Frequently Answered Questions related to Photogravitational Assists

This website lists a range of questions that have been raised and challenges that have been identified by the scientific community, reporters, and the wider public in response to the study "Deceleration of high-velocity interstellar photon sails into bound orbits at alpha Centauri" (Heller & Hippke 2017, Astrophysical Journal Letters).

1. Do you really claim that the shortest path to an orbit around Proxima actually involves first visiting A and B, both of which are further away from Earth than Proxima?

Yes. For photogravitational assists to work most efficiently toward Proxima, we propose to use alpha Cen A and B as photon bumpers first because their luminosities are so much higher than that of Proxima, which is an M dwarf. If we were to send our nominal graphene sail from Earth to Proxima directly, then Proxima's luminosity could only absorb ∼1280 km/s, which implies travel times of ∼1000 yr, compared to 95 years (+ 46 years transfer from AB) if we use alpha Cen A and B to brake down first. Indeed, very counterintuitive at first glance!

2. How long would it take to transfer from orbiting Proxima to orbiting Proxima b?

Details will depend on the exact insertion trajectory and on the characteristics of the sail, but this could be on the order of years. If one were willing to spend one more year between the AB binary and Proxima (say 47 years instead of 46 yr), then the sail would arrive at Proxima with a speed that is about 30 km/s lower than the 1280 km/s we mention in our paper. In this slightly modified scenario, the sail could be put into a fairly close orbit around Proxima (with a semi-major axis much less than an astronomical unit) right upon arrival. Then the transfer to Proxima b could be months.

3. You talk about the importance of the alignment of Proxima and Alpha Cen A and B for all this to work, and note that the next time this will be available will be in 2035. That's not a window we can hit, though, given the transit time you're discussing. So when is the next launch window for a photogravitational assist toward Proxima?

With 2035 offering a favorable A-B-C alignment, the next one will be in 2035 + 80 = 2115 since the orbital period of the AB binary is about 80 years. Thus, we would have to launch a graphene-style sail in 2020 assuming 95 years of travel. But if we were able to build a sail with a slightly lower mass-per-surface ratio than graphene, then we could (1) travel faster (and still achieve an A-B-C swingby) and/or (2) achieve larger deflection angles at alpha Cen A and B. In other words, if we could improve the fiducial sail that we discuss in the paper, then we might not need to hit one of these exact time stamps every 80 years, but we might be able to handle substantial misalignments of the A-B-C triple, and then we could work with a large tolerance in terms of a launch window. As an example, while our nominal 86 gram/(316m)2 sail would need to launch in 2020 to hit the 2115 A-B-C alignment, a 57 gram/(316m)2 sail could travel 1.22 times faster and encounter the AB binary 78 years (rather than 95 years) after launch. So it could start in 2037, or 20 years from now. But it could possibly launch even later because it could achieve a >10° deflection around alpha Cen A, so it wouldn't necessarily need to meet AB in 2115, but maybe in 2120. In this scenario, it could launch from Earth in about the year 2037 + 5 = 2042, or 25 years from now.

Bottom line, the launch window for our fiducial sail will be closing in 2020. New developments in the front of material sciences and light sail technologies, however, could soon allow us to contruct light sails that extend our launch windows by several decades.

4. Can you make a robust case for the slower trip compared to the 20 years of travel proposed by the Breakthrough Starshot Initiative?

Both the original Breakthrough Starshot and our photogravitational stopping method have their own merits. There are two aspects in particular that make photogravitational assists an interesting follow-up concept to Breakthrough Starshot.

First, one could park around Proxima b. Yes, one would need to travel ∼7 times as long (95 + 46 = 141 years instead of 20 years), but one would have years or decades of close-up exploration instead of ∼6 seconds (the time required for a Starshot-style sail to traverse the Moon's orbit around the Earth). So if we compare the ratio of in-situ exploration time over travel time for both cases, we have 6 seconds / 20 years ≈ 1/100,000,000 for Starshot compared to 1 year / 141 years for a photogravitational assist. In other words, while Starshot could use one hundred millionth of the mission lifetime for in-situ science, photogravitational assists into bound orbits could make of the order of one hundredth of the mission life time available for close-up scientific investigations, or about one million times more time than Starshot.

