The lower image is a new version I just made, doing an improved Doppler analysis of the radar telemetry.
The lower image is a new version I just made, doing an improved Doppler analysis of the radar telemetry.
Here’s a comparison of the original Soviet processing of the first Venera-15 radar swath (above) and my own processing of the data, from radio hologram to image. One thing I have not done is compensate for the antenna pattern, so my image goes more dark at the edges.
This data was acquired by the spacecraft on October 16, 1983, during a pass over the north pole of Venus.
I’ve been tinkering with some of the raw telemetry data from Venera-15, an radar imaging satellite that the Soviet Union put into orbit around Venus in 1983.
Here are some examples of the stages of processing. The radar system records 2540 complex (in-phase, quadrature) samples of a reflected radio signal. It just looks like random noise if you visualize it.
Correlation of that signal with the pseudo-random code sequence converts the continuous signal into a series of pulses, and that reveals the beginnings of an image. The curved stripes are due to phase shifting as the spacecraft moves away from the surface in its orbit.So, the next step is to correct for that, and that gives us a very noise image of a strip of the Venusian surface. There’s a lot of speckle noise, because the radar beam is coherent illumination, like laser light.
The 2540 samples are 20 looks at the surface, and we can do image stacking to get a better image (the fourth image). However, if we apply Fourier transform, we can separate the signal into 20 Doppler frequency shifts, which represent narrow strips of azimuth. These can be shifted into alignment and stacked, to produce a higher resolution image as seen on the right.
A practical concept for interstellar propulsion, based on existing technology and science. Fission power from a traveling-wave reactor generates the electricity, the waste material is ionized and accelerated to relativistic speed. This should be more efficient than the Flashlight Drive concept outlined in an earlier post.
How hard would it be to send a probe to a nearby star? I have yet to find a discussion of this problem that does not succumb to science fiction, antimatter, planet-sized lasers, etc. So let’s try to tackle this practically, assuming we just have nuclear fission as a power source.
I. The Relativistic Rocket Equation
Edwin Taylor and John Archibald Wheeler derived a relativistic form of Tsiolkovsky’s rocket equation in their 1963 book _Spacetime Physics_. The final velocity of a rocket is then given by:
(1) Boost = ExhaustVelocity * log( InitialMass / FinalMass )
(2) FinalVelocity = tanh( Boost )
II. Fission-Photon Drive
Let’s assume we take 100 kg of Uranium 235 and place it in a 2-meter spherical reactor. Let it achieve a temperature of 3100 kelvin (just below the melting point of UO2), and place this incandescent reactor at the focus of a light-weight parabolic reflector. This scheme converts mass into photons, so the Exhaust velocity is the speed of light. From the Stefan-Boltzmann law, we find the reactor core would radiate 70 megawatts for four years, giving a thrust of 0.23 Newtons (about 1/20 pounds).
The change is mass is 0.1 percent — the amount of mass turned into energy by nuclear fission. From the rocket equation, the final spacecraft velocity would be about 0.1 percent of the speed of light. At that speed, it would take about 5000 years to reach a nearby star. Not very good.
We can do a little better by carrying multiple reactor cores. Every four years, the spent core is ejected and a new one placed at the focus and activated. This is better, because we are losing the spent fission products instead of carrying them as payload. With this scheme, we can approach 0.7 percent light speed and reduce our trip time to about 700 years. Still, not really practical, and we’ve been calculating an upper bound, assuming the mass of the spacecraft’s structure is zero, just pure uranium fuel.
Instead of using photons for exhaust, let’s use the fission reactor to accelerate ions. Let’s assume 100 Kg of Uranium-235 for power, and a mass M of extra propellent to be ionized and expelled at high speed. We can calculate a necessary exhaust speed by assuming that all of the fission energy is converted into kinetic energy, M * ( cosh( Boost) – 1 ). So
(3) ExhaustVelocity = tanh( acosh ( ( M + 0.001*Uranium)/M ) )
The final mass is just the uranium, minus the mass defect coverted to energy by fission. If we plug M=0, we get the same result as our Fission-Flashlight drive.
As we add mass, we get more final velocity from the log (M1/M2) term of the rocket equation, but we also reduce the exhaust velocity. There is an optimal amount of propellent in this case. With 100 Kg of Uranium and 390 Kg of propellant, we achive a final spacecraft velocity of 3.6 percent light speed. That’s fast enough to reach the nearest star in only 140 years. Still a bit long.
Let’s try a two-stage rocket. The first stage has 1000 Kg of uranium and 5800 Kg of propellant. It carries the second stage, which is the one described above. This combination can achieve a final spacecraft speed of 6.5 percent light speed. Now we’ve reduced our mission time for a flyby to 77 years, short enough that one generation could build it, and a next generation of scientists could receive the results it reports back.
I’ll tinker a bit more and derive an optimal stage-size ratio. I’m sure a three stage spacecraft could reach something like 10 percent lightspeed.
OK, my rant about the Russian announcement they won’t send US astronauts to the ISS in a few years.
