Tag: deep space travel

Jupiter’s Moon, Europa

Jupiter’s moon, Europa, is one of the top places in our solar system where life might exist. Europa is one of the rare places in our solar system that holds all three requirements for life.

Why Explore Europa: Key Takeaways

  • A mission to Europa has a high return on investment of scientific data gathered
  • Europa is the sixth largest moon in the solar system, and one of Jupiter’s 79 known moons
  • The icy crust is between 20 and 180 million years old, relatively young for the planet’s age
  • Beneath the crust of Europa, a global ocean exists 62 miles deep – more than twice the volume of Earth’s ocean
  • The atmosphere is thin – so there is a high radiation exposure on the surface
  • The temperature is −160 °C / −260 °F at the equator

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Possible Life

The presence of water is exciting because it means the planet could possibly harbor life. Also, water is also an extremely important resource for humans in space.

hydrothermal vents
Hydrothermal vents on Earth.
source: NOAA

The excitement of a subsurface ocean of liquid water and possible presence of life brings up the question – why do we think there could be life on Europa?

As far as we know, water is one of the requirements for life – as well as a source of energy and specific chemical presence (including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur). Together, water, energy, and chemistry form the basis for life’s requirements.

Celestial bodies in the solar system often meet two out of three of these requirements. Almost nowhere has all three. Again, Europa is one of the rare places in our solar system that meets all three requirements for life.

If life were to exist within the depths of Europa’s global ocean, it may take a number of forms. To form hypotheses for what life may might look like, we can leverage what we know about the origin of life on our home planet Earth.

If anything lives in Europa’s ocean, single-celled organisms, microorganisms, and bacteria are most likely forms to be found. Given more time to evolve, there could be more complex life forms. On Earth, hydrothermal vents are home to a diverse array of life forms, which could be the same on Europa. If some hydrothermal vents exist at the bottom of Europa’s ocean, this may be a breeding ground for some life form, just like on Earth.

Life could also thrive by clinging onto the surface crust in some way, benefiting by periodic exposure to surface compounds or other non-polar molecules that accumulate near the surface.

Geological Features and Gravitational Impact

Europa’s thick ice sheet crust is beneficial for a number of reasons. For one, it protects the ocean (and any life that may inhabit it) from radiation. In addition, it allows heat from the core of the planet to stay within the, which may help the ocean maintain its liquid property by providing a layer of insulation.

Europa
source: NASA JPL
  • The force of gravity from Jupiter and its other moons affect Europa by causing tidal flexing, creating a constantly changing surface crust.
  • The constant shifting of the large glacier crust displays:
    • geysers
    • cracks
    • craters
    • volcanic activity
  • Non-synchronous rotation results in a macro movement in the liquid ocean beneath the surface.

More below on the impact that gravity has on the geological features of Europa.

Non-synchronous rotation: proof of a global ocean

Non-synchronous rotation means that the crust of the planet does not rotate at the same rate as the core. The Galileo probe found evidence of this on Europa due to the mass distribution. This rotational behavior means that there is a decoupling between the interior and exterior of the planet (between the core and the crust).

A fluidic insulation between the core and the crust means that the surface of Europa moves and changes much more than other planets.

According to Nature, because Europa spins faster than it orbits Jupiter, gravity data suggest that there may be an asymmetry in Europa’s interior mass distribution. This means that the surface of the planet is moving differently from the core of the planet. The decoupling between the rotation of the icy surface crust and the rocky core suggests that they may be separated by a layer of liquid – Europa’s global ocean. The Europan ocean is the most plausible hypothesis.

Tidal Flexing: Europa’s Forceful Energy Source

On Earth, the cause of tides is the gravitational interactions between Earth and our moon. This movement and shifting of the liquid interior mass creates tides just like in the oceans of Earth. Large movements of water within the interior of Europa cause the solid crust to crack like an eggshell, giving Europa a crust that constantly changes.

Why exactly does this happen?

