Space agencies have selected two solar missions and a “comet interceptor” for development and launch.

NASA has selected two missions — PUNCH and Tracers — to study the Sun and its connection to Earth, while the European Space Agency (ESA) has selected a cometary explorer.

Solar wind (art)
A constant outflow of solar material streams out from the Sun, depicted here in an artist's rendering.

Roll with the Solar PUNCHes

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) will investigate the Sun’s outer atmosphere (that is, the corona) and the stream of solar wind it generates. The spacecraft will also track coronal mass ejections, the solar wind tsunamis that can crash around Earth, affecting satellite communications such as GPS, damaging power grids, and creating Northern Lights in the process.

The solar wind is a plasma, whose protons and electrons fly separately along the Sun’s magnetic field lines. Earth’s magnetic field generally protects us from this stream of particles, but when the Sun’s magnetic field touches Earth’s, the two fields can connect, creating openings. When those openings are long-lived, they are called cusps.

“Photographing the sky in polarized light is the secret sauce of the mission,” says PUNCH’s principal investigator Craig DeForest (Southwest Research Institute). Sunlight becomes polarized when it bounces off the plasma’s electrons, he explains. So polarization gives 3D information, showing how features in the solar wind move through space.

PUNCH (art)
This artist's concept shows the four satellites of PUNCH, a mission that will orbit Earth and image the Sun’s outer corona.
Southwest Research Institute

PUNCH consists of four suitcase-size satellites known as microsats that will fly in formation in Earth orbit. One of them carries a narrow-field imager, while the others carry wide-field imagers. Baffles allow the wide-field imagers to gaze near the Sun and still capture space weather events that are a thousand times fainter than the Milky Way. Together, these imagers will produce polarized-light images of the entire inner solar system. Space physicists will use this information to study how the solar wind turns into turbulent gusts by the time it reaches Earth. Read more about PUNCH here.

PUNCH will aid the Parker Solar Probe, which launched in 2018 to become the closest-ever spacecraft to the Sun. Parker completed its second close approach to the Sun on April 4, 2019. The next perihelion will come on September 1st. As it swoops by the Sun, the probe directly investigates the region where the solar wind is generated, providing in situ measurements that will complement PUNCH’s measurements from afar. If PUNCH launches in 2022, the two missions will overlap for about four years.

Tracing the Sun-Earth Connection

Sharing PUNCH’s ride to space, the Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (Tracers) will study the Sun-Earth connection a bit closer to home. While PUNCH explores the region where the solar wind originates, Tracers instead investigates the cusps where the Sun’s magnetic field interacts with Earth’s field.

Physicist Craig Kletzing (University of Iowa), who leads the Tracers mission, explains that it will complement the existing Magnetospheric Multiscale (MMS) mission, launched in 2015. MMS consists of four small spacecraft that loop around Earth, observing any magnetic openings as they occur.

“One of the long-term goals of our space research to evolve toward predictive ‘space weather’ models to improve our ability to utilize space as a resource,” Kletzing says. “The science that Tracers studies will be essential to achieve this goal.”

NASA selected PUNCH and TRACERS for development and flight through its Heliophysics Small Explorers Program. The missions will cost no more than $165 million and $115 million, respectively. (PUNCH’s budget includes launch costs; TRACERS’ budget does not include rideshare costs, which are significantly smaller.) The missions could launch as early as 2022.

NASA's SIMPLEX Finalists

Also potentially launching in 2022 are small satellites (no heavier than 180 kg) selected as finalists in another program, known as Small Innovative Missions for Planetary Exploration (SIMPLEX). NASA has chosen three finalists: a reconnaissance mission to binary asteroids called Janus; Escape and Plasma Acceleration and Dynamics Explorers (Escapade), a mission to understand escape process from Mars’s atmosphere; and Lunar Trailblazer, which would directly detect and map water on the Moon’s surface.

These finalists were chosen from 12 proposals submitted in 2018 and will receive funding to develop their ideas further over the course of the next year. Read more about the finalists on NASA’s website.

The Comet Interceptor

Meanwhile, across the pond, the ESA has selected a “fast-class” mission to accompany its exoplanet-hunting satellite, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), to be launched in 2028. The three spacecraft that make up the so-called “Comet Interceptor” will ride with ARIEL to the L2 Lagrange point 1.5 million km (1 million miles) beyond Earth on the opposite side of the Sun.

Oort Cloud
Long-period comets traveling toward the Sun for the first time likely come from the Oort Cloud (shown here as an artist's concept) in the solar system's outermost reaches.
Don Davis

The Comet Interceptor will wait there while it looks for a target: either a new comet, coming to the inner solar system for the first time, or even an interstellar comet like ‘Oumuamua. Once it finds a target, all three spacecraft will fly toward it together, before separating to view the target from different vantage points.

ESA’s Rosetta made history with its explorations of Comet 67P/Churyumov-Gerasimenko, but that comet has been orbiting the inner solar system every 6.5 years for many a millennium, and solar heating has disturbed and altered its surface. A new comet, on the other hand, could provide a pristine sample of the outermost reaches of the solar system. New comets are typically discovered months to years before they approach the Sun, so building the Comet Interceptor and keeping it at the ready finally gives scientists a chance to study these interlopers.

Read more about Comet Interceptor on ESA’s website.


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