On Apr. 21, 2023, the James Webb Space Telescope team shared that one of the MIRI (Mid-Infrared Instrument) observing modes, called Medium-Resolution Spectroscopy (MRS), showed a reduction in the amount of light registered by MIRI’s detectors. Initial analysis of MIRI’s imaging mode did not show a similar effect. However, as part of the team’s investigation into the issue, additional monitoring observations were taken with MIRI imaging. Combined with earlier data, these new calibrations have revealed a reduced signal for MIRI imaging at the longer wavelengths.
This change does not substantially impact MIRI’s science capabilities but will have an impact on the exposure times needed for MIRI imaging.
There is no risk to the instrument, and the effect on imaging is less than the effect in MRS. The team is investigating the cause of this issue. Regular monitoring observations are being taken to continue measuring the response, and the team is providing updated guidance to Webb’s user community to correct for this change. MIRI’s third observing mode, Low-Resolution Spectroscopy, is currently performing normally, and the investigation of MIRI’s fourth mode, Coronographic Imaging, has not yet concluded.
The Webb team has also enacted a plan for long-term monitoring, and are exploring potential mitigations. The observatory is in good health, and each of Webb’s other scientific instruments are unaffected.
Editor’s Note: This post highlights data from Webb science in progress, which has not yet been through the peer-review process.
NASA’s James Webb Space Telescope obtained images of the Ring Nebula, one of the best-known examples of a planetary nebula. Much like the Southern Ring Nebula, one of Webb’s first images, the Ring Nebula displays intricate structures of the final stages of a dying star. Roger Wesson from Cardiff University tells us more about this phase of a Sun-like star’s stellar lifecycle and how Webb observations have given him and his colleagues valuable insights into the formation and evolution of these objects, hinting at a key role for binary companions.
“Planetary nebulae were once thought to be simple, round objects with a single dying star at the center. They were named for their fuzzy, planet-like appearance through small telescopes. Only a few thousand years ago, that star was still a red giant that was shedding most of its mass. As a last farewell, the hot core now ionizes, or heats up, this expelled gas, and the nebula responds with colorful emission of light. Modern observations, though, show that most planetary nebulae display breathtaking complexity. It begs the question: how does a spherical star create such intricate and delicate non-spherical structures?
“The Ring Nebula is an ideal target to unravel some of the mysteries of planetary nebulae. It is nearby, approximately 2,200 light-years away, and bright – visible with binoculars on a clear summer evening from the northern hemisphere and much of the southern. Our team, named the ESSENcE (Evolved StarS and their Nebulae in the JWST Era) team, is an international group of experts on planetary nebulae and related objects. We realized that Webb observations would provide us with invaluable insights, since the Ring Nebula fits nicely in the field of view of Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) instruments, allowing us to study it in unprecedented spatial detail. Our proposal to observe it was accepted (General Observers program 1558), and Webb captured images of the Ring Nebula just a few weeks after science operations started on July 12, 2022.
“When we first saw the images, we were stunned by the amount of detail in them. The bright ring that gives the nebula its name is composed of about 20,000 individual clumps of dense molecular hydrogen gas, each of them about as massive as the Earth. Within the ring, there is a narrow band of emission from polycyclic aromatic hydrocarbons, or PAHs – complex carbon-bearing molecules that we would not expect to form in the Ring Nebula. Outside the bright ring, we see curious “spikes” pointing directly away from the central star, which are prominent in the infrared but were only very faintly visible in Hubble Space Telescope images. We think these could be due to molecules that can form in the shadows of the densest parts of the ring, where they are shielded from the direct, intense radiation from the hot central star.
“Our MIRI images provided us with the sharpest and clearest view yet of the faint molecular halo outside the bright ring. A surprising revelation was the presence of up to ten regularly-spaced, concentric features within this faint halo. These arcs must have formed about every 280 years as the central star was shedding its outer layers. When a single star evolves into a planetary nebula, there is no process that we know of that has that kind of time period. Instead, these rings suggest that there must be a companion star in the system, orbiting about as far away from the central star as Pluto does from our Sun. As the dying star was throwing off its atmosphere, the companion star shaped the outflow and sculpted it. No previous telescope had the sensitivity and the spatial resolution to uncover this subtle effect.
“So how did a spherical star form such a structured and complicated nebulae as the Ring Nebula? A little help from a binary companion may well be part of the answer.”
