I recently came across a spacecraft proposal called the Interstellar Probe. This is a NASA idea to hurl a spacecraft beyond the heliopause, a region that lies well beyond Pluto’s orbit. Only two probes have ever visited this place, and both are alive: the Voyagers.
Astronomical distances are gigantic. To adjust, it’s a good idea to use Astronomical Units (AU) as the unit of length. An AU is the average distance that Earth orbits the Sun, about 150M kilometers. From the Sun, Mars is about 1.5 AU, Saturn is about 10 AU, and Pluto is 50 AU at its farthest.
The outer Solar System
In a simplified, not-to-scale picture, current models think that the Solar System’s outer regions look like this:
All stars naturally emit stellar winds, which are extremely energetic particles. These winds can travel at several hundreds of kilometers per second and gradually slow down as they move outwards. The Sun’s stellar winds, known as solar wind, is the cause of auroras on Earth. The solar winds finally become subsonic way beyond Pluto’s orbit at the termination shock. The winds continue to slow down throughout the heliosheath and eventually are stopped at the heliopause. All of this is encompassed by the heliosphere.
While the Sun’s gravitational influence extends way beyond the heliosphere, the heliopause marks some kind of “end” to the Solar System; like how 100 km above Earth’s atmosphere is considered space, despite the Moon orbiting Earth at around 400 000 km away.
The termination shock is around 100 AU away, and the heliopause is thought to lie around 120 AU away.
The Voyagers
The most distant spacecraft, Voyager 1, is currently about 160 AU from Earth and took 45 years to get there. It’s so distant that one-way communication takes 22 hours to reach the other end. Voyager 1’s initial mission was to explore Jupiter, Saturn, and their moons. This phase of its mission lasted about 3 years, and the spacecraft has been in an interstellar mission since. Voyager 1 left the heliosphere in 2012.
Voyager 1 and its sister, Voyager 2, are both recording data about the interstellar medium. They are the only probes currently there alive and are on the cutting edge of astrophysics, despite being over 45 years old. The Voyagers have a fascinating history and I highly encourage you read about their stories.
While the Voyagers are providing cutting edge data, they are approaching hard limits. Both are running out of power and are expected to shut down by around 2025. The spacecrafts disabled many instruments over time to preserve power, and soon there won’t be enough for a single one.
The only other active probe more distant than Pluto is New Horizons, the probe that took the first flyby pictures of Pluto. However, New Horizons is slower than both Voyagers, is just 56 AU from Earth, and will likely lose power before it reaches where the Voyagers are now.
All of this means that once the Voyagers go offline, there won’t be any spacecraft to continue their discoveries for decades. It’s also sad to think that such incredible machines, nearly half a century old and the only ones to have ventured into interstellar space, are close to their ends.
Interstellar Probe
This is where Interstellar Probe mentioned at the beginning comes in. Whereas the Voyagers were designed to explore the outer Solar System first and then extended to interstellar space, Interstellar Probe is designed specifically for interstellar exploration. The Voyagers were designed with a mission life of just 5 years, but Interstellar Probe is suggesting a 50-year mission, with some room for extensions. Interstellar Probe proposes to cover 400 AU in 50 years, while Voyager 1 took about 45 years for 160 AU.
This probe is basically Voyager going super saiyan.
Trajectory
Even the proposed trajectory of Interstellar Probe is cool. There are two suggestions:
- Jupiter gravity assist
- Solar Oberth maneuver
The first is a tried-and-true strategy. The probe “slingshots” around Jupiter, using the planet’s gravity, and accelerates. This has been done time and time again with many spacecrafts; the Pioneers, the Voyagers, and New Horizons are a few. Interstellar Probe is said to be able to reach its target distance just by this one assist. But the second option adds more flare to the process, quite literally.
The Solar Oberth is basically a gravity slingshot using the Sun. It sounds like something that came out of a movie. What makes this different from a plain gravity assist is the use of rocket propulsion to accelerate further when falling in. The kinetic energy gain from the greater velocity outweighs the lost fuel’s potential energy, giving the highest escape velocity. Of course, this comes at the cost of, well, absolutely insane heat.
