This story has been updated to reflect a new scheduled launch time, Weds., Sept. 24.

On Wednesday, Sept. 24, a Falcon 9 rocket will break the bonds of gravity while carrying the Interstellar Mapping and Acceleration Probe (IMAP) on its mission to explore and map the heliosphere — the invisible cosmic shield surrounding our solar system — and to answer some great unknowns about how particles accelerate in the solar wind.  

IMAP, a one-ton spacecraft, will launch from Cape Canaveral carrying 10 exquisitely sophisticated instruments to address these fundamental science questions about the nature of space. Launch is scheduled for 7:30 a.m. EDT. Princeton’s Department of Astrophysical Sciences and Center on Science and Technology are hosting a watch party in Peyton Hall, and NASA’s live broadcast will run from 6:40 a.m. till about 9:15.

The spacecraft, instruments and science preparation involved a team of institutions and major suppliers that spans 82 U.S. partners in 35 states, plus the United Kingdom, Poland, Switzerland, Germany and Japan, under the leadership of Princeton astrophysicist David McComas.

David McComas, Principal Investigator of NASA's Interstellar Mapping and Acceleration Probe (IMAP) and Professor of Astrophysical Sciences at Princeton University

Photo by

Matthew Raspanti, Office of Communications

“IMAP will revolutionize our understanding of the outer heliosphere,” said McComas, IMAP principal investigator and professor of astrophysical sciences at Princeton University. “It will give us a very fine picture of what’s going on out there by making measurements that have roughly 30 times higher combined resolution and sensitivity than ever before.”

McComas has been shepherding this mission since its inception, just as he led its predecessor NASA mission, IBEX (the Interstellar Boundary Explorer), and other spacecraft and instruments. IBEX was much smaller — about the size of a truck tire — while IMAP is 10 feet across and 4 feet tall.

In addition to mapping the wonders of the heliosphere, IMAP will also monitor the Sun for dangerous solar storms to help protect our planet and astronauts from them.

“It’s really an extraordinary mission, and it’s being led out of Princeton,” McComas said. “We’re not known as a big space physics place, a big NASA heliophysics place, but we should be.”

Protecting satellites and astronauts from solar storms

Over the next three months, IMAP will fly toward the Sun, then settle into orbit around a point in space called the first Lagrange point (L1), the position where the Sun’s gravity is perfectly balanced by Earth’s. IMAP will remain at L1 indefinitely, orbiting the Sun at the same rate the Earth does, always remaining roughly a million miles Sun-ward of our planetary home. 

From there, IMAP has an unobstructed view of the Sun and the solar wind, the million-mile-per-hour stream of plasma constantly pouring outward from the Sun in all directions. IMAP spins on its central axis every 15 seconds so that each of its instruments sees its full field of view four times per minute. 

When IMAP’s instruments detect a disturbance, such as heated plasma, compressed magnetic fields, or energized particles from large solar eruptions called coronal mass ejections (CMEs), the IMAP Active Link for Real-Time (I-ALiRT) system will downlink critical information to the space weather community about a half hour prior to the storm’s arrival at Earth. 

That’s enough time for satellites to react with protective measures and also warn any astronauts who might be in harm’s way. Everything from GPS systems to streaming football games to critical national defense infrastructure can be disrupted by space weather events if satellites are not prepared.

The IMAP solar array is tested at the Astrotech Space Operations Facility near NASA's Kennedy Space Center. When IMAP is in position at L1, 1 million miles Sun-ward of Earth, it will rotate four times per minute while keeping the solar array pointed at the Sun to generate approximately 500 watts of power.

Ed Whitman, Johns Hopkins APL, NASA

Additionally, as NASA sends astronauts to the Moon and Mars, IMAP data becomes ever more crucial to help protect our human explorers. 

“Many people think that solar flares are the main source of space weather, but that’s not true,” said McComas. “The main source of the biggest, most difficult space weather, are large geomagnetic storms that are usually made by large coronal mass ejections that come off of the Sun.” 

These CMEs travel “sometimes two, three, four times faster than the solar wind,” McComas said. “That causes them to generate big shocks in front of them, just like the shockwave in front of a supersonic airplane.” 

When these shocks and CMEs pass over IMAP, they’re traveling between 1 and 3 million miles an hour, McComas said, “and they’re still a million miles away from us. We take the data there and use radio waves — which of course, travel at the speed of light — to send that information to Earth. The data takes only seconds to get back to Earth, but the CME takes anywhere from 20 to 30 minutes, so that’s how you get the advance warning.”

The heliosphere: The solar system’s invisible cosmic shield 

Some astrophysicists, including IMAP team member Hakeem Oluseyi, like to describe the heliosphere’s outer edge, known as the heliopause, as a “cosmic shield” or “galactic shield” because it blocks roughly 90% of the harsh interstellar radiation that would otherwise pour into the solar system. 

“It’s like Captain America’s shield but for Earth and the Moon and Jupiter and the whole solar system, protecting us from the harmful radiation streaming through our galaxy,” said Oluseyi, a Clarence J. Robinson Professor of Astrophysics at George Mason University and a visiting research scholar at Princeton.

McComas and NASA like to think of the heliosphere as “the Sun’s neighborhood.” Despite its importance, scientists don’t fully understand the heliosphere’s shape or structure. What we do know has been pieced together using measurements taken at Earth and throughout the solar system from spacecraft like Ulysses, IBEX and Parker Solar Probe, and the twin Voyagers that have traveled out beyond the heliosphere. 

Over the past decade, McComas and other space physicists have refined their understanding of how the heliosphere shields us from cosmic radiation from distant exploded stars — and learned that the front edge of the heliosphere is much closer than the trailing edge. 

