darkest portion of the universe. CEERS is a research program aligned with the first of Webb’s four science goals, “First Light in the Universe.”
Dr. Steven Finkelstein, Professor of Astronomy, The University of Texas at Austin, presented research to lawmakers collected from CEERS during the first five months of Webb space science operations. Finkelstein describes “red-shift” as the universal phenomenon of the ongoing movement of galaxies away from each other. This constant movement means that the light waves generated by receding galaxies observed by Webb are redder–or more “red shifted”–than they were when the waves were first emitted.
One of the first Webb discoveries presented by Finkelstein is, “Maisie’s Galaxy,” which has a redshift of 12 allowing Finkelstein and others to pinpoint it at “over 97 percent of the way back in our cosmic history,” according to Finkelstein’s testimony.
On November 15, STScI opened its call for proposals for General Observer time for Cycle 2. There are up to 5,000 GO hours available in the second cycle, with the team requiring proposals to be submitted by January 27, 2023. The Cycle 2 Telescope Allocation Committee plans to announce the GO operations selected for Cycle 2 in May 2023.
The planned dates for Cycle 2 observations are July 1, 2023 - June 30, 2024.
This article was written by Woodrow Bellamy III, Editor, Photonics & Imaging Technology, SAE Media Group (New York, NY).