Europa orbits Jupiter in an elliptical pattern, meaning that it moves nearer and further from Jupiter throughout one full orbit. When Europa is closer to Jupiter, the force of gravity between the two celestial bodies is stronger. The larger gravitational attraction causes fluidic turbulence in Europa’s liquid interior. This causes Europa to elongate like a bouncing rubber ball making impact with the floor. As Europa moves further from Jupiter, the force of gravity is lower, and the oval shape relaxes back into more of a spherical shape.

tidal flexing
tidal flexing
source: astronomynotes.com

This process, called tidal flexing, is similar to how a water balloon behaves haphazardly as it is tossed through the air. Fluid moves – and water does not simply maintain a completely spherical shape.

The constant tidal flexing motion of Europa’s interior causes macro-level friction and pressure, providing a source of heat, allowing its ocean to stay liquid while affecting geological features on the surface.

Europa’s Dynamic Crust

To determine the age of a celestial body, humans observe asteroid and meteor impacts on the surface. The record of craters creates a historical map, allowing us to date the age of planets.

Because Europa’s crust is made of massive global shell of ice, these ice sheets behave similarly to glaciers in that they are moving a little bit each year. The tectonic liveliness of these ice sheets effectively erases meteor craters and other surface impacts within about a hundred million years. This means that no Europan surface features last much longer. On a geological scale of billions of years (Earth for example 4.6 billion years old), a hundred million years is a relatively short amount of time.

On a shorter time-scale, the behavior of these large glacier like ice sheets surrounding the planet is not dissimilar from the plate tectonics on Earth. The ice sheets are constantly moving and shifting, and volcanic activity occurs from within cracks and pores in the surface just like we have volcanoes on Earth.

Because of its constant shift and changing ice crust, Europa is the smoothest surface of any other object in our solar system. There are no real massive mountains, nor are there big canyons like on Earth.

Again, tidal shifting of the ocean, drives this, and the smoothness is more proof that a water ocean exists beneath the crust.

Europa’s Chemistry

Aside from the water ice crust and liquid ocean, Europa is composed mostly of silicate rock. This is most similar in core structure to the rocky planets like Mercury, Venus, Earth, and Mars. We believe that it has an iron-nickel core, which is radioactive and produces some amount of internal heat.

There is also evidence for hydrogen peroxide on the surface. This is a significant finding because hydrogen peroxide can react with water to produce an oxygen byproduct. Oxygen, of course, is yet another requirement for life as we know it, and this process could be an explanation for the presence of oxygen in the atmosphere.

Although Europa has an oxygen-based atmosphere, it is very thin and blocks almost no radiation. Based on the data we have, we still know a relatively small amount about the surface of Europa. Future missions to the moon would help us learn about the chemical composition of the surface and interior.

Craters

There are not many craters on Europa because the surface changes too quickly, removing most evidence of surface impacts. Quick, tectonically dynamic changes mean that any surface features relatively young, around 100 millions years old (as opposed to billions on planets with less dynamic surfaces).

One of the few craters that exists is the Pwyll crater, and is thought to be one of the moon’s youngest features, remaining the surface from a surface impact 26 km or 16 miles wide. Below is a picture of the Pwyll crater, taken by the Galileo orbiter.

europa pwyll crater
source: NASA JPL

Cracks

As tidal shifting is constantly a force of change, the slow flowing, shifting, and disruption of the solid icy crust cracks produce incredible streaking lines along the surface.

The image below is a 250 by 200 kilometer close-up photo (also taken by Galileo probe) that shows in detail some of the cracks on the surface of Europa. This image is taken about 1000km to the north of the Pwyll crater.

The reddish areas are associated with more recent internal geological activity.

europa linea
These lines are called linea
source: NASA JPL

The way the cracks are aligned in different directions has lead researchers to hypothesize that Europa’s axis of rotation has not been constant over time. At some point in the past, Europa may have spun around a tilted axis.

