The Challenges of an Alien Spaceflight Program: Escaping Super Earths and Red Dwarf Stars

In a series of papers, Professor Loeb and Michael Hippke indicate that conventional rockets would have a hard time escaping from certain kinds of extra-solar planets. Credit: NASA/Tim Pyle

Since the beginning of the Space Age, humans have relied on chemical rockets to get into space. While this method is certainly effective, it is also very expensive and requires a considerable amount of resources. As we look to more efficient means of getting out into space, one has to wonder if similarly-advanced species on other planets (where conditions would be different) would rely on similar methods.

Harvard Professor Abraham Loeb and Michael Hippke, an independent researcher affiliated with the Sonneberg Observatory, both addressed this question in two recentlyreleased papers. Whereas Prof. Loeb looks at the challenges extra-terrestrials would face launching rockets from Proxima b, Hippke considers whether aliens living on a Super-Earth would be able to get into space.

The papers, tiled “Interstellar Escape from Proxima b is Barely Possible with Chemical Rockets” and “Spaceflight from Super-Earths is difficult” recently appeared online, and were authored by Prof. Loeb and Hippke, respectively. Whereas Loeb addresses the challenges of chemical rockets escaping Proxima b, Hippke considers whether or not the same rockets would able to achieve escape velocity at all.

Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser

For the sake of his study, Loeb considered how we humans are fortunate enough to live on a planet that is well-suited for space launches. Essentially, if a rocket is to escape from the Earth’s surface and reach space, it needs to achieve an escape velocity of 11.186 km/s (40,270 km/h; 25,020 mph). Similarly, the escape velocity needed to get away from the location of the Earth around the Sun is about 42 km/s (151,200 km/h; 93,951 mph).

As Prof. Loeb told Universe Today via email:

“Chemical propulsion requires a fuel mass that grows exponentially with terminal speed. By a fortunate coincidence the escape speed from the orbit of the Earth around the Sun is at the limit of attainable speed by chemical rockets. But the habitable zone around fainter stars is closer in, making it much more challenging for chemical rockets to escape from the deeper gravitational pit there.”

As Loeb indicates in his essay, the escape speed scales as the square root of the stellar mass over the distance from the star, which implies that the escape speed from the habitable zone scales inversely with stellar mass to the power of one quarter. For planets like Earth, orbiting within the habitable zone of a G-type (yellow dwarf) star like our Sun, this works out quite while.

This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: Pale Red Dot

Unfortunately, this does not work well for terrestrial planets that orbit lower-mass M-type (red dwarf) stars. These stars are the most common type in the Universe, accounting for 75% of stars in the Milky Way Galaxy alone. In addition, recent exoplanet surveys have discovered a plethora of rocky planets orbiting red dwarf stars systems, with some scientists venturing that they are the most likely place to find potentially-habitable rocky planets.

Using the nearest star to our own as an example (Proxima Centauri), Loeb explains how a rocket using chemical propellant would have a much harder time achieving escape velocity from a planet located within it’s habitable zone.

“The nearest star to the Sun, Proxima Centauri, is an example for a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, it was discovered that this star has an Earth-size planet, Proxima b, in its habitable zone, which is 20 times closer than the separation of the Earth from the Sun. At that location, the escape speed is 50% larger than from the orbit of the Earth around the Sun. A civilization on Proxima b will find it difficult to escape from their location to interstellar space with chemical rockets.”

Hippke’s paper, on the other hand, begins by considering that Earth may in fact not be the most habitable type of planet in our Universe. For instance, planets that are more massive than Earth would have higher surface gravity, which means they would be able to hold onto a thicker atmosphere, which would provide greater shielding against harmful cosmic rays and solar radiation.

Artists impression of a Super-Earth, a class of planet that has many times the mass of Earth, but less than a Uranus or Neptune-sized planet. Credit: NASA/Ames/JPL-Caltech

In addition, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and shallower oceans – an ideal situation where biodiversity is concerned. However, when it comes to rocket launches, increased surface gravity would also mean a higher escape velocity. As Hippke indicated in his study:

“Rockets suffer from the Tsiolkovsky (1903) equation : if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive.”

For comparison, Hippke uses Kepler-20 b, a Super-Earth located 950 light years away that is 1.6 times Earth’s radius and 9.7 times it mass. Whereas escape velocity from Earth is roughly 11 km/s, a rocket attempting to leave a Super-Earth similar to Kepler-20 b would need to achieve an escape velocity of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times as much fuel as a rocket on Earth to get into orbit.

