PUNCH is a group of four satellites that will orbit Earth above the day-night terminator to study the Sun. Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab
On March 2, a SpaceX Falcon 9 rocket is scheduled to launch the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission into low Earth orbit. From this location, its four satellites will have nearly constant views of the Sun to help researchers answer questions about how activity near our star propagates through the inner solar system, influencing the space weather we experience here on Earth.
A broad view
Developed by the Southwest Research Institute and other partners, the suitcase-sized satellites contain imagers sensitive to polarized light — light waves in which the direction of the oscillating electric fields are aligned. From their orbital placements, the PUNCH satellites will have fields of view that extend 1.5° to 45° from the Sun. This will give them a view from the corona — the Sun’s superheated outer atmosphere — to nearly the distance of Earth’s orbit. That’s a range spanning six to 185 times the Sun’s radius or equivalently 2.5 million to 80 million miles (4 million to 129 million kilometers).
Their combined images will be able to monitor, with observations every four to 32 minutes, virtually the entire inner solar system to keep watch for changes in the solar corona and solar wind of particles constantly flowing outward from the Sun. More importantly, they will be able to see how dynamic changes in the inner corona propagate outward in the solar wind as far from the Sun as Earth’s orbit.
The electrons in the corona and solar wind scatter sunlight through a process called Thompson scattering, and the resulting light is polarized. The PUNCH imagers, connected together into a vast virtual imaging system, are sensitive to polarized light and will produce the first-ever movies of coronal disturbances evolving into solar wind features, such as coronal mass ejections and other phenomena yet to be discovered. The polarization data will also provide a three-dimensional probe of the structure of the solar wind and corona.
Envisioning the Sun
The four PUNCH satellites will orbit Earth at an altitude of 385 miles (620 km). Their polar orbit is Sun-synchronous, which will keep them constantly above the terminator dividing night from day on our planet as it rotates.
Three of the 110-pound (50 kilograms) PUNCH observatories host a Wide Field Imager (WFI) instrument each, and one observatory hosts a Narrow Field Imager (NFI). The NFI, developed by the Naval Research Laboratory, monitors an annular, or doughnut-shaped, field of view around the Sun between 6 and 32 solar radii — similar in coverage to the LASCO C3 coronagraph on the Solar and Heliospheric Observatory (SOHO) satellite. The view is dark in the center because coronagraphs use an occulting disk to block the intense sunlight from the disk of the Sun itself, which allows us to view the much fainter material around the Sun.
The NFI also has sophisticated stray-light suppression technology to reduce scattered light from the Sun, Moon, and Earth inside the telescope. It is most sensitive to visible light at wavelengths between 450 and 750 nanometers, and has a resolution of about 30” per pixel across its 2148 x 4200-pixel CCD sensor area. The telescope’s optical resolution is about 1.5 arcminutes, which coincidently is close to human visual acuity. So, the images returned by PUNCH will resemble what the human eye might see if it were sensitive enough to the exceptional faintness of the corona.
The three WFI observatories will simultaneously acquire data that will look like a trefoil (three-pointed) pattern on the sky when the data from each observatory is combined into a virtual image. As the spacecraft orbit Earth, the trefoil pattern rotates on the sky and builds up the full 90° circular PUNCH field of view with a size of 18 to 180 solar radii.
Each spacecraft also carries an X-ray sensor called the Student Thermal Energetic Activity Module (STEAM), designed and built by a team of 50 students at Colorado University through the Space Grant Consortium. The cubical module is about 4 inches (10 centimeters) on a side and is designed to detect X-rays in the soft (0.5–15 keV) and hard (5–40 keV) spectral ranges. (Softer X-rays have lower energies, while harder X-rays have higher energies.) The students had planned to use this data to study solar flare events and deduce how coronal plasma is heated during flare events.
Unfortunately, an issue with STEAM occurred just before launch and the instrument was shut off to minimize any possible risks to the mission.
Seeing it all
The goal of this remarkable observatory network is to track individual disturbances in the region closest to the Sun — called the inner heliosphere — from their origins in the corona all the way out to nearly Earth’s orbit. This region of the solar wind is believed to a complex pastiche of organized and tangled magnetic fields and plasmas. Scientists have been trying to understand how coronal irregularities form and how they evolve in time and space as they travel through the inner solar system. The relationship between small-scale coronal features measured in tens of thousands of miles (hundreds of thousands of kilometers) and the vastly larger solar wind features measured in millions of miles (tens of millions of kilometers) is a regime of physics that remains data-poor. Scientists expect that PUNCH will fill in this missing information and allow better physics-based modeling of solar phenomena, with consequent gains in our ability to forecast space weather near Earth.
Other goals of the mission including understanding how coronal mass ejections evolve and propagate as they reach Earth’s orbit, and how shock fronts generated in the inner corona travel outward and accelerate particles along the way. Earlier missions such as STEREO-A and B provided stereoscopic views of coronal mass ejections using similar polarized light imaging, but were limited by smaller fields of view. This will not be the case for PUNCH, which will watch the cradle-to-grave development of coronal and solar wind disturbances in a movielike mode across the full scope of the inner solar system.
But wait… there’s more!
The PUNCH mission includes an integrated outreach program with the theme of ancient and modern Sun-watching.
All humans have Sun-watching ancestors. In the U.S., for example, there is evidence for ancient Sun-watching in Chaco Canyon, New Mexico. So by observing the Sun today, we are connecting to people of our past. The outreach program seeks to convey NASA’s exploration of the Sun as a natural extension of humanity’s age-old dedication to observing and learning about the Sun’s rhythms and mysteries.
