In my Far Centaurus* probe, we enter the Alpha Centauri system at 30,000 km/sec. I don’t think gravitational slingshotting is going to gain us all that much, we are already way past the escape velocity of all the stars and planets in that system. We are entering that system so fast that in order to change direction even slightly with our “secondary propulsion system” we’d have to start boosting months, or even years before entering the system. The probe will be in the cis-Centauri environment for only a few hours at most, and the local gravities would have little effect on it.

Your comments on interferometers are interesting, but its been a long time since I studied them, and that was when they were used in radio astronomy. Two separate telescopes were linked together, at first by a cable or microwave link. Later, atomic clocks were used to synchronize the two. But the problem of different parts of the interferometer being light-hours apart, with the distance known only imperfectly, may be insuperable.

Fortunately, parallax work is not the same as interferometry, there is no wave interference involved, the process is just pure geometry, in fact, no real physics is involved at all.

But astronomers have experience with much longer baselines than solar system-sized ones. In the technique known as a secular parallax, the base line used is not the diameter of earth’s orbit around the sun, but of the sun’s orbit around the galactic center. The sun orbits the galactic nucleus at much higher speeds than the earth orbits the sun, and you can wait months or years to give yourself a really long baseline.

Check this out, from the Wikipedia article on “Stellar Parallax”:

The motion of the Sun through space provides a longer baseline that will increase the accuracy of parallax measurements, known as secular parallax. For stars in the Milky Way disk, this corresponds to a mean baseline of 4 AU per year, whereas for halo stars the baseline is 40 AU per year. After several decades, the baseline can be orders of magnitude greater than the Earth–Sun baseline used for traditional parallax. However, secular parallax introduces a higher level of uncertainty because the relative velocity of other stars is an additional unknown. When applied to samples of multiple stars, the uncertainty can be reduced; the precision is inversely proportional to the square root of the sample size.[15]

* A shout-out to A E van Vogt, author of the short story “Far Centaurus”, about astronauts who use suspended animation on a sub-light spaceship to travel to Alpha Centauri. When they get there, after hundreds of years in transit, they find planets in the system long settled by Earth colonists. FTL travel was discovered after our protagonists left Earth, and by the time they got to their destination the Centaurus system had been thoroughly explored and occupied. They are treated as honored guests, but are totally out of place in the future. Their science/tech skills are obsolete, the customs of the future are alien and disturbing, and even the English language has changed so it is unintelligible to their ears. The colonists (who had been long expecting their arrival) had to educate linguists in antique English so our sub-luminal crew would have someone to talk to.

I did some more digging, so let me simplify my idea further: What if the 4 Nomads were used in conjunction with Earth as an astronomical interferometer? We can do fantastic chart work from home, but we can only ascertain so much.

Concerning the Centauri Flyby, I have an idea there. Why not angle the approach so that the probe flys between Proxima and Alpha/Beta until gravitational assistance successfully slows it down to orbital speeds? With constant monitoring and course correction, this would be possible.

Personally, I think we should be sending out deep space telescopes just for this purpose. Imagine what we could discover with synthetic apertures the size of the solar system…

Alpha Centauri is a triple system. The A and B components are listed as 4.36 ly away at RA 14h 40m, DEC -60d 50′. The third component, Proxima, is considerably distant from the pair at 4.24 ly
from us, it is in a highly elliptical orbit around them. Its geocentric coordinates are RA 14h 30m, DEC -62d 41′. This separation is easily noticeable, even to the naked eye, Proxima and Alpha are about two degrees apart in the sky. Even though these stars are all gravitationally connected, they are also extremely close to us as stars go. By the way, keep in mind that Dec is in angular units, 360d to a full circle. RA is always given in time units, 24h to a full circle. A minute of time = 15 minutes of arc. There is a good reason for this, but I won’t go into it now.

Incidentally, the catalog positions are for Epoch 2000.0, so you’ll have to precess the coordinates for whatever year your probe will be launched. In addition, at launch time, your position for the system will be four years out of date because of the light travel time to the system. You’re not seeing it where it is now, but where it was four years ago. Also keep in mind, it will also have moved even further off position in the 40-odd years it will take your probe to get there. All this has to be taken into account or you will miss your destination. It is a moving target.