To put it into perspective, Hippke considers specific payloads being launched from Earth. “To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships,” he writes. “For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400,000 t.”

Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org

While Hippke’s analysis concludes that chemical rockets would still allow for escape velocities on Super-Earths up to 10 Earth masses, the amount of propellant needed makes this method impractical. As Hippke pointed out, this could have a serious effect on an alien civilization’s development.

“I am surprised to see how close we as humans are to end up on a planet which is still reasonably lightweight to conduct space flight,” he said. “Other civilizations, if they exist, might not be as lucky. On more massive planets, space flight would be exponentially more expensive. Such civilizations would not have satellite TV, a moon mission, or a Hubble Space Telescope. This should alter their way of development in certain ways we can now analyze in more detail.”

Both of these papers present some clear implications when it comes to the search for extra-terrestrial intelligence (SETI). For starters, it means that civilizations on planets that orbit red dwarf stars or Super-Earths are less likely to be space-faring, which would make detecting them more difficult. It also indicates that when it comes to the kinds of propulsion humanity is familiar with, we may be in the minority.

“This above results imply that chemical propulsion has a limited utility, so it would make sense to search for signals associated with lightsails or nuclear engines, especially near dwarf stars,” said Loeb. “But there are also interesting implications for the future of our own civilization.”

Artist’s concept of a bimodal nuclear rocket making the journey to the Moon, Mars, and other destinations in the Solar System. Credit: NASA

“One consequence of the paper is for space colonization and SETI,” added Hippke. “Civs from Super-Earths are much less likely to explore the stars. Instead, they would be (to some extent) “arrested” on their home planet, and e.g. make more use of lasers or radio telescopes for interstellar communication instead of sending probes or spaceships.”

However, both Loeb and Hippke also note that extra-terrestrial civilizations could address these challenges by adopting other methods of propulsion. In the end, chemical propulsion may be something that few technologically-advanced species would adopt because it is simply not practical for them. As Loeb explained:

“An advanced extraterrestrial civilization could use other propulsion methods, such as nuclear engines or lightsails which are not constrained by the same limitations as chemical propulsion and can reach speeds as high as a tenth of the speed of light. Our civilization is currently developing these alternative propulsion technologies but these efforts are still at their infancy.”

One such example is Breakthrough Starshot, which is currently being developed by the Breakthrough Prize Foundation (of which Loeb is the chair of the Advisory Committee). This initiative aims to use a laser-driven lightsail to accelerate a nanocraft up to speeds of 20% the speed of light, which will allow it to travel to Proxima Centauri in just 20 years time.

Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

Hippke similarly considers nuclear rockets as a viable possibility, since increased surface gravity would also mean that space elevators would be impractical. Loeb also indicated that the limitations imposed by planets around low mass stars could have repercussions for when humans try to colonize the known Universe:

“When the sun will heat up enough to boil all water off the face of the Earth, we could relocate to a new home by then. Some of the most desirable destinations would be systems of multiple planets around low mass stars, such as the nearby dwarf star TRAPPIST-1 which weighs 9% of a solar mass and hosts seven Earth-size planets. Once we get to the habitable zone of TRAPPIST-1, however, there would be no rush to escape. Such stars burn hydrogen so slowly that they could keep us warm for ten trillion years, about a thousand times longer than the lifetime of the sun.”

But in the meantime, we can rest easy in the knowledge that we live on a habitable planet around a yellow dwarf star, which affords us not only life, but the ability to get out into space and explore. As always, when it comes to searching for signs of extra-terrestrial life in our Universe, we humans are forced to take the “low hanging fruit approach”.

Basically, the only planet we know of that supports life is Earth, and the only means of space exploration we know how to look for are the ones we ourselves have tried and tested. As a result, we are somewhat limited when it comes to looking for biosignatures (i.e. planets with liquid water, oxygen and nitrogen atmospheres, etc.) or technosignatures (i.e. radio transmissions, chemical rockets, etc.).

As our understanding of what conditions life can emerge under increases, and our own technology advances, we’ll have more to be on the lookout for. And hopefully, despite the additional challenges it may be facing, extra-terrestrial life will be looking for us!

Professor Loeb’s essay was also recently published in Scientific American.

Further Reading: arXiv, arXiv (2), Scientific American

Why Do Rockets Need Stages? The Quest to Build a Single Stage to Orbit (SSTO)

Single Stage To Orbit!


