Earth exists in a bubble. Our atmosphere forms a protective barrier between everything on the planet's surface and the near-empty vastness of space. But it's not the only bubble Earth sits inside.
Beyond our familiar atmospheric cocoon lies a much larger bubble, an invisible boundary carved by the Sun itself.
This bubble, known as the heliosphere, is enormous. It encompasses the entire Solar System, spanning such a vast distance that only two spacecraft have ever managed to leave it.
Launched in 1977, NASA’s Voyager 1 and 2 passed beyond the heliosphere into interstellar space – the region between stars – in 2012 and 2018 respectively.
They’re the first man-made objects to ever travel beyond the Sun’s protective bubble, and are still transmitting data about the charged particles and plasma waves that can be found there.
But astronomers want to learn more about the heliosphere and what lies beyond it.
What’s its exact size and shape? How do solar particles emitted by the Sun interact with interstellar space once they pass through it? And how effective is it at protecting us from the high-energy cosmic rays coming in from outside?
“We’re still putting together a lot of pieces about how the local interstellar medium really interacts with the heliosphere,” says Dr Ralph McNutt, a physicist at Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Maryland, in the US.
It’s hoped a new $782 million (£580 million) NASA mission will help put more of those pieces together.
The Interstellar Mapping and Acceleration Probe (IMAP) spacecraft was launched on 24 September 2025.
And instead of spending decades travelling to the edge of the heliosphere, over the next few months it’ll travel to a position about 1.5 million kilometres (one million miles) from Earth called Lagrange Point 1 (L1).
Once there, unhindered by interference from Earth, IMAP can carry out its purpose: study the heliosphere, from afar.
Mission objective
IMAP is a hexagonal, solar-powered spacecraft that’s 2m (6.5ft) wide and looks a bit like a lollipop.
It’s equipped with 10 instruments designed to study the heliosphere, the Sun and incoming particles from interstellar space. It spins about four times a minute, letting its detectors scan the entire sky.
Over the course of two years, it’ll conduct its primary missions, continuously sending data back to Earth for scientists to pore over.
“February 2026 is the official start of the science phase of the mission,” says Prof Dave McComas, mission lead at Princeton University in New Jersey, US.
“The mission is nominally two years, although we have propulsion and gas to last for at least five years and probably many decades.”
The purpose of the IMAP mission is to map the heliosphere around the Sun.
We know from Voyager 1 and 2 that the boundary of the heliosphere and interstellar space, called the heliopause, is about 120 astronomical units (AU) away (120 times the Earth-Sun distance), or about 18 billion kilometres (11 billion miles).
But we don’t know if this is the same distance in all directions.
If it isn’t, the heliosphere may not actually be a sphere. It might be more of an egg shape with a long tail. Some scientists even think it could be a croissant shape, with the Sun in the middle and two tails on either side.
“We believe it’ll be possible to resolve [the heliosphere’s shape] with IMAP,” says Dr Matina Gkioulidou, the project scientist for IMAP at JHUAPL.
To do that, instruments onboard IMAP will look at neutral atoms, tiny particles with no electric charge, coming from the heliopause. These are created when outgoing charged particles from the Sun’s solar wind – a steady outflow of plasma from the Sun's outer atmosphere – crash into interstellar gas.
Because neutral atoms have no charge, they travel in straight lines. Hotter neutral atoms travel towards us faster and come from more energetic regions of interstellar space, while colder atoms originate from calmer areas.
By measuring their energies and angle of arrival, scientists can model how long the atoms took to reach us.
With this information, they can map the heliopause in different directions and reveal its true shape to a distance of about 300–400 AU, says McComas.
“We should be able to get the shape” in most directions, he says.
In regions where the heliopause stretches beyond 400 AU, however, such as at the end furthest from the Sun, “we just won’t know what that looks like,” says McComas, but we should get at least an idea of the rough shape in that direction.
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Life on the outside
The nature of the heliosphere is crucial to our understanding of how we came to exist on Earth.
“I’m trying to understand the properties of today’s heliosphere to say if the shape has changed,” says Prof Merav Opher, an astronomer and heliosphere expert at Boston University.
Her research suggests Earth might have found itself beyond the heliopause in the past, during periods when the Sun was less active.
“If we were, at times, outside of the heliosphere, how did life evolve on Earth?” she asks. “It’s really critical for us to understand which type of radiation Earth would have encountered.”
A prior mission has already explored the shape of the heliosphere. NASA’s Interstellar Boundary Explorer (IBEX) launched in 2008 and is still operational today.
It gave us our first hint that the heliosphere wasn’t a perfect sphere and looked slightly lopsided, but its instruments are low resolution, giving us only a blurry image of the heliosphere. IMAP should give us a much clearer picture.
“IMAP is a big step up over IBEX,” says McComas. “[IBEX is] a very small mission. But you need a big, strong mission with the resolution and sensitivity to put together the full picture.”
IBEX also revealed something unusual: there seems to be a band of neutral atoms surrounding the heliosphere, called the IBEX ribbon, possibly caused by the interstellar magnetic field.
“Some of these energetic neutral [atoms] are in the shape of a ring in the sky,” says McNutt. IMAP should give us a better understanding of this ribbon and exactly how it’s produced.
Dust in the wind
It’s not just the heliopause that IMAP will study, however. One of its other instruments, the Interstellar Dust Experiment (IDEX), will be trying to collect dust grains arriving from interstellar space.