This is a blessing in disguise. Mr. Rogozin should do us a favor and shut off access to ISS right now. ISS and the Space Shuttle sucked the life out of NASA’s scientific space exploration budget. $50 billion to keep people in low earth orbit, where most of the science and physiology is already known, or can be found with satellites, or was learned by the Russians decades ago. Instead we have bullshit like growing tomatoes in space, music videos from space, experiments designed by cute school kids, diplomatic stroking of allies who don’t have their own space program. This for $50 billion.
Look at NASA’s $17 billion annual budget in detail and weep. They have all but stopped future planetary probes, shutting down a pipeline of infrastructure and human expertise that will be lost if these guys leave JPL. All because the money is eaten up by an unfocused manned space program with no vision or commitment. We don’t even have manned spaceflight capability now, thanks to total bungling and poor long-term planning. And yet the manned program still consumes a big slice of NASA’s budget. And we have billions sucked up maintaining giant techno-burocratic empires left over from the Apollo era (Marshall Space Flight Center, Goddard, etc.). Billions sucked up by administrative overhead.
Look at how much people enjoy the Mars rover, the images, the science learned. We should be doing more of that, landing on Europa, rovers on Titan, seismometers on Venus. Instead, we will lose that capability if we cut back on the activity of places like the Jet Propulsion Laboratory. The people who know how to build those complex spacecraft systems will leave and nobody will take their places.
ESPACE Memo #1
D. P. Mitchell
ESPACE: Environment for Secure Public Assembly and Commercial Exchange
ESPACE is a project to explore certain advanced concepts in electronic communication service. The intent is to take a large step beyond electronic mail and electronic bulletin boards. We believe that a sort of “virtual space” can be created in which people carry out a variety of associations including mail, elections, signing of documents, contracts, and fund transfers.
Recent work in the area of cryptography has resulted in a number of very interesting algorithms for accomplishing these tasks, but these advanced algorithms have never been integrated into a system, and a model for such a system does not yet exist.
II. Related Work
British Telecom and three British universities are currently working on a project called COSMOS. This is a formal study of group communication. They are interested in issues like planning assistance, multi-media electronic mail, and active messages.
A collection of European universities is working on a project called AMIGO. This is also a formal study of group communications, geared primarily for office automation. They are heavily committed to the massive OSI message-handling and directory systems.
The ESPACE project differs considerably in intent. We are interested in the application of advanced cryptographic protocols, and we want to build a system with more general goals than office automation.
III. Public Assembly
Our model for organizing communication is based on the concept of a Group. A Group is more than a distribution list for mail or a bulletin-board discussion group. A Group can behave like a first-class communicator in ESPACE. It can receive messages, and it can send and sign messages.
The key concept is capability of signing messages, which then convey group authority. The way in which a group signs a message is determined by its Constitution. There may be a group leader who holds the official seal, an election may be required to generate a signature, etc.
A simple example of an application might be an electronic journal. People submit papers to an editor group. The editors vote on papers, and if paper is accepted, the editor’s signature validates it as an entry in the journal. Publication of the journal might be a completely separate process. Perhaps a journal group would exist simply as a publically readable mailbox which only accepts messages signed by the editors.
IV. Commercial Exchange
A number of standards for electronic fund transfers are forming now, mostly based on highly centralized algorithms which model electronic cheques or credit cards. These are important, but in addition there are interesting cryptographic which support the concept of unmarked cash. The service offered in that case is customer privacy. It is interesting that these electronic-cash algorithms offer a number of protections from fraud and black-marketing, while still preventing customers from divulging personal patterns of commerce.
Many distributed system contain the concept of “capability”, getting permission to access an object by showing a certificate. We think this can be extended easily to allow accounting by granting access permission in exchange for “cyberbuck” certificates.
One possible example of use for this would be to control message congestion in large popular bulletin boards. It is possible that user may have to pay to publish a message widely, and that they may (or may not) receive payment in return if the message is deemed useful by its readers. This is an example of a closed cyberbuck economy, where the currency only has meaning and value within ESPACE.
V. Message Handling System
ESPACE rests on a simple and reliable binary message handling system. This system is inspired by X.400 and Grapevine, and the ISO Directory Service, but it is greatly simplified. It provides name service and message routing in a unified manner. It will also allow users to access ESPACE from any location.
VI. User Interface
Some user interface will be required. We would like to support multi-media messages. Recent work on “virtual reality” has suggested that the user interface to a system like ESPACE could be a highly advanced 3D visualization of the objects and processes of the system. We prefer to concentrate on designing those objects and processes before considering any far-out “cyberspace” interfaces.
We have access to new “super smart cards” which may be an interesting part of a user interface, particularly important if users can enter ESPACE from any location.
We hope to build a prototype system and test a user community of reasonable size. One way to do that would be to test the system in the ARPANET environment. This is one of the purposes of the ARPANET, and we should make use of this valuable resource. In particular, we have thought about the possibility of starting a serious electronic journal in the ARPANET.
(note: this short-lived project took place inside AT&T Bell Labs)