Now, don’t get me wrong, Science Fiction is awesome. Like almost everyone working in the field of space and astronomy, I was deeply influenced by science fiction. For me, it was Star Trek and Star Wars. I had a toy phaser that made this awesome really loud phaser sound, and I played with it non-stop until it disappeared one day. And I was sure I’d left it in the middle of my floor, like I did with all my toys, but I found it a few years later, hidden up in a closet that I couldn’t reach. And I always wondered how it got there.

Anyway, back to science fiction. For all of its inspiration, science fiction has put a few ideas into our brains which aren’t entirely helpful. You know, warp drives, artificial gravity, teleportation, and rockets that take off, fly to space, visit other planets orbiting stars, land again.

The Millennium Falcon, Firefly, and Enterprise Shuttles are all examples of single stage to orbit to orbit spacecraft, or SSTOs.

Consider the rockets that exist in reality, you know, the Atlases, Falcons and Deltas. They take off from a launch pad, fly for a bit until the fuel is used up in a stage of the rocket, then they jettison that stage and thrust with the next stage. The mighty Saturn V was so powerful that it had three stages, as it made it’s way to orbit.

Diagram of Saturn V Launch Vehicle. Credit: NASA/MSFC

As we discussed in a previous article, SpaceX is working to make the first stage, and maybe even the second stage reusable, which is a vast improvement over just letting everything burn up, but there are no rockets that actually fly to orbit and back in a single stage. In fact, using the technology we have today, it’s probably not a good idea.

Has anyone ever worked on a single stage to orbit? What technological advances will need to happen to make this work?

As I said earlier, a single stage to orbit rocket would be something like the Millennium Falcon. It carries fuel, and then uses that fuel to fly into orbit, and from world to world. Once it runs out of fuel, it gets filled up again, and then it’s off again, making the Kessel Run and avoiding Imperial Blockades.

This concept of a rocket matches our personal experience with every other vehicle we’ve ever been in. You drive your car around and refuel it, same with boats, airplanes and every other form of Earth-based transportation.

But flying into space requires the expenditure of energy that defies comprehension. Let me give you an example. A Falcon 9 rocket can lift about 22,800 kilograms into low-Earth orbit. That’s about the same as a fully loaded cement truck – which is a lot.

SpaceX Falcon 9 poised for Jan. 14, 2017, Return to Flight launch from Vandenberg Air Force Base in California carrying ten Iridium NEXT comsats to orbit. Credit: SpaceX

The entire fueled Falcon 9 weighs just over 540,000 kg, of which more than 510,000 kgs of it are fuel, with a little extra mass for the engines, fuel tanks, etc. Imagine if you drove a car that was essentially 95% fuel.

The problem is specific impulse; the maximum amount of thrust that a specific kind of engine and fuel type can achieve. I’m not going to go into all the details, but the most efficient chemical rockets we have, fueled by liquid hydrogen and oxygen, can just barely deliver enough thrust to get you to orbit. They have a maximum specific impulse of about 450 seconds.

Because the amount of fuel it takes to launch a rocket is so high, modern rockets use a staging system. Once a stage has emptied out all its fuel, it detaches and returns to Earth so that the second stage can keep going without having to drag along the extra weight of the empty fuel tanks.

After stage separation of the Falcon 9 rocket, flames are barely visible around nozzle as the second stage engine ignites and the first stage falls back to the Earth below. Credit: SpaceX

You might be surprised to know that many modern rockets are actually capable of reaching orbit with a single stage. The problem is that they wouldn’t be able to carry any significant payload.

At the end of the day, considering the chemical rockets we have today, the multi-staged profile is the most efficient and cost-effective strategy for carrying the most payload to space for the lowest cost possible.

Has anyone tried developing SSTOs in the past? Definitely. Probably the most widely publicized was NASA’s X-33/VentureStar program, developed by Lockheed Martin in the 1990s.

The proposed X-33 spacecraft. Credit: NASA

The purpose of the X-33 was to test out a range of new technologies for NASA, including composite fuel tanks, autonomous flight, and a new lifting body design.

In order to make this work, they developed a new kind of rocket engine called the “aerospike”. Unlike a regular rocket engine which provide a fixed amount of thrust, an aerospike could be throttled back like a jet engine, using less fuel at lower altitudes, where the atmosphere is thickest.

