Monthly Archives: February, 2016

PopSci: Lasers Could Send A Wafer-Thin Spaceship To A Star

Lasers and photon drives have been a staple of science-fiction for 150 years.

Now finally, laser-driven star probes are in the main-stream of science.

Lasers are now advanced enough to help launch interstellar space probes, researchers say.

Scientists calculate that a gram-sized laser-propelled space probe could reach more than 25 percent of the speed of light and arrive at the nearest star in about 20 years.

The Voyager 1 spacecraft launched in 1977 is finally leaving the solar system after 37 years of flight at a speed of roughly 38,000 miles per hour or less than 0.006 percent the speed of light. This suggests that with conventional propulsion technology, humanity will never reach even the nearest stars, says experimental cosmologist Philip Lubin at the University of California, Santa Barbara.

Lubin and his colleagues suggest that, instead, lasers could accelerate small probes to relativistic — that is, near-light — speeds, reaching nearby stars in a human lifetime. “No other current technology offers a realistic path forward to relativistic flight at the moment,” Lubin says.

The problem with all thrusters that current spacecraft use for propulsion is that the propellant they carry with them and use for thrust has mass. Interstellar spacecraft require a lot of propellant, which makes them heavy, which requires more propellant, making them heavier, and so on.

Photon drives instead involve equipping spacecraft with mirrors and depending on distant light sources for propulsion. Solar sails rely on light from the sun, while laser sails count on powerful lasers.

Lubin acknowledges that photon drives are nothing new — in a letter to Galileo Galilei in 1610, Johannes Kepler wrote, “Given ships or sails adapted to the breezes of heaven, there will be those who will not shrink from even that vast expanse.” What is new, Lubin says, is that recent, poorly appreciated breakthroughs in laser technology suggest they can now accelerate spacecraft to relativistic speeds.

Breakthroughs in laser technology suggest they can now accelerate spacecraft to relativistic speeds.

The advance that Lubin’s approach depends on involves laser arrays. Instead of building one extremely powerful laser — a technologically challenging feat — researchers now can build phased arrays that are made of a large number of relatively modest laser amplifiers that can sync up to act like a single powerful laser. This strategy also eliminates the need for a single giant lens, replacing it with a phased array of smaller optics.

The researchers envision a phased array of currently existing kilowatt-scale ytterbium laser amplifiers that can scale up gradually, adding lasers over time. For instance, a current 1- to 3-kilowatt ytterbium laser amplifier is about the size of a textbook and weighs roughly 5 kilograms.

Eventually, the scientists calculate that a 50- to 70-gigawatt array that is 10 kilometers by 10 kilometers large in Earth orbit could propel a gram-sized wafer-like spacecraft with a 1-meter-wide sail to more than 25 percent of the speed of light after about 10 minutes of illumination, which could reach Mars in 30 minutes and Alpha Centauri in about 20 years. The researchers suggest this array could launch roughly 40,000 relativistic wafer-sized probes per year — each “wafersat” would be a complete miniature spacecraft, carrying cameras, communications, power and other systems.

The same array could propel a 100-ton spacecraft — about the mass of a fully loaded space shuttle, sans rockets — with a 8.5-kilometer-wide sail to about 0.2 percent of the speed of light after about 15 years of illumination. However, it would take about 2,200 years to reach Alpha Centauri at those speeds. Lubin suggests a larger array would make more sense for a human interstellar trip in the distant future, “but I personally do not see this as a priority until many robotic probes have established a need to do so.”

A major problem with this strategy is braking — the researchers currently have no way to slow down these laser-driven spacecraft enough for them to enter into orbit around the distant planets that they are dispatched to. The first missions that accelerate to relativistic speeds may have to simply fly by targets and beam back their data via lasers, Lubin notes.

Lubin notes there are many additional uses for such a laser array other than space exploration. For example, it could deflect asteroids away from Earth, or blast debris out of orbit to prevent it from threatening spacecraft, astronauts and satellites.

They are currently testing to show that small lasers can stop asteroids from spinning.

The researchers stress that they are not proposing to immediately build the largest system. They are currently testing small lasers on asteroid-like rock samples to show that such systems can stop asteroids from spinning, work that could help one day wrangle asteroids for exploration.

If lasers are the only practical route for interstellar travel, Lubin and his colleagues suggest that alien civilization may currently use lasers to help explore the cosmos. They suggest that SETI projects should look for telltale signs of such technology.

Lubin presented his latest work in a talk on January 25 at Harvard.

Lubin however fails to mention how the Military-Industrial-Complex invested billions and billions of dollars to make lasers into a combat grade weapon, which lasers of this type obviously are.

Which begs the question of “Will the government allow space probes of this type to be used?”

And who or whom would be allowed to construct them?

Original article

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Crowlspace: Journey to Planet 9

For years “planet 9” referred to Pluto.

Unfortunately, Pluto has been downgraded to dwarf-planet status, (in-spite of the spectacular fly-by of New Horizons).

Now there is much speculation that Planet 9 is a cold gas giant, perhaps even a small brown dwarf.

