SHIELDS! NASA rocket launches to inspect the windshield of our solar system

An illustration of the heliosphere bombarded with cosmic rays coming from outside our solar system. Credit: Goddard Space Flight Center / NASA Conceptual Imagery Lab

Eleven billion miles away – more than four times the distance from us to Pluto – is the limit of the magnetic bubble of our solar system, the heliopause. Here, the Sun’s magnetic field, which stretches across space like an invisible spider’s web, is extinguished to nothing. Interstellar space begins.

“It really is the biggest such frontier that we can study,” said Walt Harris, a space physicist at the University of Arizona at Tucson.

We still know little about what lies beyond this border. Fortunately, fragments of interstellar space can reach us, cross this border and enter the solar system.

A new ">Nasa The mission will study light from interstellar particles that have drifted through our solar system to learn more about areas closest to interstellar space. The mission, called Spatial Heterodyne Interferometric Emission Line Dynamics Spectrometer, or SHIELDS, was successfully launched at 4:30 a.m. EDT on April 19, 2021 from the White Sands Missile Range in New Mexico. NASA’s Brant IX black probe rocket carried the payload to a climax of 177 miles before parachuting and landing at White Sands. Preliminary indications show that the vehicle’s systems performed as expected and that the data was received.

Our entire solar system is adrift in a cloud cluster, an area cleared by ancient supernova explosions. Astronomers call this region the local bubble, an oblong space about 300 light years long in the spiraling arm of Orion of our Milky Way galaxy. It contains hundreds of stars, including our own Sun.

Local bubble illustration

Illustration of the local bubble. Credit: NASA’s Goddard Space Flight Center

This interstellar sea is our trusty vessel, the heliosphere, a much smaller (albeit gigantic) magnetic bubble blown by the Sun. As we circle the Sun, the solar system itself, enclosed in the heliosphere, travels through the local bubble at a speed of about 52,000 miles per hour (23 kilometers per second). Interstellar particles project the nose of our heliosphere like rain against a windshield.

Our heliosphere looks more like a rubber raft than a wooden sailboat: its environment shapes its shape. It compresses at pressure points, expands where it gives way. Exactly how and where the liner of our heliosphere deforms gives us clues to the nature of interstellar space outside. This limit – and all the distortions it entails – is what Walt Harris, principal investigator of the SHIELDS mission is looking for.

SHIELDS is a telescope that will be launched aboard a sounding rocket, a small vehicle that flies into space for a few minutes of observation before falling back to Earth. Harris’s team launched an earlier version of the telescope as part of the HYPE mission in 2014, and after changing the design, they are ready to launch again.

SHIELDS will measure the light of a special population of hydrogen atoms originating in interstellar space. These atoms are neutral, with a balanced number of protons and electrons. Neutral atoms can cross magnetic field lines, so they seep through the heliopause and into our solar system almost unfazed – but not completely.

Sound rocket launch from NASA's Wallops flight facility

Observers witness the launch of a sounding rocket from NASA’s Wallops Flight Facility in April 2015. Credit: NASA / Patrick Black

The small effects of this border crossing are the key to the SHIELDS technique. Charged particles circulate around the heliopause, forming a barrier. Neutral particles from interstellar space must pass through this glove, which changes their trajectory. SHIELDS was designed to reconstruct the trajectories of neutral particles to determine where they came from and what they saw along the way.

Within minutes of launch, SHIELDS will reach its maximum altitude of about 186 miles (300 kilometers) from the ground, well above the absorbent effect of Earth’s atmosphere. Pointing its telescope at the nose of the heliosphere, it will detect the light of incoming hydrogen atoms. Measuring how the wavelength of this light stretches or contracts reveals the speed of the particles. In total, SHIELDS will produce a map to reconstruct the shape and varying density of matter at the heliopause.

The data, Harris hopes, will help answer some tantalizing questions about what interstellar space looks like.

For example, astronomers think the local bubble as a whole is about 1/10e as dense as most of the rest of the galaxy’s main disk. But we don’t know the details – for example, is the matter in the local bubble distributed evenly, or clustered in dense pockets surrounded by nothingness?

“There are a lot of uncertainties about the fine structure of the interstellar medium – our maps are pretty rough,” Harris said. “We know the general outlines of these clouds, but we don’t know what’s going on inside.”

Astronomers don’t know much about the galaxy’s magnetic field either. But that should leave a mark on our heliosphere that SHIELDS can detect, compressing the heliopause in a specific way based on its strength and orientation.

Finally, learning what our current interstellar space plot looks like could be a useful guide for the (distant) future. Our solar system is just passing through our current piece of space. In some 50,000 years, we’ll be on the verge of breaking out of the local bubble and into something or other.

“We don’t really know what that other cloud looks like, and we don’t know what happens when you cross a border in that cloud,” Harris said. “There is a lot of interest in understanding what we are likely to experience as our solar system makes this transition. “

Not that our solar system hasn’t done this before. Over the past four billion years, says Harris, Earth has passed through a variety of interstellar environments. It’s just that now we’re here, with the scientific tools to document it.

“We’re just trying to figure out our place in the galaxy and where we’re headed in the future,” Harris said.


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Arline J. Mercier

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