Geysers

Another unique occurrences that comes with having a global ocean beneath the surface of the moon is volcanic activity – in this case, geysers or plumes.

Hubble space telescope detected large geysers of water vapor from Europa, similar to the ones we know to exist on Saturn’s moon, Enceladus.

The volcano-like plumes of Europa are reach than twenty times as high as mount Everest. Since these periodic events expel a large amount of vapor and compounds high into the atmosphere, this provides an opportunity for future missions to capture samples and more readily analyze the constituents in the search for life.

The advantage of this sampling technique is that we don’t have to land a spacecraft on the surface to get samples. This is much less energy intensive than landing, drill through ice and rock to collect material, and then launching from the surface again. Because of the relative ease with which this sampling process can be done, a mission to Europa has a higher return on investment for the gathering of scientific data compared to other destinations in the solar system.

Missions to Europa

Humans have observed the Europa moon during flybys of space probes since the 70s.

Past Missions

  • The first space probe flybys were Pioneer 10 and 11, which captured low resolution images of the icy surface in the 1970s.
  • Voyager 1 and 2 have visited destinations never before seen, in addition to Europa. The data sent back show images of the ice cracks and lines on the surface. Launched in 1977, these probes are still actively transmitting data back to Earth after 40 years.
  • Galileo orbiter probe orbited Jupiter for 8 years and was able to observe Europa’s surface. Galileo provided significant information about Europa’s icy surface, as well as data supporting evidence of the global ocean. It discovered important evidence for a sub-surface ocean. Overall, the total dollars invested in the probe was 1.39 billion.
  • NASA’s JUNO spacecraft captured data about Europa and the other moons of Jupiter in orbit.
  • Cassini-Huygens spacecraft flew by Europa on its way to Saturn and Saturn’ moon, Titan.
  • New Horizons mission flew by Europa on the way towards Pluto.

Planned Missions

  • ESA’s Juiper Icy Moon Explorer, which will also study Ganymede.
  • The Europa Clipper will be launched by NASA in 2025.
  • So far, we have never landed a spacecraft on Europa, but perhaps we will do so in the not too distant future.

This is part of a series where we discuss various Moons and Planets in our solar system, and why we might want to explore them. See more on Saturn’s moons: Titan and Enceladus.

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sources:

https://www.nature.com/articles/34869
https://www.nature.com/articles/s41550-018-0450-z
life possibility on Europa: https://ui.adsabs.harvard.edu/abs/2001EOSTr..82..150S/abstract
chemistry for life on Europa: https://www.nasa.gov/topics/solarsystem/features/europa20130404.html
https://books.google.com/books?id=8GcGRXlmxWsC&pg=PA427#v=onepage&q&f=false
http://www.astronomynotes.com/solarsys/s14.htm
https://europa.nasa.gov/europa/life-ingredients/

Deep Space Travel: X3 Ion Thruster 2021 update

Ion propulsion is one of the top technologies that will enable deep space exploration.

The X3 ion thruster is currently the most advanced of its kind and capable of producing greater power and thrust. The X3 will further advance human space travel technology and our ability to embark on missions into the far depths of outer space.

The X3 Nested Channel Hall thruster is being developed in collaboration between NASA, the University of Michigan PEPL, Aerojet Rocketdyne, and the Air Force Research Laboratory.

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X3 Ion Thruster Updates as of 2021

Additional developers include NASA GRC, NASA JPL and the Air Force Office of Scientific Research.

Ion propulsion uses electrostatic fields to ionize and accelerate a propellant.

More on ion thrusters here.

Specs for the X3 ion thruster:

the x3 ion thruster
source: Journal of Propulsion and Power, 2020
  1. Type: Hall-effect ion thruster
  2. Size: 80 cm diameter
  3. Weight: 230 kg (500 pounds)
  4. Specific Impulse: 1800–2650 seconds
  5. Force/Thrust: 5.4 Newtons
  6. Power: 100mw
  7. Discharge Current: 247 A
  8. Discharge Voltage: 500 V at peak efficiency
  9. Propellant: Krypton or Xenon compatible
  10. Lifetime: over 50,000 hours
  11. Speed: 40 km/s = 89,000 mph

What’s so special about the X3 ion thruster?