A conceptual illustration showing the heliosphere — the vast, comet-shaped 'solar neighborhood' protecting our Sun and solar system — that is generated by the solar wind pushing out against the interstellar medium.

Image courtesy of NASA's Goddard Space Flight Center Conceptual Image Lab

“It’s probably more like a comet shape than a sphere,” said Jamie Rankin, a research scholar and lecturer in astrophysical sciences at Princeton who is a co-investigator on IMAP and the instrument lead on SWAPI, the Solar Wind and Pickup Ion instrument onboard IMAP.

The charged outgoing solar wind materials and charged incoming cosmic materials do not easily intermix. The zone of balance that separates the two materials marks the outermost boundary of the heliosphere.

McComas and the IMAP team will generate a high-definition heliosphere map with the data that their new spacecraft will provide. They hope to resolve the detailed structure of the heliosphere; to determine exactly what is happening at the front edge of the heliosphere, where we are pushing up a bow wave of charged cosmic particles as our solar system plows through interstellar space; and to finally understand what causes solar wind particles to accelerate — some of them to nearly the speed of light. 

The acceleration question

Astrophysicists still don’t fully understand how the Sun can accelerate particles so dramatically. A typical solar wind particle has energy measured in the thousands of electron-volts, but when it’s accelerated, it suddenly has millions and sometimes even billions of electron-volts in very short order. 

The faster a particle is moving, the more deeply it can penetrate and the more damage it can cause, so these accelerated particles pose a significant risk to spacecraft and astronauts. Many of IMAP’s instruments are designed to measure and analyze these particles to help astrophysicists understand this acceleration and prepare to keep our astronauts and instruments protected from it — just one of the many ways NASA innovation is benefiting humanity. 

Technicians perform tests on the Solar Wind and Pickup Ions (SWAPI) instrument, which was designed and built in Princeton's Space Physics Lab. On IMAP, SWAPI will collect and count particles from the solar wind flowing from the Sun and particles called pick-up ions that have entered the heliosphere from outside the solar system and traveled inwards where IMAP orbits near Earth.

Ed Whitman, Johns Hopkins APL, NASA

Scientists have seen particles get this kind of energy boost from astrophysical processes throughout the universe, from CME shock waves to the heliosphere’s termination shock (where particles bump into our cosmic shield) to distant supernovas and black holes. They also know that accelerated particles have deposited energy in Earth’s atmosphere (and the atmospheres of other planets) and into solid material, including soils and organic tissue. Some have theorized that chemical changes induced by accelerated particles contributed to forming the building blocks of life.

“The heliosphere serves as a local laboratory that gives us the opportunity to have a close-up look at the sorts of energization processes that occur throughout the universe,” said Rankin. 

The particles are the explorers

IMAP itself will stay at the first Lagrange point (L1), only a million miles from Earth. The Sun is 93 million miles away, and the heliopause is around 120 times farther than that, so how can IMAP map anything? 

“The particles are doing the exploring,” said McComas. When a charged solar wind particle reaches the edge of the solar system, it can do what’s called a “charge exchange,” he explained. “It finds a loosely bound electron and says, ‘I’ll take that!’ When it grabs that electron, the particle becomes neutral, and it can move freely across any magnetic field.” 

In other words, these neutral particles can travel right through the heliopause, unaffected by the electromagnetic gas of the heliosphere. At that point, the particle’s direction of movement is preserved — it continues whichever way it was going when it stole the electron, whether that’s onward into the galaxy or back into our solar system. In a tiny handful of cases, that particle finds its way right into an IBEX or IMAP instrument, where its precisely measured speed and direction will help space physicists extrapolate the structure and nature of the heliosphere.

In addition, the spacecraft instruments can directly detect interstellar neutral atoms — particles from other stars that did their own charge exchange, became neutral, then sailed through our galactic shield. “It’s really amazing that our instruments find and measure particles raining in from outside of the solar system,” said McComas. 

Each of these particles bears an imprint from outside the heliosphere — a message in a bottle — describing where it has been and some of what it is like out there. 

‘A snapshot of our cosmic shield’

The IMAP suite of instruments is building the map of our cosmic shield by collecting these, one pixel at a time. Think of it like a camera, said Rankin, “but instead of using light to provide the information, we collect neutral atoms, and that gives us a snapshot of our cosmic shield and provides clues about interstellar space.”

Grains of interstellar dust also travel through the heliopause, and they will be measured by their own instrument aboard IMAP. The composition of this cosmic dust is a fingerprint of where it comes from in the galaxy, giving us glimpses into the compositions of distant stars. By studying cosmic dust, IMAP scientists hope to piece together what the material between stars is made of. 

“IMAP allows us to get a complete picture of the full life cycle of the particles: how they come out from the Sun, pass over the region of space around the Earth, how they then go out and interact at the boundaries of the heliosphere, and then some fraction come back as energetic neutral atoms,” said McComas. “Previous work was like touching the different parts of the elephant. This is like actually getting to see what an elephant fully is instead of, ‘Oh, that’s an energetic particle. I wonder what that’s related to…’”

McComas said he is looking forward to finding surprises in the data gathered by the extraordinary suite of instruments aboard IMAP. 

“In addition to all the great science we know we’ll be doing with this mission, there’s also a huge new opportunity for what we like to call discovery science, which is the science that you get that you haven’t thought of yet, because the measurements are so much more sensitive and capable than we’ve ever taken before,” he said. “Whenever you do that in the space environment, you find a bunch of new, unexpected phenomena, so you make new discoveries. That part really excites me, too!”

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Photo by

Jamie Rankin, Princeton University