NASA’s James Webb Space Telescope is nearly 1 million miles (1.5 million kilometer) away from Earth, orbiting around the Sun-Earth Lagrange point 2. How do we send commands and receive telemetry – the science and engineering data from the observatory – from that far away? We use the DSN (Deep Space Network) to communicate with the observatory. We receive data when we have a contact with Webb using a DSN antenna
Sandy Kwan, the mission interface manager for Webb within the DSN, notes that “each mesmerizing Webb image that has graced our screens would not have been possible without the support of the DSN antennas and personnel, the backbone of interplanetary communication.”
The DSN has three sites around the world, each positioned 120 degrees apart. There are antennas in Goldstone, California; Canberra, Australia; and Madrid, Spain. This allows us to communicate with Webb at any time of day, as the Earth rotates. The DSN is managed by NASA’s Jet Propulsion Laboratory (JPL) in Southern California. Kari Bosley, the lead Webb mission planner at the Space Telescope Science Institute (STScI), walks us through more of this communication process between Webb and the DSN.
“How do we plan contact time with Webb? It’s not as simple as picking up the phone and calling the telescope. In order for Earth to connect with Webb there are a few things that happen prior to scheduling a contact. On average, the Webb mission operations center connects with the observatory at least 2-3 times in a 24-hour period. There are mission planners at STScI where the Mission Operations Center (MOC) is located, mission schedulers at JPL, and of course at the DSN complexes. The mission planners at STScI work together with the mission schedulers at JPL to create contacts with Webb.
“How do we know when we can contact Webb? The Flight Dynamics Facility at NASA’s Goddard Space Flight Center sends the MOC at STScI the view periods in which the observatory is visible from those three different DSN sites. The mission scheduler compares those times to what is available in the scheduling system where other missions are competing for time with their spacecraft. All missions require specific amounts of time to communicate with their spacecraft, and the timing depends on where the spacecraft are in space. There are times when conflicts between multiple missions request the same resource at the same time. When this happens, our mission scheduler at JPL will negotiate with other missions to come to a compromise that satisfies all of the missions. Once all negotiations are complete, schedules are sent to the mission planners up to 6 months in advance. The scheduling for the first 8 weeks is fixed, with no changes allowed unless there is an emergency or important event with a spacecraft. The later periods are subject to continuing negotiations.
“Each of the DSN complexes has different types of antennas, including 70-meter (230-foot in diameter), 34-meter (111-foot in diameter), and 26-meter (85-foot in diameter) antennas. The DSN complexes use the 34-meter antennas to talk with Webb with the 70-meter antennas as a backup. The DSN supports different radio frequency allocations, such as the S-band and Ka-band frequencies that Webb uses. S-band has a lower bandwidth, and we use that to send commands to the spacecraft (e.g., start recorder playback), to receive engineering telemetry to monitor the health and safety of the observatory, and for ranging. Ranging is the process of determining Webb’s position and trajectory by the delay between when the signal is sent up and when it is received back on the ground.
“We use Ka-band to downlink stored science and engineering data, and some telemetry from the spacecraft. If we used S-band to downlink data, it would take many days to download each day’s data. With Ka-band, it takes much less time, and we can usually complete download all of the stored data in a couple of hours. The high gain antenna on Webb is used for Ka-band downlink and the medium gain antenna is used for S-band uplink and downlink when both antennas are pointed directly at the complex for a contact. Most of our contacts are 2-6 hours in length. Normally, we request at least 4-hour contacts. Since DSN hosts almost 40 different missions, scheduling is complicated.
“There are times when our contacts are very short and times when they are longer. In each contact, it is important to downlink as much data as we can since the telescope continually makes science observations and acquires more data. When we are not in contact, the telescope continues to autonomously perform science observations. These data are stored on a solid-state recorder and downlinked on our next contact. After the Webb MOC at STScI receives the data and ingests them into the Barbara A. Mikulski Archive for Space Telescope for processing and calibration, the observers will receive the data from their observations.
“Those interested in seeing the downlink and uplink between NASA missions and the DSN can visit the ‘Deep Space Network Now’ website at https://eyes.nasa.gov/dsn/dsn.html. You can view the missions and resources that are actively being used at DSN.”
About the author:
Kari Bosley is the lead mission planner in the Ground Systems Engineering Branch at the Space Telescope Science Institute. She schedules the activities that are executed onboard the James Webb Space Telescope. She also collaborates with other mission planners and schedulers to obtain contact time for Webb through Deep Space Network. Kari thanks Carl Hansen (Webb spacecraft systems engineer at STScI) for providing information on the subject of ranging and data rates.