Although I’ve written on a number of occasions about the project called Interstellar Probe, the effort to create what we might call a next-generation Voyager equipped to study space beyond the heliosphere, it’s always been in terms of looking back toward the Solar System. What is the shape of the heliosphere once we see it from outside, and how does it interact with the local interstellar medium? The Voyagers have given us priceless clues, but they were never designed for this environment and in any case will soon exhaust their energies.
Pontus Brandt (JHU/APL), who is project scientist for the Interstellar Probe effort, takes us beyond these heliosphere-centric ideas as he talks to Richard Stone in a fine article about the mission called The Long Shot that ran recently in Science. Because when you launch something moving faster than Solar System escape velocity, you just keep going, and while 1000 AU is often cited as a target for this mission, it’s really only a milestone marker telling us how long we’d like the spacecraft to fly with all its equipment functioning and robust. Beyond the heliosphere, though, we’re looking at interstellar clouds we know fairly little about, and in the long-term view, future interstellar missions will have to know this terrain.
When stars are born, clouds of gas and dust that were not incorporated into the final stellar system remain. Moving on an orbit around the Milky Way that takes some 230 million years to complete, the Solar System encounters these clouds, one of which is the Local Interstellar Cloud, although as Brandt told Richard Stone in the Science article, we really know so little about the cloud environment that our conception is on the order of a child’s sketch. According to the sketch, the Sun has been in the Local Interstellar Cloud for thousands of years (Brandt cites 60) but we’re on its boundaries and appear to be approaching the edge of the so-called G Cloud now. I should add that, as we’ll see in a moment, there are scientists who disagree.
Image: Our solar journey through space is carrying us through a cluster of very-low-density interstellar clouds. Right now the Sun is inside of a cloud (Local cloud) that is so tenuous that the interstellar gas detected by the IBEX (Interstellar Boundary Explorer mission) is as sparse as a handful of air stretched over a column that is hundreds of light-years long. These clouds are identified by their motions, indicated in this graphic with blue arrows. Credit: NASA/Goddard/Adler/U. Chicago/Wesleyan.
What happens when, in perhaps as little as two millennia, we make this crossing? It would be useful to know more about the heliosphere to answer the question. And we also need to know more about the temperature and density of a cloud like this, because the heliosphere itself seems malleable, capable of being deformed by the medium through which it moves. Compress the heliosphere and there are implications for life on Earth, for we are protected from dangerous cosmic rays – at least a high percentage of them – by its protective embrace. It would be good to know just how far the heliosphere can be compressed. All the way down to Earth’s orbit?
Here I want to quote from the article:
There’s evidence of such an event around the time early hominids were just beginning to pick up stone tools, and Brandt muses on a possible connection. “Let that creep up your spine for a moment,” he says. In recent years, scientists have discovered iron-60 isotopes in ocean crust samples dating from 2 million to 3 million years ago. Iron-60 is not found naturally on Earth: It’s forged in the cores of large stars. So, either a nearby supernova blasted the heliosphere with the iron dust, or the heliosphere drifted through a dense cloud laden with iron-60 from a previous supernova. Either way, Brandt says, “The heliosphere was way in, and we had a full blast of galactic cosmic rays and interstellar matter for a long, long time.” To look for relics of other such events, IP could use plasma wave antennas to essentially take the temperature of nearby electrons. Hot regions might mark the blast paths of material from past supernovae.
And here’s an interesting factoid, which I’m pulling from the Harvard & Smithsonian Center for Astrophysics: About half of the interstellar gas, almost entirely hydrogen and helium, is spread through 98% of the space between stars, hot but extremely low in density. The other half of the interstellar gas is compressed into 2% of the volume, and we observe it as interstellar clouds, the densest of which are molecular clouds, primarily formed of molecular hydrogen though including carbon monoxide and some organic compounds, with higher concentrations of dust than in the rest of the ISM.
We know about the interstellar medium both by astronomical measurements and spacecraft within the medium – our Voyagers again – that move amidst neutral gas and dust grains, some of which penetrate the heliosphere and can also be measured by spacecraft like New Horizons. Clearly, a craft designed from scratch to make these measurements outside the heliosphere would free us from the uncertainties of astronomical observation looking through the heliosphere to the medium outside.
The Local Interstellar Cloud moves toward us from the direction of Scorpius and Centaurus. All of this movement through clouds and voids is a reminder that the Sun orbits the galaxy and moves through different environments all the time. JPL notes that interstellar densities ranging from 10-5 to 105 atoms/cm3 can be observed near our system in the Milky Way.
