In February, 2017 the Space Horizons workshop at Brown University focused on sending spacecraft to the Alpha Centauri stellar / planetary system. The first questions that come to mind include: getting there in some relevant amount of time, i.e. less than a human lifespan; communicating back over the 4+ light years distance; surviving the space environment en route; What do you do when you get there, and; how much is all that going to cost? We also asked: what could we learn by going there vs. wouldn’t we learn more directing those resources to developing instruments here in the earth and near-earth environment to observe the star system (and probably other star systems) and exoplanets remotely? Would going there have some special meaning to people on earth as America’s landing of people on the moon did, versus the Soviet robotic missions? And what might we learn along the way - thinking our way to another star - that might change how we do other possibly completely unrelated things - closer to home? We barely managed to even address all those questions in a day and a half at Brown, but undaunted I will do the best I can to distill that concentrated dose of interstellar travel down another factor of 30 into a talk 0.0014% of the trip time to Alpha Centauri at the speed of light. I.e. 30 minutes.

2. A S E L F I E F R O M A L P H A C E N TA U R I ?
(Ourselves
from Proxima Bb)

3. A S E L F I E F R O M A L P H A C E N TA U R I ?
(Ourselves
from Proxima Bb)
Rick Fleeter
Brown University
La Sapienza
Space-Point

5. Space for the rest of us V1.0

6. http://www.youtube.com/
watch?v=erdeWSaqce0
Micro
meets
Macro
feb. 21:

7. THE OPPOSITE
OF MISSION:

8. Re-
Architecting
Space Systems:
Infrastructure
Changes
Everything Space Horizons 2014
Space: 0 infrastructure Earth: infrastructure inescapable

9. Space Horizons 2016
International City on the Moon

10. without Sponsors
doing Science
Charles Darwin
and the HMS Beagle
S p a c e H o r i z o n s Wo r k s h o p
B r o w n U n i v e r s i t y
Fe b r u a r y 1 0 , 2 0 1 8

11. H O W FA R A WAY I S I T?
paperthickness
=earthtomoon
stackheight
=earthtoalphaCentauri
106000000 x
farther
thanthemoon

12. WHY CHIPSATS AND LASER PROPULSION TO ALPHA CENTAURI
HOW MUCH ENERGY
▸ 60,000 km/s instead of 6000 m/s
velocity ratio 10,000:1
energy ratio 100,000,000
▸ if the spacecraft is 20g instead of 2000 kg
mass ratio 100,000
energy ratio 100,000
▸ net energy ratio 100,000,000 / 100,000 = 1000x
1000 Atlas rockets pushing a 20 gram spacecraft

18. 50,000 km/s means:
- the Moon in 6 seconds (instead of a week)
- Mars in 25 minutes (instead of 6 months)
- the Sun in 1 hour
- Titan (Saturn) in 6 hours (instead of 3 years)
- Alpha Centauri in 20 years (instead of 100,000 years)
Enabled by laser photon propulsion:
- geosensor swarms on the moon - and student chips
- constellations at earth, venus, mercury, mars,
without on board propulsion
- Power beaming, heating, high bandwidth comms
with little or no on-board energy

19. Alpha Centauri
• 4.36 light years
• 4.13 × 1016 m
• Loss calculation
• 2.2 × 10-48 at 780 nm
• 476 dB loss at 780 nm
• Gain?
• 120 dB w/ 25 cm Dtx
• 192 dB w/ 1 km Drx
• 212 dB w/ 10 km Drx
• Need another 144 dB…
19http://davidmalin.com/fujii/image/af1-06_72.jpg
Alpha
Centauri
Southern
Cross
λ
4πR
⎛
⎝
⎜
⎞
⎠
⎟
2
πDtx
λ
⎛
⎝
⎜
⎞
⎠
⎟
2
and
πDrx
λ
⎛
⎝
⎜
⎞
⎠
⎟
2
250 TW?
What dB offset w/receive sensitivity?

20. One

21. Receive Sensitivity
Received
Signal
Local
Oscillator
Power
Detection
10
1
0.1
Photonsperbit
Coherent
Photon
Counting
Optically
Preamplified
LO shot noise
ASE noise
“noiseless”
16.851 lecture, D. Boroson

22. 1 AU Link margin
1 AU is 149.6 million km
(typical link to Mars)
(to Saturn /Titan figure 11 AU)
(20 dB worse = 0.5 bits per sec)
Assume 34 meter DSN
(70m => 6 dB = 4x data rate)
Spacecraft:
X-band 28 dB antenna @ 1 Watt
=> 10 bit/s with margin
pushing it, 40 bit/s maybe….

23. hey, that’s a watt of photons!
using the 1.06 µ atmospheric window (~80%)
(the classic for Neodymium - YAG)
abd
1 eV = 1.602 x 10-19
J
at 1.06µ, e/photon = 1.87 x10-19
J (1.17 eV)
1 W = 5.35 x1018
photons/sec
Ideal Nd-YAG divergence 0.34 mrad = 0.019˚
spot size at 1 AU = 0.00034 x 149,600,000,000 = 50,864,000 m

24. a watt of photons (continued)
for a (modest) 3 meter collector compared to a spot size of
50,864,000 m, the collection area ratio is
(3/50,864,000)2
= 3.5 x10-15
(why I like corn)
of the 5.35 x1018
photons/sec transmitted we collect
18611 per second.
at 10 photons per bit,
> 1000 bit/s
for a 30m Ø light bucket,
>100 kbit/s !!!
(10,000 x better than radio)

25. Alpha Centauri
• 780 nm = 2.5 x 10-19 J/photon
• 1 W = 4 x 1018 phot/sec
• RX sensitivity: 2 bits/photon
• 1 W = 8 x 1018 bit/s = +189 dB
• Total link:
120 dB (TX gain)
- 476 dB (space loss)
+212 dB (RX gain)
+189 dB (RX sensitivity)
= +45 dB = 30 kbps (per Watt)
– TW transmitter not necessary!*
25http://davidmalin.com/fujii/image/af1-06_72.jpg
Alpha
Centauri
Southern
Cross
λ
hc
Ephot =
*With a 10 km diameter receive aperture

26. OTHER PROBLEMS
▸ 20+ years at 4K
▸ 20+ years of deep space radiation
▸ all of that energy focused on a 1m mirror
▸ So What - what do we learn if we get there with our 1mm
aperture vs. virtual apertures of 10,000 km from here?

28. A S E L F I E F R O M A L P H A C E N TA U R I ?
(Ourselves
from Proxima Bb)

29. NEXT YEAR - SCIENCE IN NEW SPACE
BEYOND ALPHA CENTAURI
WHAT CHANGES DOWN HERE FROM GOING OUT THERE
▸ chipsats - siliconization of spacecraft
▸ laser propulsion
▸ not carrying the energy on board
(terrestrial transport, cell phones…)
▸ infrastructure
▸ the goal: (mimicking ebooks:)
missions at cost 0 in time 0