I’ve done some digging around in the official mission concept study, and the summary is that the rocket needs to get within about 2M km from the Sun’s surface. That is ludicrously close. Anything travelling that close to the Sun will need impeccable thermal shielding. The Parker Solar Probe needs to survive an irradiance of 650 kW/m2 during its closest approach to the Sun at ~7M km.
Using a little math and assuming:
- A perihelion of 2M km
- Solar flux is constantly proportional to the inverse of distance squared
That means Interstellar Probe’s rocket will need to survive about 8000 kW/m2 of flux. Now, the real flux will likely be lower than this; the Parker Solar Probe receives less energy than the inverse square law gives when starting from Earth’s orbit. But even if the flux is just half of the calculated value, that’s still an insane heat load. Personally, I think this trajectory is only practical when we find Unobtanium. Even the authors of the mission concept study concluded that the Solar Oberth maneuver is not feasible for something as big as a rocket. There would be too many constraints and risks trying to slingshot an active rocket and probe around a star. It’s a bit of a shame because the idea is fascinating.
Ice giant flybys
Now to discussion points that are more realistic. To me, one of the most exciting opportunities this probe would provide is flyby opportunities. Since Interstellar Probe is exiting the Solar System, why not have it visit Uranus or Neptune, or both? These two planets are the least explored in the Solar System. They’ve only been visited once each by Voyager 2 back in the late 1980s, so having a much newer spacecraft visit would provide a lot of data.
It seems like there’s always a new mission headed for Mars, Jupiter, or Saturn’s moons. The sheer distance of Uranus and Neptune make them hard to reach, and it’s hard to see them clearly with Earth-based telescopes. Having an opportunity to see new images of the ice giants would be cool. Interstellar Probe shouldn’t make this a priority, it’s better to focus on its primary objectives rather than visiting worlds that might cause it to slow down. I’d prefer to have dedicated missions to visit Uranus and Neptune, but if Interstellar Probe can do double duty that would be great.
Touching the Oort Cloud
Comets 101
Comets are chunks of icy material and debris like dust or rock. They don't just contain water ice, but have other materials like methane, carbon dioxide, or ammonia. When they come close to the Sun, the warmth melts the ices into vapour and there are ejected from the comet. These form the long tails that we see.Because a comet melts a bit every time it completes an orbit, a comet will eventually disintegrate or become an asteroid. Comets are classed as short-period or long-period, based on their orbital period. Short-period comets complete an orbit every 200 years or less, while a long-period comet can take any time longer than that. Some orbit so far away that they take millions of years to complete a lap.
The Oort Cloud is a theorized volume full of comets and icy chunks of material orbiting the Sun. It’s supposed to explain how long-period comets consistently show up in the Solar System without being exhausted over time. The cloud is thought to start at 1000 AU away at the minimum, maybe up to 2000 AU. 1000 AU is so distant that Voyager 1 will take another 300 years to reach it. Even sunlight takes 5 days to travel that far.
Astronomy is always about inhumanely large magnitudes. Stars live for millions to trillions of years. The Milky Way probably has over 100 billion stars. The closest star is over 1000 times more distant than the Oort Cloud. Even something small, like Pluto’s orbit, takes 250 years to complete. The fact that, theoretically, a functional spacecraft can travel 1000 AU is mind-boggling. That’s on the magnitude of a human lifespan.
If the Oort Cloud starts at 2000 AU or farther away, then Interstellar Probe’s power source will die long before reaching it. But, if the cloud is closer than expected, there’s a very small chance that Interstellar Probe will be able to explore it. Now that would be one hell of a milestone.
A multi-generational spacecraft
So by now you should know that Interstellar Probe’s mission is ambitious. It’s planned to go far, get there fast, stay alive for a long time, and even had an insane and impractical trajectory planned for its launch. It might be the second spacecraft to visit Uranus or Neptune, and just might be able to touch the Oort Cloud. Its mission length of 50 years means that someone my age might have already died by the time the mission ends. And that’s just the primary mission.
While the power source of this spacecraft will fail over time from radioactive decay, an extended mission might honestly outlive myself. That’s fascinating to think of. You can even see this in the header image of this article, where it shows Gen Z bottoming out 50 years from the mission start. I was born too late to explore the world, and born too early to explore the galaxy. The meme part aside, at least I get to say I was born in time to explore interstellar space.