The test of twin Linear Aerospike XRS-2200 engines, originally built for the X-33 program, was performed on August 6, 2001 at NASA’s Sternis Space Center, Mississippi. The engines were fired for the planned 90 seconds and reached a planned maximum power of 85 percent. Credit: NASA’s Marshall Space Flight Center

Lockheed Martin was working on a 1/3rd scale prototype, but they struggled with many of the new technologies. In the end, their failure to be able to build a composite fuel tank that could contain the liquid oxygen and hydrogen forced them to abandon the project.

Even if they could get the technology working, so the X-33 was fully reusable, its ability to carry a payload would have been dramatically lower than a traditional multi-staged rocket.

In order to really achieve the dream of single stage to orbit, we need to step away from chemical rockets and move to a type of engine that can deliver thrust more efficiently.

We know that jets work more efficiently than rockets, because they only need to carry fuel. They pull oxygen in from the atmosphere, to burn the fuel. So one intriguing idea is to make a rocket that acts like a jet engine while in the atmosphere, and then acts like a rocket once it’s out in space.

And that’s the plan with the British Skylon rocket. It would take off from a regular runway, accelerate to about 6,600 km/h reaching an altitude of 26 kilometers. All this time, its SABRE engine would be pulling in oxygen from the atmosphere, combining it with hydrogen fuel.

An artist’s conception of Reaction Engines’ Skylon spacecraft. Credit: Reaction Engines

From this point, it would switch over to an internal liquid oxygen tank to provide oxidizer, and complete the flight to orbit. All the while using the same flexible SABRE engine. Once in orbit, it would release its 15-tonne payload and then return to Earth, landing on a runway like the space shuttle orbiter did. It’s a really creative idea.

Unfortunately, the development of the Skylon has taken a long time, with shrinking budgets limiting the amount of tests they’ve been able to do. If everything goes well, the first prototype might fly within a few years, so stay tuned to this story.

Another idea which has had some testing is the idea of a nuclear rocket. Unlike a chemical rocket, which burns fuel, and blasts it out the back for thrust, a nuclear rocket would carry a reactor on board. It would heat up some kind of working fuel, like liquid hydrogen, and then blast it out the back for propulsion.

The key elements of a NERVA solid-core nuclear-thermal engine. Credit: NASA

NASA did some tests a few decades ago with a nuclear thermal rocket called NERVA, and found that they could sustain high levels of thrust for very long periods of time. Their final prototype, provided continuous thrust for over 2 hours, including 28 minutes at full power.

NASA calculated that a nuclear-powered rocket would be roughly twice as efficient as a traditional chemical rocket. It would have a specific impulse of more than 950 seconds. But flying a nuclear rocket into space comes with a significant downside. Rockets explode. It’s bad when a chemical rocket explodes, but if a nuclear reactor detonated while making its way up through the atmosphere, it would rain down radioactive debris. For now, that’s considered too much of a risk; however, future interplanetary missions may very well use nuclear rockets.

There’s one more exotic fuel system that’s really exciting – metallic hydrogen. This solid form appears naturally at the heart of Jupiter, under the incredible pressure of the planet’s gravity. But earlier this year, researchers at Harvard finally created some in the lab. They used a tiny vice to squeeze hydrogen atoms with more force than the pressures at the center of the Earth.

Microscopic images of the stages in the creation of atomic molecular hydrogen: Transparent molecular hydrogen (left) at about 200 GPa, which is converted into black molecular hydrogen, and finally reflective atomic metallic hydrogen at 495 GPa. Credit: Isaac Silvera

It took an enormous amount of energy to squeeze hydrogen together that tightly, but in theory, once crafted, it should be relatively stable. And here’s the best part. When you ignite it, you get that energy back.

If used as a rocket fuel, it would provide a specific impulse of 1700 seconds. Compare that to the mere 450 from chemical rockets. A rocket powered by metallic hydrogen would easily get to orbit with a single stage, and travel efficiently to other planets.

Single Stage to Orbit rockets would be awesome. Science fiction has foretold it. That said, at the end of the day, whatever gets the most amount of payload into orbit for the lowest price is the most interesting rocket system. And right now, that’s staged rockets.

However, a bigger issue might be reliability and reusability. If you can get a single vehicle that takes off, travels to orbit and then returns to its launch pad, you can’t get anything simpler than that. No rockets to restack, no barges to navigate. You just use and reuse the same system again and again, and that’s a really exciting idea.

Right this moment, reusable staged rockets like SpaceX has the edge, but if and when the Skylon gets flying, I think we’ll have some serious competition.

Once we master metallic hydrogen, spaceflight will look very very different. Science reality will nearly match science fiction, and I’ll finally be able to fly my own personal Millennium Falcon.