There are two key technological factors that make the X3 Ion thruster better, faster, and more efficient:

1. Hall Effect Thruster Technology

First of all, there are multiple types of ion thruster designs.

The best is the Hall effect ion thruster. The X3 Ion Thruster is designed based on the Hall effect.

Hall thrusters have been identified as the best approach to building better ion drives because of their longer lasting characteristic, as opposed to other plasma based ion thrusters.

Hall-effect ion thruster – What is it?

The Hall Effect describes how an electromagnetic field occurs perpendicular to the flow of current.

By using electricity to create a current in a circular shape, depending on whether current flows clockwise or counter-clockwise, the vector of the magnetic field will point either up or down.

The electromagnetic field gives ionized, or charged, particles kinetic energy, resulting in a force and causing the particles to accelerate in the given direction.

Based on Newton’s third law, the force of particles leaving the engine ultimately causes the spacecraft to move forward.

Why Hall effect ion thrusters last longer than plasma ion thrusters:

  • Hall thrusters feature an innovative magnetic field configuration which prevents interaction and disturbances between ionized propellant and the engine components.
  • In the case of plasma based ion thrusters, the ionized particles tend to quickly erode engine components after a year.
  • The magnetic configurations in Hall thrusters produce a shielding mechanism so this does not happen.

2. Nested Ion Propulsion Channels

In addition to using the Hall effect, the second innovative differentiator in the X3 design is nested channels. The X3 has multiple rings, or discharge channels.

The X3 ion thruster
source: Michigan PEPL

The nesting approach places multiple propulsion channels in a concentric-circle arrangement around a center-mounted cathode. Electric current flows around three circular pathways of different sizes, each producing the electromagnetic field perpendicular to the flow of current. By featuring additional channels, the magnetic field is stronger and thus produces more force to move a spacecraft.

From the 2017 tests at the NASA Glenn Research Center, the X3 demonstrated the ability to produce 5.4 newtons of thrust, which is almost 40% more than the previous best ion thruster, which was capable of producing 3.3 newtons.

Nested Hall Thrusters (NHTs) have a larger throttling range than traditional single-channel thrusters. By only engaging a single channel, a minimum amount of force can be produced. Alternatively, by engaging all three, more powerful configurations are possible.

As of 2018, the project is at a Technology Readiness Level 5, (TRL 5) meaning “component and/or breadboard validation in relevant environment”. This is a significant step on NASA’s 9 TRL levels, with number 9 being that the system is flight proven through successful mission operations.

From the Latest Journal Articles:

the x3 ion thruster during test
source: Michigan PEPL

One of the main things that the July 2020 ion thruster paper found was that the X3 is likely able to operate more efficiently than expected.

In technical terms, the paper discovered that cathode flow as a fraction of anode flow can be as low as 4% in the X3 without having significant impact.

According to the paper, “due to the reduced flow rates, the total efficiency is slightly increased (although all values are within the measurement uncertainty)”.

Also, “These results suggest that low-TCFF operation is feasible for high-power Hall thrusters and can offer increased system efficiency as well as improved cathode lifetime, and can do so with little impact on thruster operation.”

Since the X3 Nested Hall Effect Thruster is more efficient, this means that it can be more conservative with fuel propellant, essentially getting more “miles per gallon”, to put it in terms used with automobiles. Saving propellant means that missions can go longer, further, and faster.

The article did not underplay the importance of unanswered questions that have yet to be resolved.

Why is the X3 ion thruster a big deal?

The short answer: the X3 is more powerful while at the same time, more efficient.

Nested-channel Hall thrusters have been identified as a means to increase Hall thruster power levels above 100 kW.

Given the X3’s capacity to produce a greater amount of force, the engine itself is also larger.

