2079: Alpha Centauri
Title text: And let's be honest, it's more like two and a half stars. Proxima is barely a star and barely bound to the system.
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However, the offscreen person is against her idea, for the strange logic that "Alpha Centauri sucks". He says that he looked "online" and that the system "only has three stars". This is a pun regarding online reviews. Online rating systems, such as Yelp, often use star rating systems, with more stars indicating higher quality, up to an arbitrary maximum, such as five stars to indicate the best rating. Thus 3 stars out of 5 stars in a 5-star rating system would theoretically be a "middling" rating, equating to a C grade, whereas in a 10-star rating system 3 stars out of 10 stars would be very poor quality. The Alpha Centauri star system has 3 physical stars: Alpha Centauri A, Alpha Centauri B, and Proxima Centauri. The offscreen person has misconstrued this fact of the system as some kind of review.
A previous comic, 1098: Star Ratings, points out that star ratings below 4 out of 5 tend to be seen as "crap".
The title text furthers the pun. Some online star rating systems also allow partial stars, such as a half-star, to allow more precision in rating (i.e. rating 2.5 stars instead of forced to chose 3 stars or 2 stars), or display an average collective rating as partial stars (i.e. showing 2.5 stars when five people have rated 3 stars and five people have rated 2 stars). Alpha Centauri's "half star" refers to Proxima Centauri, a red dwarf, which is a type of low-mass star. According to the offscreen person, this barely qualifies it to be a star. Furthermore, Proxima Centauri is nearly 13,000 AU (0.21 light years) away from the other 2 stars in the system, so it was long unknown whether Proxima Centauri was gravitationally bound to the Alpha Centauri star system.
All numbers are rounded after subsequent calculations.
4.367 light years / 35 years = 0.12477 light years/year
0.12477 light years/year * 5.879e+12 miles/light year = 733,484,000,000 miles/year
733,484,000,000 miles/year / 365 days/year / 24 hours/day = 83,000,000 Miles/hour / 1.60934 miles/kilometer = 134,000,000 Kilometers/hour
According to space.com the fastest spacecraft ever will be the Parker Solar Probe which will reach 430,000 mph (692,000 km/h) as it reaches its closest point orbiting the sun. This is just over half of 1% of the needed speed of the Alpha Centauri vehicle proposed in the comic. The Voyager 1 spacecraft, launched in 1977, is currently traveling at about 38,000 mph (61,000 km/h).
The above math assumes a constant speed, and requires a speed of ~0.0001c. Assuming a constant acceleration from rest (non-relativistic math follows):
35*365.25*24*60*60 = 1.10e+9 seconds in 35 years
4.367 * 5.879e+12 = 2.57e+13miles, 4.13e+13 km, 4.13e+16 m.
x = 1/2*a*t2
a = 2*x*t -2
Assuming constant acceleration to the halfway point and constant deceleration to the destination, (otherwise you streak through the system, barely observing anything):
ttrip = 2*thalfway
a = 2*2.06e+16*(5.50e+8) -2 = 0.136 m/s2, roughly 1/80 gravity.
vhalfway = a*thalfway.
Top Speed: 75,000,000 m/s ~ 1/4*c.
Assuming E = F*d, 0.136*1*4.13e+16 = 5.37e15 Joules will be required for each kilogram carried to Alpha Centauri in 35 years.
This would require an unimaginable amount of mass for a conventional chemical rocket, is a completely impractical power requirement for any sort of passive solar sail concept.
Further, the top speed is fast enough to require a recalculation using relativistic physics to model the problem. This means that the energy budget will need to increase, as the relativistic mass of the probe will increase, requiring more force (and thus more energy) to accelerate and decelerate near its top speed than this calculation returns.
Active, laser based propulsion methods require currently non-existent and purely specualtive laser and materials technologies, as well as a powerplant equivalent to 12,500 of the World's Largest Nuclear Plant to transport sub-gram masses on this timescale. This also assumes that any probes can be steered accurately enough across interstellar distances to come close enough to image with any resolution the bodies they will be passing at a non-trivial fraction of c.
Short of FTL travel or near-perfect mass-energy conversion technology, transporting more than fraction of a gram of material to Alpha Centauri in a human lifetime will be unachievable. Short of an enormous breakthrough in power generation, transporting even a fraction of a gram is impossible.
Nonetheless, Breakthrough Starshot is attempting to send many gram-sized probes to Alpha Centauri within the century. Following current technological trends, they expect the efficiency of laser-based propulsion to increase by launch time, allowing launches driven by an unreasonably-large-but-achievable amount of power. The top speed needed is halved by refraining from slowing at all at the destination: the probes will aim a distance away from the target, so that it traverses by slowly enough for a camera to rotate and track it, even at near-light speeds. To account for error and space dust, the plan is to launch many tiny probes simultaneously. They may only be able to accomplish their goal if they can get enough funding to actually affect the global economy enough to make the technologies they require more efficient to produce. Launches would additionally burn incredible quantities of natural gas.
- [Ponytail stands on a podium giving a presentation in front of a slide with an image of a Voyager-like spacecraft.]
- Ponytail: Our probe can reach Alpha Centauri in under 35 years.
- Offscreen voice: We should go somewhere else. Alpha Centauri sucks.
- Ponytail: Huh? It's the closest, most convenient system!
- Offscreen: Yeah, but I checked online and it only has three stars.
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