Once all these purely geometrical effects are corrected for, you still have to deal with the kinematics of the Alpha Centauri system. Proxima has a cataloged proper motion of 3853 milliarcseconds/yr in a Position Angle of 281.5d (measured clockwise from N). Since it is in orbit around the center of gravity of the A-B pair, the numbers for A and B are slightly different than those for Proxima (and keep in mind they are also orbiting around each other). A’s proper motion is 3710 mas/yr, PA 277.5d. B’s proper motion is 3724 mas/yr, PA 284.8d. The radial velocities (motion IN the line of sight) for Proxima, A and B are all AWAY from us, at 16, 26 and 18 km/sec, respectively.

So how do we approach a system like this? We want to get a good look a Proxima, because it has a planet, but we might also want to study the stellar A and B components in detail. It may not be possible to get as close as we want to both objectives because our approach velocity will be extremely fast (0.1c) and we may not have enough capability to deploy sufficient delta-v to do both. We will have to think long and hard on which target we intend to favor, and by how much, and it may be wise to delay that decision and make careful measurements on our final approach, allowing for time to radio those observations to mission planners on earth so that final maneuver instructions can be transmitted back.

And remember, at these speeds, and with our limited secondary propulsion system capability, the earlier you communicate maneuvering instructions to the spacecraft the better. If you get hailed by the local Centaurians as you fly through the system the message will never get relayed back to earth in time for you to do any braking or course changes. You won’t hear about it until long after the encounter is over.

I’m not too familiar with modern methods of astrometry, but when I was a student it was done photographically. Star positions on modern photographs were compared with carefully measured positions taken from old photographs. These measurements were made with high precision micrometers and microscopes, using dimensionally stable glass photographic plates taken many decades apart. But this only gave us the stars ‘proper motion’, its movement across our line of sight. A star’s motion toward or away from us was determined in a completely different way, by Doppler measurements of its spectrum. Unscrambling all these different measurements and allowing for our own sun’s orbital path around the galaxy was the problem.

Remember, the “motion” of an object is a meaningless term. Its always the relative motion between two objects that we measure–there is no preferred coordinate system in space. All motion is specified as relative to something else. There are techniques to measure motion in a coordinate system, but these reference frames must be defined by either a physical object (like the earth) or some relationship between physical objects (like the Vernal Equinox, where ecliptic crosses equator). And these are also in motion.

Measuring the distance to an object can be done only by triangulation, as is described in the article. But this method is limited to only the nearest stars due to the technical difficulty of measuring very tiny angles. However, we can make good estimates of greater distances if we apply our knowledge of the nature of these objects. For example, if we know stars of a certain spectral class are of a certain true brightness, then by measuring the apparent brightness of a star we can make a good estimate of its true brightness, and hence, its distance. But these are, at best, estimates, and the distance scales must be calibrated eventually on some actual measurement. Everything eventually relies on those stars actually close enough to show a measurable parallax.

When you look up a star’s position in a catalog, or scale it off a chart, you come up with a pair of coordinates, The Declination and Right Ascension. DEC is the number of degrees N or S of the equator, RA is the number of hours east of the Vernal Equinox. But due to precession, the Vernal Equinox is in constant motion as the earth wobbles every few thousand years on its axis. Right now, catalog positions are referenced to where the Equinox was in the year 2000.0. But if you should need an accurate position for today (like to point your telescope) you need to do the math and
“precess” the catalog values to the present day.

And remember, a “position” isn’t really a position. Its only a direction. You may know exactly where in the sky a star’s direction is, but that doesn’t necessarily mean you have any idea of how far away it is.

By “apparent star positions”, what I mean is the apparent position of the stars themselves in the field of view. Seeing as stars move throughout space just like everything else, it would be interesting to use the apparent positions and parralax given by terrestrial observations, and measure how much the same objects deviate from said positions at the probe locations; thus allowing for more accurate star charting. What their light doesn’t show at one location, math can yield from both.