Second, by using the sun rather than a ground-based laser system at departure from the solar system, the energy required to launch the sail would be limited to the energy required for a usual liftoff of a small satellite. This avoids the need for a > 10 billion dollar 100 GW laser array completely.

5. How realistic is the proposed design of a graphene sail?

The peculiar tensile and electrical properties of graphene have only been discovered a little more than a decade ago though graphene had already been described in the early 20th century. These more recent experiments have quickly been acknowledged with the Nobel Prize in Physics in 2010. Hence, the modern science history of graphene is still young. That said, huge progress has been made at the experimental and technological fronts of graphene, and the largest graphene structures that have been built were substantially larger than cm2-sized wafers. Graphene is the strongest material ever tested. According to Wikipedia, "the Nobel announcement illustrated this by saying that a 1 square meter graphene hammock would support a 4 kg cat but would weigh only as much as one of the cat's whiskers, at 0.77 mg (about 0.001% of the weight of 1 m2 of paper)". One critical issue for the design of an interstellar graphene-based light sail, however, would be to incorporate a very light coating that is highly reflective to the stellar light and that guards the graphene structure from electron bombardment in the stellar vicinity.

In our context, we propose that the solar radiation could be used to accelerate a space-based graphene-like sail to interstellar velocities. Hence, the critical question will be whether the development of a space-based (≈100 m)2-sized graphene sail would be more cost-effective to build than a 100 GW ground-based laser array. Keeping in mind that nuclear plants generate typically of the order of one GW of power, the power output of about 100 nuclear power stations would have to be combined to launch a laser-riding sail, whereas a solar sail would cost as much energy as a usual liftoff of a small satellite.

6. Could a graphene-based sail survive a close stellar approach as required for an effective photogravitational swingby?

Graphene can be heated up to 623K (350°C) before it burns. A graphene-sail that is covered with a thin and light coating that reflects 99.99% of the incoming stellar irradiation would absorb about 242 W/m2 at a distance of five stellar radii around alpha Cen A. This would correspond to a thermal equilibrium temperature of 256K (-17°C) of the sail. So that's safe. However, the electron gas around a star has a temperature of several 100,000 K, so the sail would need to be shielded against electron bombardment. The Solar Probe Plus (planned to launch in mid-2018) is expected to withstand these conditions for tens of hours, although the shielding technology for an interstellar sail would need to be entirely different, e.g. using an electromagnetic shielding to deflect electrons and ions.

7. The thickness of your sail must be only a few atoms, which is orders of magnitude thinner than the wavelength of light that it aims to reflect. How could it possibly have a reflectivity near 100%?

It has been shown both in computer simulations and in the laboratory that so-called all-dielectric metamaterials, e.g. made up of a single layer of cylindrical 500 nm-sized silicon resonators (Moitra et al. 2014, Applied Physis Letters), are nearly perfect reflectors across a bandwidth of up to several 100 nm wavelength. In some regions of the electromagnetic spectrum, subwavelenght materials can have reflectivities of up to 99.999% (Slovick et al. 2013, Physical Review B). If a graphene-based sail could be covered with such a mono-layer, then maybe the mass-per-area ratio could be orders of magnitude lower than 1 g/m2.

8. Assuming Proxima b is inhabited by intelligent observers, could they detect incoming photon sails from Earth?

Yes. As the sail approaches their stellar system, they would notice a new star in their skies, which would have almost precisely the same electromagnetic spectrum as their host star, except that it would be blue-shifted and that any time variability of their host star's spectrum would be delayed in that star — initially by years, later only by months, weeks, and finally just days or seconds. This new star would also become brighter as the sail approaches Proxima b, and the blue-shift would decrease until, upon the sail's arrival at Proxima b, the blueshift would disappear and the time delay would be very short, e.g. seconds only. At some point, when the sail would reorient itself into an oblique angle to transfer into an orbit at Proxima b, this fake star would suddenly disappear for an observer on Proxima b. As the sail would orbit the planet over the next months or years, it could occasionally reappear for just a few seconds as a very bright star-like dot in the sky. In principle, if these potential inhabitants of Proxima b were able to identify the sail as being artificial, they might conceive of a way to deliberately betray their presence to the cameras aboard the sail.

