by Prof. Pierre Kervella of Universidad de Chile and Observatoire de Paris.

Edited by James S. Jenkins and Zaira M. Berdiñas

Alpha Centauri and Proxima are located respectively at 4.37 and 4.24 light-years, a bit more than 40,000 billion kilometers. As a consequence, speed is the primary requirement for an in situ exploration. In fact, an incredibly high speed is required. The Breakthrough Starshot initiative aims at launching a miniature spacecraft to Alpha Cen at a velocity of 0.2 times the speed of light (~60.000 km/s) using a powerful laser. The technological challenges to achieve this velocity are considerable, but a conceptual difficulty is that after a multi-decadal flight, the probe will travel a distance equivalent to the diameter of a planet in less than one second. A very short time to obtain accurate scientific measurements, e.g., images and spectra.

Achieving a gravitationally bound state has numerous advantages for a long-term study of the stellar system and its planets. The Alpha Cen system is formed by two Sun-like stars orbiting each other in approximately 80 years. As of today, no confirmed planet has been detected in the Alpha Cen AB system, although their presence is plausible1. Proxima on the other hand has the telluric planet Proxima b in its habitable zone2. Proxima is a different type of star, a very small M star, and its brightness is only 1/1000th of that of its more massive neighbors. We propose to use the radiation pressure of the star’s light to slow down the probe, symmetrically to its initial acceleration using laser light. However, slowing down at Proxima is much more difficult than at Alpha Cen. Due to the faintness of the red dwarf, its radiation pressure is insufficient to stop a sail craft flying at 20% of the speed of light before it collides with its surface.

To inject a probe at Proxima, we thus propose to use the light of Alpha Cen A and B to slow down the interstellar sail, and inject the probe in orbit around Proxima3. The encounters with Alpha Cen A and B will progressively slow down the probe, so that it arrives at Proxima with a velocity compatible with its deceleration into a bound orbit, and ultimately enters an orbit around its telluric planet Proxima b. The principle of this slow down is to “bounce” on the radiation field of Alpha Cen A and B, to bring the velocity of the probe down from its cruise velocity of 17.000 km/s to 8.400 km/s after the approach of Alpha Cen A, and to 1300 km/s after B.

Among its main drawbacks, this scheme increases the travel time to Proxima from ~20 years (direct fly-by at 0.2 times the speed of light) to 121 years (full stop at Proxima). It also implies heat and particle hardening requirements for the spacecraft (sail and electronics) as it will penetrate deep into the coronae of Alpha Cen A, B, and Proxima at very high speed.

Knowing the orbits of Alpha Cen and Proxima is critical to be able to inject the sail in the narrow funnel in which it will be subject to the expected photogravitational slow down. We have calibrated the orbital parameters of the AB pair, as well as their proper motion, masses and distance, but the achieved accuracy is still insufficient for the targeting of the Starshot probe4. Nevertheless, we made an important step forward by finally confirming that Proxima is in fact in a gravitationally bound orbit around the AB pair5. We found that the escape velocity of Alpha Centauri at the distance of Proxima (545 ± 11 m/s) is larger than the velocity detected for Proxima with respect to Alpha Centauri (273 ± 49 m/s, close to 1000 km/h, the velocity of a commercial airliner), meaning that they are bound. Our calculations indicate that Proxima has a very long period of 550,000 years and a moderate eccentricity of 0.5. Projected on the plane of the sky, the orbit presents a very large angular size of more than 3 degrees, that corresponds approximately to the width of two fingers at arm’s length. The European satellite Gaia will soon deliver an extremely accurate measurement of the distance and proper motion of Proxima, that we will use to improve its orbital parameters.

References:

Quarles & Lissauer. “The long-term stability of planets in the α Centauri System”, AJ, 151, 111 (2016). Anglada-Escudé G. et al. “A Terrestrial Candidate in a Temperate Orbit Around Proxima Centauri”, Nature, 536, 437 (2016). Heller, Hippke & Kervella, “Optimized trajectories to the nearest stars using lightweight high-velocity photon sails”, AJ, in press. Kervella et al. “Close stellar conjunctions of α Centauri A and B until 2050 . An m K = 7.8 star may enter the Einstein ring of α Cen A in 2028″. A&A, 594, A107 (2016). Kervella, Thévenin & Lovis. “Proxima’s orbit around α Centauri “, A&A, 598, L7 (2017).

About the author

Professor Pierre Kervella has a double affiliation at the Universidad the Chile and the Observatoire de Paris (CNRS UMI 3386, Universidad de Chile, and LESIA, Observatoire de Paris). Pierre finished his PhD in 2001 at the European Southern Observatory (ESO) at Garching, which was focused on the topic of Optical interferometry with the Very Large Telescope (VLT). Since then he has worked on Stellar Physics and has published more than 300 articles. In 2009 he was honored with the Prix Jeune Chercheur of the Société Française d’Astronomie et d’Astrophysique (SF2A). Recently, he was the first author of a study confirming the long held notion that Proxima is in a gravitationally bound orbit around the Alpha Centauri AB pair (Kervella,Thévenin & Lovis, A&A, 598, L7).