This will enable deep space travel:

  • According to NASA’s technology roadmap, “This higher-power category [of ion drives] will be pertinent to human space exploration missions beyond LEO, and for rapid-transit science missions to the outer solar system and deep space destinations.”
  • According to the research paper by Scott James Hall, if an ion propulsion system could produce over 300 kW of power, it would enable possible space missions to near-Earth asteroids as well as Mars.

So far, however, it has been challenging to reach this level of power. But the X3 has pushed the limits on what’s possible – although not yet in the 300 kW range, the technology is slowly progressing to higher levels, which may one day be attainable.

X3 Ion Propulsion Reducing Launch Mass

When you look at a traditional chemical rocket, the majority of the mass that is used to send it into orbit is fuel.

The large amounts of chemical fuel required for space missions is less efficient than the amount that would be needed by utilizing electric propulsion.

“high-power electric propulsion was key to allowing affordable travel to asteroids and near-Earth destinations by reducing launch mass”

NASA / Michigan PEPL

In the quotation above, “reducing launch mass” is referring to the absence of more heavy rocket fuel propellant as part of the payload. Ion thrusters carry a comparatively tiny amount of inert gas as propellant that allows the launch mass payload to be reduced.

“Large-scale cargo transportation to support human missions to the Moon and Mars will require next-generation, high-power Solar Electric Propulsion (SEP) systems capable of operating between 200 and 400 kW.” – American Institute of Aeronautics and Astronautics, Inc., 2018.

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x3 ion thruster test
source: Journal of Propulsion and Power, 2020

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Deep Space Travel with Ion Thrusters – an Overview

Electrically powered ion thrusters are one of the most common propulsion systems for the vacuum of space.

Ion Propulsion Key Takeaways:

ion propulsion system as Xenon exits the engine
source: NASA
  • Ion thrusters allow spacecraft to travel further, faster, and cheaper than other systems.
  • 11.5 times as efficient as chemical propellant.
  • Though efficient, the system provides very little thrust, a fraction of 1 newton. The tiny force, over time, eventually results in big changes in velocity.
    • Cannot be used to launch a rocket from Earth.
    • Are excellent for maintaining satellite orbit, sending smaller probes on long distance voyages to asteroids or outer planets.
  • Can operate continuously for years. Ion thrusters can be used over very long periods of time. For example, Dawn, the spacecraft that was the first to reach a dwarf planet Ceres, used ion thrusters to reach a speed of 10 km/s.
  • Could be used in the near future to power additional missions to Saturn’s moon, Titan, or other places proximate to our solar system, for example.
  • Could one day be used to send people on a thousand year voyage to other stars.

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Even though the amount of force that an Ion Thruster Engine provides is barely the weight of a piece of paper, these systems allow spacecraft to reach enormous speeds.
Although going from 0-60 takes about 4 days, the compounding acceleration of running these engines for years allows them to cover distances of billions of miles through space.

Ultimately, ion engines are the perfect system for long-distance, deep space travel.

What is an Ion Thruster?

An artist’s concept of Dawn, propelled by ion propulsion, approaching Ceres.
source: NASA

We now know that an ion thruster is a method of powering a spacecraft during flight through outer space.

As ions are ejected from the back of the engine, each ion generates a tiny force which slowly begins accelerating the spacecraft.

In the absence of air, there is no friction or wind resistance to slow down a spacecraft’s trajectory. Thus, the tiny, consistent force generated is successful at increasing the speed of a vehicle in space when maintained for long periods of time – often years.

Ion thruster engines are the most useful known method of movement in space, particularly over long durations.

Why are ion thrusters used?

Ion propulsion is specifically valuable for long distance space travel because less propellant is needed to increase speed.

With traditional rockets, chemical-based fuel like methane or hydrogen is used. In these systems, fuel tends to be such a large fraction of a spaceship’s total mass.