Thus the interest in what happens next, as Brandt notes. For one thing, our future hopes for interstellar exploration focus particularly on the nearest stars, and Alpha Centauri is within the G-cloud the Sun now moves toward. We rely on hydrodynamic simulations to estimate the effects of the Solar System moving into a cloud of denser material. A spacecraft like Interstellar Probe could be launched to move ‘upstream’ of the Sun’s motion, essentially exploring the future environment through which we will pass. We’ll someday send much faster exploratory missions to sample the G Cloud in situ.
The Interstellar Probe concept study goes to the National Academies of Sciences, Engineering, and Medicine, which essentially prioritizes where we are going in space exploration over ten year periods. We won’t know how the panel enjoined with making these decisions will come down until 2024, and remember that competing ideas involving space beyond the heliosphere are out there, the most visible of which is the Solar Gravitational Lens (SGL) mission now in advanced study at the Jet Propulsion Laboratory. Be aware as well of a Chinese effort known as Interstellar Express.
One way or another, and this is true with or without the endorsement of a Decadal study, we will get spacecraft fully designed for the interstellar environment out beyond the heliosphere. I make no predictions on timing other than to say that the earliest we might expect a launch of this kind of mission is in the 2030s, and who knows what other factors may come into play if none of the current studies is funded? Nonetheless, the long-term picture I embrace makes robotic exploration beyond our Solar System inevitable whether time to launch is 10 or 100 or 1000 years from now. I think we’re wired to do it.
Space missions always bring surprises, and it’s only fair to note that the model of the Sun’s nearby cloud environment has been challenged in recent work. What alternate outcomes might an Interstellar Probe mission alert us to? Here is a snip from an interesting 2014 paper by Cécile Gry and Edward Jenkins proposing a model that is:
…fundamentally different from previous models (e.g., Lallement et al. 1986; Frisch et al. 2002, RL08) where the LISM is constituted of a collection of small clouds or cloudlets that are presented as separate entities moving as rigid bodies at different velocities in slightly different directions. In particular, in our picture, the LIC, the G cloud, and other distinct clouds of the RL08 model, are unified in a single local cloud.
And this, of course, would become apparent to any future mission pushing upstream from the heliosphere. We may have to send such a mission to make this call.
Back to the present, though. Pontus Brandt now takes his heliophysics expertise into an ongoing mission, with a new role as part of the New Horizons science team at JHU/APL. This is clearly a good fit, given that this spacecraft is already out there, operating more than 50 AU from the Sun with a suite of plasma and dust instruments that is exploring the dust and charged particles in the full flow of the solar wind. We have only one operating spacecraft in the Kuiper Belt, and this is it, as Brandt notes:
“New Horizons remains a pathfinder on a historic journey, and since we’re equipped with instrumentation not flown on Voyager, we will be able to answer some of the big questions about what upholds our vast heliosphere as it plows through the interstellar medium. Leaving the foreground ‘haze’ of the solar system’s dust and gas, New Horizons is also in a position to make some game-changing discoveries that not only give us glimpses into our changing local interstellar medium, but also discoveries on cosmological scales.”
Image: Pictured in the New Horizons mission operations center at the Johns Hopkins Applied Physics Laboratory, Pontus Brandt brings a new kind of expertise to the New Horizons science leadership team — heliophysics, where the team expects to make breakthroughs that no other mission can, with new capabilities never before available so far from the Sun. Credit: Johns Hopkins APL/Craig Weiman.
Brandt’s work at New Horizons is a reminder that the deeper we push into the Solar System, the more we also explore basic interactions between our star and an interstellar medium we are learning how to map. All of this couples with a continuing effort from Earth to locate future flyby targets for close observation. Going interstellar demands knowing where we’re coming from as much as knowing where we’re heading.
And once we do get to the point of sending a spacecraft all the way to another star? Let me quote something Ian Crawford said on this topic in a paper some years back:
If the Sun does lie within the LIC, then a mission to ? Cen would sample the outer layers of the LIC, an interval of low density LB material, the edge of the G cloud, and the deep interior of the G cloud. This would sample one of the most diverse ranges of interstellar conditions of any mission to another star located with 5 pc of the Sun, as most other potential targets lie within the LIC… Even if the Sun lies just outside the LIC…, the trajectory to ? Cen would still permit detailed observations of the boundary of the G cloud (and its possible interaction with the LIC), and determine how its properties change with increasing depth into the cloud from the boundary.
The paper on a new model for nearby interstellar clouds that I mentioned above is Gry & Jenkins, “The interstellar cloud surrounding the Sun: a new perspective,” Astronomy & Astrophysics 567 (2014), A58 (abstract). For further background on the interstellar medium, see Ian Crawford’s “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight,” published in the Journal of the British Interplanetary Society Vol. 62 (2009), pp. 415-421 (preprint). The definitive book on the matter is Bruce Draine’s Physics of the Interstellar and Intergalactic Medium (Princeton University Press, 2011).