In this article by Adam Crowl, he dishes on potential rocket systems that could get probes like New Horizons there in decades, not centuries:

Power, Distance and Time are inextricably linked in rocketry. When leaving the Earth’s surface this is not so obvious, since all the sound and fury happens for a few minutes, and silence descends once the rocket enters orbit, free-falling indefinitely, at least until drag brings it back down. For slow journeys to the Moon, Near Earth Asteroids, Mars, Venus etc. the coasting Hohmann Transfer orbits and similar low-energy orbits, are all typically “sudden impulse” trajectories, where the engines fire for a few minutes to put a spacecraft on a months long trajectory.

For trips further afield – or faster journeys to the nearer planets – the acceleration time expands to a significant fraction of the total journey time. Ion-drives and solar-sails accelerate slowly for months on end, allowing missions like “Dawn” which has successfully orbited two Main Belt objects, Ceres and Vesta, all on one tank of propellant. Given more power an electrical propulsion system can propel vehicles to Mars in 2-3 months, Jupiter in a year and Saturn in under 2. Exactly how good the performance has to be is the subject of this post.

Firstly, an important concept is the Power-to-Mass ratio or specific power – units being kilowatts per kilogram (kW/kg). Any power source produces raw energy, which is then transformed into the work performed by the rocket jet. Between the two are several efficiency factors – the efficiency of converting raw heat into electricity, then electricity into jet-power, which includes the ionization efficiency, the nozzle efficiency, the magnetic field efficiency and so on. A solar array converts raw sunlight into electricity with an efficiency of between 20-25%, but advanced cells exist which might push this towards 40-50%.

Let’s assume a perfect power source and a perfect rocket engine. What’s the minimum performance required for a given mission? The basic minimum is:

Power/Mass is proportional to (S^2/T^3)

That is the Power-to-Mass ratio required is proportional to the displacement (distance) squared, and inversely proportional to the mission time cubed. For example, a 1 year mission to Jupiter requires 1,000 times the specific power of a 10 year mission.

The minimum acceleration case is when acceleration/deceleration is sustained over the whole mission time. When acceleration is constant, it means a maximum cruise speed (i.e. actual speed of vehicle) of 2 times the average speed (defined as total displacement divided by total mission time).

Another result, from a mathematical analysis I won’t go into here, is that the minimum specific power mission requires a cruise speed that is 1.5 times the average speed and an acceleration+deceleration time, t, that is 2/3 the total mission time T.

Remember that kinetic energy is 1/2.M.V^2, thus specific kinetic energy per unit mass is 1/2.V^2.

The power required – which is work done per unit time – is a trade off between acceleration time and mission time. Say the mission time is 10 years. If all the acceleration is done in 1 year, then the cruise speed required is 1/0.95 times the average speed, but power is proportional to the speed squared divided by the acceleration time: P = (1/2).V^2/t = (1/2).(1/0.95)^2/1 ~ 0.55, whereas in the case of constant acceleration, the average specific power is (1/2).(2)^2/10 = 0.2. For the case of minimum power it’s (1/2)*(3/2)^2/(2/3*10) = 0.16875 – just 84.375% the constant acceleration case and ~31% the 1 year thrust time.

So what does it take to get to Planet 9? If we use the distance of 700 AU to Planet 9, and a total trip time of 10 years, that means an average speed of 70 AU per year. To convert AU/yr to km/s, just multiply by 4.74 km/s, thus 331.8 km/s is needed. Cruise speed is then 497.7 km/s and the specific jet-power is 1.177 kW/kg, if we’re slowing down to go into orbit. Presently there are only conceptual designs for power sources that can achieve that sort of specific power. If we take 20 years to get there, the specific power is 0.147 kW/kg, which is a bit closer to possible.

Vapor Core Reactor Schematic

Space reactor designs typically boast a specific electrical power output of 50 W/kg to 100 W/kg. Gas-core nuclear reactors could go higher, putting out 2,000 – 500 W/kg, but our applied knowledge of gas-core reactors is limited. Designs exist, but no working prototypes have ever flown. In theory it would use uranium tetrafluoride (UF4) gas as the reacting core, which would run at ~4000 K or so and convert heat to electricity via a magnetohydrodynamic (MHD) generator. Huge radiators would be required and the overall efficiency of the power source would be ~22%. In fact there’s a theorem that any thermal power source in space has its highest specific power when the Carnot efficiency is just 25%, thanks to the need to minimise radiator area by maximising radiator temperature.

More exotic options would be the Fusion-Driven Rocket or a space-going stellarator or some such fusion reactor design with a high specific power. In that case it’d be operated more as a pure rocket than powering an electrical rocket. Of course there’s the old Orion option – the External Nuclear Pulse Rocket – but no one wants to put *potential* nuclear warheads into orbit, just yet.

– See more at: http://crowlspace.com/#sthash.hYmlMqQ6.dpuf

Posts coming…etc…etc…

The bad news is I have not had much time to post. The good news is I have had a lot of time to watch stuff so expect several posts coming starting tomorrow.

If I’m not watching my grandkids that is.

Source: Posts coming