We know that there are tiny grains of dust drifting between the stars because we’ve seen their infrared glow – some spacecraft have even captured interstellar dust before – but we only have a small number of detections.
It’s hoped IMAP will collect hundreds of incoming grains of dust, both from interstellar space and from the interplanetary space between the planets of the Solar System. That’s possible thanks to the size of IDEX, which has a collection area comparable to an A4 sheet of paper.
“We have this huge dust collector,” says McComas. “We should get about 100 [interstellar] dust grains in the first year alone. That would be more dust grains than all of humanity has collected in the space age.”
IDEX will measure the size, velocity and energy of the incoming dust, telling us more about its formation and journey to us.
Each grain will be broken up inside the instrument, striking an ultra-pure gold surface where they become vaporised, breaking down into atoms with either a positive or negative electrical charge.
A detector will then identify the elements present in the dust fragments, helping scientists deduce where they likely came from – exploding stars, for example, or other cosmic explosions.
The last key goal of IMAP is to measure the solar wind at L1, including how it interacts with Earth’s magnetic field, and give us a better understanding of so-called space weather.
Several of its instruments are designed to track energetic particles from the Sun that could damage satellites or cause geomagnetic storms and threaten our way of life here on Earth.
“The solar wind flies everywhere in space,” says Gkioulidou. “IMAP will measure all these particles as they’re coming out of the Sun.”
Those measurements could be useful not just for tracking space weather on Earth, but for future human missions to the Moon and Mars (both China and the US have plans to land humans on the lunar surface by 2030).
Astronauts travelling that far will spend prolonged amounts of time outside Earth’s protective magnetic field and will be subjected to more solar radiation.
“If there’s an event that could be dangerous to astronauts on the surface of the Moon, [IMAP’s data] will feed into that forecast,” says Jamie Favors, director of NASA’s Space Weather Program in Washington, DC.
For humans going further afield, “on that journey to Mars, there may be months in deep space [with] no protection,” he says. “There are things about the [space] environment we don’t know.”
Interstellar exploration
It’s the observations of the heliosphere, however, that are perhaps the most enticing aspect of IMAP’s mission. “It’s extremely exciting,” says Opher. “It’s a state-of-the-art mission to study the heliosphere.”
Aside from the Voyager 1 and 2 spacecraft, only one other active spacecraft is set to cross the boundary into interstellar space.
NASA’s New Horizons spacecraft, which famously provided the first high-resolution images of the dwarf planet Pluto in 2015, is expected to cross the heliopause in the 2040s.
It wasn’t designed to study this region, however, so while it’ll teach us a lot about it, scientists are hopeful of one day sending new, dedicated missions into interstellar space.
One such proposal led by McNutt is called Interstellar Probe. It’s an idea to send a long-lived spacecraft into interstellar space, designed to operate for generations and travel an incredible 1,000 astronomical units in about 50 years.
That's roughly ten times farther than the Voyager probes have currently reached.
The mission would harness a gravity assist from Jupiter to fling itself out of the Solar System, carrying a suite of advanced instruments to study interstellar dust, neutral atoms, magnetic fields and more beyond the heliopause.
“I’ve been convinced this is a good idea for a long time,” says McNutt. “It’s just a question of getting other people convinced.”
In 2024, the National Academies of Sciences, Engineering and Medicine in the US – a public body that advises the US government on science, engineering and health – announced its proposal for the next decade of NASA’s research into the Sun and its surrounding environment, known as the Heliophysics Decadal Survey.
The survey didn’t directly recommend building Interstellar Probe, but it did say it was a compelling future mission that should be considered in the next decadal survey in the 2030s.
Having such a mission to collect in-situ data (data directly from the source) would be invaluable, says Opher. “We still want a new Voyager,” she says. “There’s nothing like in-situ data.”
It’s not just NASA considering a new mission to interstellar space. China is investigating a possible mission called Interstellar Express, or Shensuo.
It would send two or more probes beyond the heliopause, carrying out flybys of Neptune and other interesting objects (like the dwarf planet Quaoar in the outer Solar System) along the way. The status of the mission currently remains uncertain, but could be hugely exciting if it goes ahead.
There are more speculative ideas, too. Opher is part of a team that has proposed using a fusion-based propulsion system to explore the heliosphere in a way that’s simply not possible with conventional chemical spacecraft engines.
“We could launch six different spacecraft in all directions of the heliosphere based on fusion propulsion,” says Opher. “Something like that can bring us fast to the edges. We’re trying to get to the edge in five years and then slow down to get all the data.”
Whether humans, rather than just our robotic explorers, will ever venture into interstellar space seems like a far more daunting prospect.
Such a mission would almost certainly carry a one-way ticket.
Perhaps a bold attempt could be made to reach one of our closest stars, such as Proxima Centauri, about four light-years (approx 40 trillion kilometres) away, where astronomers are hopeful there might be interesting planets to visit.
For the time being, however, it’s IMAP that’s at the forefront of everyone’s minds, as it sets out to study the heliosphere from a vantage point, not in the empty expanse of interstellar space, but from a quiet corner of the Solar System.
Will it reveal that the heliosphere is shaped like an egg, a croissant, or something else entirely? In the coming years, we should finally have a solid answer to this question, and plenty of others, as well as a better understanding of the boundary between our Solar System and interstellar space.
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