Untrue. Project Longshot, using nuclear pulse propulsion, did not rely on non-existent or impractical technologies. And it was developed in the 1980s.
Similar to Project Daedalus, Longshot was designed with existing technology in mind, although some development would have been required. For example, the Project Longshot concept assumes "a three-order-of-magnitude leap over current propulsion technology".
Quoting from the Wikipedia article on the subject:QuoteSimilar to Project Daedalus, Longshot was designed with existing technology in mind, although some development would have been required. For example, the Project Longshot concept assumes "a three-order-of-magnitude leap over current propulsion technology"."Some development" is quite the understatement. We don't have inertial confinement fusion of the kind of efficiency necessary for Longshot, and we don't have ANY machine that can be expect to keep working for 100 years without any kind of human intervention, much less a 300 kw fission reactor at full blast. So I would say Project Longshot very much relied on technology that does not exist even now, 30 years later.Regarding the technical difficulties, Breakthrough Starshot seems at least as doable ...
(Sorry haven't read the entire thread. If you know it has already been discussed just say so and I will trawl for it. If you know who by that could help locate it maybe.)Has there been discussion of the vast range of nearer targets this could apply to? Obviously you would not just run it once and there are hundreds of known dwarf planets and a possible new ninth planet in our solar system that are still well out of practical reach. Perhaps such lasers could have duel uses also such as powering thermal or electric propulsion near earth or a probe far from the sun?Another possible application of this array: space junk removal?(I know RobotBeat doesn't like the beamed power idea compared to SEP but I can't remember the issue. Sorry RB.)
...Surprisingly, regardless of the material choice, the optimal structures turn out to be simply one-dimensional subwavelength gratings, exhibiting nearly 50% improvement in acceleration distance performance compared to prior studies. ...
How is the 100 watts for the laser generated? Most of the transmission will be far from the star.
Battery design is one of the most challenging aspects of the mission. Currently under consideration for the energy source onboard are plutonium-238, which is in common use, or Americium-241. 150mg has been allocated for the mass of the battery. This includes the mass of the radioisotope and the ultra-capacitor. As the isotope decays it will charge the ultra-capacitor. Then, at the appropriate time, the StarChip components will be switched on and begin to operate.
A Starshot Communication DownlinkBreakthrough Starshot is an initiative to propel a sailcraft to Alpha Centauri within the next generation. As the sailcraft transits Alpha Centauri at 0.2 c, it looks for signs of life by imaging planets and gathering other scientific data. After the transit, the 4.1-meter diameter sailcraft downlinks its data to an Earth-based receiver. The present work estimates the raw data rate of a 1.02 {\mu}m, 100 Watt laser that is received at 1.25 {\mu}m by a 30-meter telescope. The telescope receives 288 signal photons per second (-133 dBm) from the sailcraft after accounting for optical gains (+296 dBi), conventional losses (-476 dB), relativistic effects (-3.5 dB), and link margin (-3.0 dB). For this photon-starved Poisson channel with 0.1 nm equivalent noise bandwidth, 90% detector quantum efficiency, 1024-ary PPM modulation, and 10^-3 raw bit error rate, the raw data rate is 260 bit/s (hard-decision) to 1.5 kbit/s (ideal) raw data rate, which is 8-50 Gbit/year. This rate is slowed by noise, especially starlight from Alpha Centauri A scattered into the detector by the atmosphere and receiver optics as sailcraft nears the star. Because this is a flyby mission (the sailcraft does not stop in the Centauri system), the proper motion of Alpha Centauri relative to Earth carries it away from the sailcraft after transit, and the noise subsides over days to weeks. The downlink can resume as soon as a day after transit, starting at 7-22 bit/s and reaching nearly full speed after 4 months. By using a coronagraph on the receiving telescope, full-rate downlink speed could be reached much sooner.https://arxiv.org/abs/2005.08940
Quote from: daedalus1 on 05/23/2020 08:39 amHow is the 100 watts for the laser generated? Most of the transmission will be far from the star.Breakthrough starshot has an article on the power system:QuoteBattery design is one of the most challenging aspects of the mission. Currently under consideration for the energy source onboard are plutonium-238, which is in common use, or Americium-241. 150mg has been allocated for the mass of the battery. This includes the mass of the radioisotope and the ultra-capacitor. As the isotope decays it will charge the ultra-capacitor. Then, at the appropriate time, the StarChip components will be switched on and begin to operate. https://breakthroughinitiatives.org/forum/16?page=4Although this would suggest the power source only generates .02 watts. So, you could run your 100 watt laser for 100 minutes per year. At a data rate of 1500 bits per second, that is only about a megabyte per year. Probably should add photo compression to the research list.
It may be possible to coat the lightsail with a thin film of photovoltaic material, which was demonstrated on the IKAROS mission. This could be extremely useful during approach to the host star. The photovoltaics will be able to supply significant energy when they are within 2AU of the target star. Even with just 10% efficient photovoltaics, the energy supplied would be nearly 2kW at 1AU. This is more than 100,000 times the power of the radioactive energy source, and could conceivably allow much higher data rates for laser communication. This is one option that will be explored.