Ion thrusters, on the other hand, are effective in generating the most thrust for a given mass of fuel. A lower fuel mass is analogous to a car getting more miles per gallon.

Traditional chemical rockets release energy stored in molecular bonds of propellant (Methane, CH4 for example) and are limited by their low specific impulse, no matter what type of fuel or design is used.

For missions that require a large acceleration (like deep space travel over millions of miles), chemical propulsion is no good because the low specific impulse requires fuel to take up a larger percentage of the payload.

According to the Tsiolkovsky Rocket Equation, as acceleration increases, the amount of fuel needed increases exponentially.

Specific impulse describes how effectively propellant is converted into thrust.

Key differences between ion and chemical propulsion:

ion thruster
source: NASA
  • Compared to chemical engines, ion powered spaceships are able to reach speeds that are more than 11 times as fast.
  • Greater efficiency; high specific impulse, which means its able to generate larger force for a given mass of propellant.
  • Less powerful
  • Ionizes atoms rather than reacting molecules in an exothermic combustion reaction.
  • Acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.
  • Less propellant is required, which means we can send smaller, cheaper vehicles on missions.

What are the drawbacks of ion thrusters?

There are a few tradeoffs to using an ion thruster engine. Although sustainable for months or even years, ion thrusters produce only a small amount of force, so it takes a long time to reach high speeds.

Additionally, ion thrusters can only be used in the vacuum of space. They don’t work in the presence of air particles, and the tiny force cannot overcome air resistance.

An ion thruster won’t work for launching a rocket from a planet’s surface, but it can be used for steering, orientation and acceleration once you’re in space.

How do Ion Thrusters work?

What is an ion and how is it different from an atom? | Socratic
source: socratic.org

Instead of igniting propellant, ion propulsion works by taking inert, unreactive gas atoms (such as xenon or krypton), and inducing a positive charge by removing an electron.

An inert gas is used as the propellant because it is unreactive and non-corrosive. Xenon and Krypton are quite unreactive because they contain a full 8 electrons in the valence shell. Elements with a full outer shell are called noble gases.

Xenon is a slightly better propellant than Krypton because it has a larger atomic mass, producing more force.

The process of removing electrons requires electricity. Often, solar panels are a good option to power the ionization. However, for deep space missions where less solar power is available, the energy is too small and acceleration is slowed. Because of this, alternative energy sources are required. Nuclear is one possible option. NASA and Livermore labs working on a nuclear system for electricity production.

Once electrons are ionized (electrically charged), the ion can be accelerated. This is done by applying a voltage to create an electrical field, causing them repel one another (similar to the way magnets behave when you hold them next to each other). The large amount of ions repelling each other produce momentum and force to accelerate the spacecraft.

The voltage causes the ions to leave the engine at up to 90,000 miles per hour. Each individual ion provides a small amount of thrust for the spacecraft.

ion thruster
source: NASA JPL

With a large number of these ions being expelled, a constant force is generated that can move the spacecraft forward, to the left in the image above.

Given the amount of force is small, for reference it takes four days to accelerate from 0 to 60 miles per hour. Though small, when sustained over many years, continual acceleration can cause the spacecraft to reach up to 200,000 miles per hour, fast enough for deep space travel.

Who works with ion thrusters today?

SpaceX Starlink satellites use ion thrusters. Each SpaceX Starlink satellite is able to propel and orient itself to ensure it doesn’t run into other satellites or orbital debris. Starlink uses krypton for the inert gas because it is cheaper (though less efficient) and better suited for their large amount of satellites.

NASA Glenn Research center has done tests on a Hall Effect thruster, known as HERMeS, which is three times as powerful as other systems.

ion thruster
source: NASA

The University of Michigan is developing the X3 Ion thruster, which is also a type of Hall Effect thruster capable of generating 5.4 Newtons of thrust, and has set numerous records.

NASA Dawn Mission has used ion thrusters to travel 4.3 billion miles towards Ceres, a planet in the asteroid belt.

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sources