NASA rocket to analyze the windshield of the solar system

Eleven billion miles away – more than four times the distance between us and Pluto – is the boundary of our solar system’s magnetic bubble, the heliopause. Here the Sun’s magnetic field, stretching through space like an invisible spider’s web, disintegrates. Interstellar space begins.

“It’s really the biggest frontier we can study,” said Walt Harris, a space physicist at the University of Arizona in Tucson.

We still know little about what lies beyond this border. Fortunately, pieces of interstellar space can come to us, passing right through this border and making their way into the solar system.

A new NASA mission will study the light from interstellar particles that have entered our solar system to find out about the nearest areas of interstellar space. The mission, called the Heterodyne Space Interferometric Emission Line Dynamics Spectrometer, or SHIELDS, will have its first opportunity to launch aboard a suborbital missile from the White Sands missile range in New Mexico on April 19, 2021.

Our entire solar system is drifting in a group of clouds, an area cleared of ancient supernova explosions. Astronomers call this region the Local Bubble, an elongated plan of space about 300 light-years long in the spiral arm of Orion in our Milky Way galaxy. It contains hundreds of stars, including our own Sun.

We traverse this large interstellar in our trusty ship, the heliosphere, a much smaller (though still gigantic) magnetic bubble blown by the Sun. As we orbit the Sun, the solar system itself, wrapped in the heliosphere, passes through the local bubble at about 52,000 miles per hour (23 kilometers per second). Interstellar particles throw the nose of our heliosphere like rain on the windshield.

Our heliosphere looks more like a rubber raft than a wooden sailboat: its surroundings shape its shape. It compresses at pressure points, expands where it yields. Exactly how and where the lining of our heliosphere deforms gives us clues about the nature of interstellar space outside it. This boundary – and any distortions in it – are being pursued by Walt Harris, the lead investigator for the SHIELDS mission.

SHIELDS is a telescope that will launch aboard a sound rocket, a small vehicle that flies in space for a few minutes of observation time before falling back to Earth. Harris’ team launched a previous iteration of the telescope as part of the HYPE mission in 2014 and, after the design change, they are ready to launch again.

SHIELDS will measure light from 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 infiltrate through heliopause into our almost tireless solar system – but not completely.

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

A few minutes after launch, SHIELDS will reach a maximum altitude of about 300 kilometers from the ground, well above the absorbing effect of the Earth’s atmosphere. Aiming his telescope at the nose of the heliosphere, it will detect light from the arriving hydrogen atoms. Measuring how the wavelength of light stretches or contracts reveals the speed of the particles. All this, SHIELDS will produce a map to reconstruct the variable shape and density of matter at heliopause.

The data, Harris hopes, will help answer tempting questions about interstellar space.

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

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

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

Finally, learning what our current interstellar space plot is like could be a useful guide for the (distant) future. Our solar system only passes through our current space. In about 50,000 years, we will leave Bubble Local and find out who knows what.

“We don’t really know what the other cloud is like and we don’t know what happens when you cross a border into that cloud,” Harris said. “There is a great deal of interest in understanding what we will probably experience as our solar system makes this transition.”

Not that our solar system hasn’t done it before. In the last four billion years, Harris explains, the Earth has gone through a variety of interstellar environments. It’s just that we’re around now, with the scientific tools to document it.

“We’re just trying to figure out where we are in the galaxy and what we’re heading for in the future,” Harris said.

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