9. Aiming Accuracy — How could the sail perform course corrections during its decades of interstellar journey?

Our concept involves an autonomous active sail, which would need to calculate its trajectory and steering maneuvers (involving photogravity assists) live on-the-fly. During its interstellar travel, it could use an onboard laser to perform course corrections. A 10-Watt laser aboard a 100 gram probe (86 gram of total weight in our paper) that would fire perpendicular to the direction of travel for the duration of 10 minutes would add a tangential velocity of about 350 m/s. After 95 years of travel to alpha Cen A, it could achieve course corrections of almost 10 astronomical units. A 100 Watt laser firing for 100 minutes could yield course corrections of almost 100 AU etc. Once it approaches its target star, the sail could start to use the strengthening stellar photon pressure for its photogravitational assist.

Just as critically maybe, the position and proper motion of alpha Cen A (the first star in our sequence of photograv assists) would need to be determined very accurately prior to launch.

10. The minimum mass necessary to store enough energy (based on some energy density) to transmit a detectable signal to Earth could be much more than the 10 grams of fiducial payload.

To reduce the sail's energy consumption, communication during the decades of interstellar travel could be minimized. Once the sail arrives at alpha Cen, it could use the stellar radiation to refuel its batteries (whatever technology would be installed) or it could directly convert the stellar photon radiation into energy to be used for the laser and other onbaord electronics such as science instruments, CPU power, steering, etc.

11. Could variations of the stellar radiation, magnetic fiels, unforseen stellar flares etc. fatally affect the sail's trajectory and make a precise encounter with the stars impossible?

The sail's trajectory as well as net effect of the external perturbations would need to be permanently measured with onboard instruments, e.g. via forces measurements on the sail and/or positional changes of reference stars. These observations would then be used for updated calculations of the optimal trajectory.

12. Could drag from interstellar material (e.g. hydrogen) slow the sail down during its interstellar journey?

The sail would fly edge-on between the solar system and the stars, so with the sail being likely much thinner than a milimeter, interstellar drag should be negligible.

13. Mission Duration — When you propose launching a probe on a 100 year journey, you should imagine scientists and engineers in 1917 coming up with a plan that would still be useful and sensible for our purposes today!

Assuming that 1917 engineers would have found a way to send a steam-driven locomotive on a 100 year journey (and that there would have been rails wherever the crew had wanted to go) at a speed of, say, 50 km/h, then this vehicle would have travelled over 114 times the distance between the Earth and the Moon today or over 1100 times the length of the Earth's equator. Actually, at that speed it would have reached the Moon within less than a year, or decades before a much faster vehicle finally did it. However poor an example this is, the journey of such a longliving locomotive would still be incredibly useful and inspiring for us today. Most important, such a longterm interstellar journey would have to continuously deliver results, e.g, remote observations of the solar system, using the sun as a gravitational lense, measurements of the interstellar medium etc. Thus, one wouldn't have to wait a millenium for the scientic return to be delivered. And even if the probe would not reach its final destination, it would deliver highly valuable scientific observations.

14. Do you consider your proposed mission and concept of autonomous active sails with photogravitational assists as an alternative to Breakthrough Starshot?

No. The fundamental goal of Breakthrough Starshot is a proof of capability for interstellar flight within a human generation. Our new mission format, acting on a time scale of a century rather than decades, would be more plausible as a follow-up on Starshot or as a companion mission. Its technologies could be developed along with the Starshot technologies because many tech features will be very similar. However, our scenario involves a much more autonomous active sail, which needs to maneuver itself independently through multiple stellar systems. This concept would benefit from the experiences of a Starshot predecessor/companion mission and from the data collected during Starshot fly-bys at the alpha Cen system. A few sails of the Starshot armada could be sent to each of the alpha Cen stars to probe their stellar astrophysical properties (magnetic fields, high-energy ion/electron radiation, temperatures in the chromosphere etc.) to prepare later fly-bys of autonomous active sails for photogravitational assists.

Further reading

R. Heller & M. Hippke (2017) Deceleration of high-velocity interstellar photon sails into bound orbits at α Centauri [pre-print PDF]
P. Lubin (2016) A Roadmap to Interstellar Travel [pre-print PDF]