Marc G. Millis, Jeff Greason, Rhonda Stevenson of the Tau Zero Foundation provided a 69-page review to NASA of the Interstellar Flight Challenges and Prospects.

A lot of the focus is on the massive speed, distance, and power challenges.

We Are Crippled Because We Cannot Really Build in Space

The most technically feasible ways to start making much faster progress to making travel around the solar system routine and fast and then to build a foundation for interstellar flight is to build large and light space structures.

It will be easier to build bigger in the low gravity of space. We need robots and new construction systems. All of our Earth-based megastructures will look tiny in comparison to the space-based structures.

Fully reusable rockets are the game changer that SpaceX is creating now. The next steps are robotic construction capabilities and megawatt and gigawatt power. No matter what the large power source is we have to build large in space to radiate the heat from the large power systems.

Going 400 Times Faster to Get to 10% of the Speed of Light

To convey the challenge of reaching 10% lightspeed, consider the improvements between the 1977 Voyager and the 2011 Juno missions. In roughly three decades there was a four-fold increase in speed. At that rate, it would take another 130 years to reach 10% lightspeed. The gap between achieved speeds and the goal of 0.1c is a factor of 400 (Juno achieved 0.00025c). The technical challenge is to increase spacecraft ∆V by at least 400 times more than presently possible with chemical rockets.

Most of the systems for useful interstellar missions would need gigawatts or terawatts of power.

Passenger Jet Travel Volumes at 1% of the Speed of Light Around the Solar System

The edge of our solar system can be defined as beyond around 200 AU. The next destination of interest past that point is 550 AU, where the gravitational lensing effect of our sun can be used to magnify images of whatever is on the opposite side of the sun. It has been proposed that this solar gravitational lens has the magnification to be able to image an exoplanet with enough resolution to distinguish land features.

This figure shows the correlation between long timescales, interstellar distances, and average flight speed. Both the distance and timescales are logarithmic. The horizontal scale spans the radius of the Milky Way galaxy (50,000 ly), while the time scale extends all the way to the certain end of Earth’s habitability (~1 billion years). The assumed upper limit for the operational duration of a space probe (200 years) is shown. The diagonal lines represent different speeds, starting on the left with Voyager’s 0.00006 c. The faster Juno spacecraft (0.00025 c) is also shown. The other diagonal lines are in terms of fractional lightspeed, shown in increasing factors of 10 all the way up to lightspeed. For each factor of 10 increase in speed, the required energy goes up by at least a factor of 100.

Going to Another Star is 500 Times Further than Gravitational Lensing Points

Beyond that point, the next targetable object is almost 500 times farther away, specifically the Centauri star systems (270,000 AU, 4.2 ly). In the vast void between those points of interest there exist only sparse densities of comets and asteroids; the Hills cloud (2,000 AU), Oort cloud (10,000 AU), and the G-cloud (41,000 AU). These features are difficult to discern using Earth-based astronomy, but probes passing through them could make direct in-situ measurements of the fields and particles.
Once past the Centauri systems, there are already eight potentially habitable planets detected within 41 ly, half of which are within 22 ly.

This chart compares the amount of energy likely to be made available for interstellar missions
(one-millionth of total world energy) to the energy required for interstellar missions. The central diagonal line is the nominal energy growth from extrapolating data spanning 1980-2007. The upper and lower diagonal lines are ± one standard deviation of that data. The horizontal lines represent the energy requirements for the following missions:
1. Ten StarShot Probes (0.01 kg total) at 20% c – kinetic energy only (1.9 x 10^13 J)
2. Ten StarShot Probes (0.01 kg total) at 20% c – energy beamed from lasers (1.8 x 10^14 J)
3. Flyby Probe (100 kg) at 10% c – kinetic energy only (4.5 x 10^16 J)
4. Rendezvous Probe (100 kg) 10% c – kinetic energy only (9.1 x 10^16 J)
5. Flyby Rocket Probe (100 kg) at 10% c, with 1 million sec Isp – rocket energy (eq. 1) (9.8 x 10^16 J)
6. Rendezvous Rocket Probe (100 kg) at 10% c, with 1 million sec Isp – rocket energy (eq. 1) (2.2 x 10^18 J)

To estimate when the energy will be available for such missions, look at the calendar year beneath the intersection of that mission energy to predicted energy availability trend lines.

SOURCES- Tau Zero Foundation, NASA

Written By Brian Wang