FOCAL: A Call for Papers

by Paul Gilster on December 18, 2009

The Tau Zero Foundation is announcing a call for papers related to the FOCAL mission. The venue: The 61st International Astronautical Congress in Prague, which convenes on the 27th of September, 2010 and runs to October 1. Specifically, we are looking for papers for session D4.2, “Interstellar Precursor Missions,” whose focus is “…missions that significantly expand science — using existing and emerging power and propulsion technologies.”

Long-time Centauri Dreams readers are well aware of Claudio Maccone’s FOCAL concept, a mission to the Sun’s gravitational lens at 550 AU and beyond. FOCAL would make possible studies of astronomical objects at unprecedented magnifications. The electromagnetic radiation from an object occulted by the Sun at 550 AU (i.e., on the other side of the Sun from the spacecraft), would be amplified by 108. Moreover, whereas with an optical lens light diverges after the focus, light focused by the Sun’s gravitational lens stays fixed along the focal axis. Every point along the straight trajectory beyond 550 AU remains a focal point for any vehicle we put on this trajectory.

Imagine, then, two possible FOCAL mission targets. The first option would be to launch the probe toward the heliopause in the place where it is closest to the Sun, the direction of the incoming interstellar wind. This would allow useful studies of the heliosphere itself, but the deeper goal would be to reach 763 AU, the place where the cosmic microwave background will be focused by the Sun’s gravitational lens upon the spacecraft. As Maccone has shown, detecting lower frequencies pushes the focus further from the Sun — the focal distance, in other words, changes as a result of frequency.

We’ve learned how valuable information about the CMB is to cosmologists. Now imagine the result of examining the CMB with the vast magnifications possible through a FOCAL probe. But a second choice is also available. FOCAL could be optimized for close study of the Alpha Centauri stars, especially if current efforts pay off and we do find interesting planets around Centauri A or B. Centauri demands a different kind of mission because it is far from the ecliptic. The flight path is problematic because the Centauri stars are so close, requiring ion propulsion to achieve the necessary spiral trajectory.

Addendum: So many readers have mentioned Dr. Maccone’s recent SETI Institute lecture that I want to go ahead and link to it now, although I was planning a separate piece on it next week. When I met with Claudio recently in Austin, he was getting ready to leave for the West Coast to make this presentation before concluding his US trip and heading back to Italy. What a pleasure it was to talk to him at leisure about FOCAL.

But all of these are matters that now need to be taken to the next step at the International Astronautical Congress, where they will gain further visibility in the scientific and industrial community. Papers are solicited on the propulsion problem — is a solar sail optimal? Nuclear-electric? Perhaps VASIMR? We also hope for submissions on the scientific return from a FOCAL mission, on telecommunications technologies, on computing requirements, and perhaps on the social and cultural value of a concept that would take human technologies further from the Sun than any previous missions.

focal_cover

Image: The FOCAL mission as currently envisioned by Claudio Maccone. The image is taken from the cover of his book Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer/Praxis, 2009), and shows two 12-meter antennae operating through a tether which is gradually released, allowing a field of view much larger than that offered by a single antenna. Credit: Claudio Maccone/Springer.

The preliminary program for the Prague IAC has already been posted. The deadline for submitting abstracts to the Congress is 5 March 2010. Let me quote from the IAC documentation on what the criteria for selection will be:

Paper selection

Submitted abstracts will be evaluated by the Session Chairs on the basis of technical quality. Any relevance to the Congress main theme of ‘Space for human benefit and exploration’ will be considered as an advantage.

The criteria for the selection will be defined according to the following specifications:

* Abstracts should specify: purpose, methodology, results and conclusions.

* Abstracts should indicate that substantive technical and/or programmatic content is included

* Abstracts should clearly indicate that the material is new and original; explain why and how.

* Prospective authors should certify that the paper was not presented at a previous meeting and that financing and attendance of an author at the respective IAC at Prague to present the paper is assured.

Full information about the meeting and the submission process is available through the official Call for Papers & Registration of Interest.

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NS December 18, 2009 at 15:00

A lecture by Maccone describing the FOCAL mission is shown here (YouTube):

http://www.youtube.com/watch?v=ObvKVe5H8pc

Administrator December 18, 2009 at 15:40

Thanks, NS. I had planned a separate article on the lecture next week, but a number of people noted it and asked for the link, so I’ve added it in, as you suggested.

Enzo December 18, 2009 at 22:30

“Moreover, whereas with an optical lens light diverges after the focus, light focused by the Sun’s gravitational lens stays fixed along the focal axis. Every point along the straight trajectory beyond 550 AU remains a focal point for any vehicle we put on this trajectory.”

I have heard this many times, but there must be a limit to the distance you can use a star as gravitational lense, otherwise we could use nearby stars from Earth. What am I missing ?

Administrator December 18, 2009 at 22:46

Enzo, I want Claudio to answer this himself, and have just forwarded your comment to him, so I’ll hope to have his thought on this soon.

Eniac December 18, 2009 at 22:48

It would be good to know what the flight path really requires for observing nearby stars and planets. Since we need a lot of propellant just to get there, there should be sizable leftovers for tracking. But we need to know what the requirements are. Has anyone done flightpath calculations on FOCAL, at all?

Eniac December 19, 2009 at 1:14

I much appreciated Claudio’s lecture. Thank you for posting the link. One thing that concerns me is the point he makes about Alpha Centauri being much too large to observe. What was the size scale used for this assertion?

Clearly, the stellar orbits, and even a planetary orbit would be much too large to fit into a FOCAL field of view. But imaging at that scale would not be useful, anyway, because your scanning spiral would be unlikely to ever hit a star or planet, and you would be observing empty space, mostly.

It seems much more appropriate to image a stellar or planetary disk, and that appears quite possible at distances tonearby stars. Alpha Centauri is about 280,000 AU from the sun, i.e. the ratio between object distance and focal length is 280000/550 ~ 500. Thus a planet of Earth size (r=6000 km) would require a tether of 24 km to cover with a single “snapshot”, i.e. one run through the spiral. With a shorter tether, we’d get more of a close-up, but that is not necessarily a bad thing. Over time, we can go through many spirals on different spots and patch the pictures together.

Claudio seems focussed entirely on radio astronomy, and he did not make clear to me why. From what I understand, optical wavelengths are better. Higher resolution, higher gain, less coronal interference, and smaller image sensors. Is there any reason at all to start with microwave frequencies?

Given the above distance ratio (Object / image ~ 500), we can estimate the acceleration necessary to keep the spacecraft aligned with an accelerating target, such as an orbiting planet. Using the equation for centripetal acceleration v^2/r and an orbital velocity of 30 km/h and r = 1 AU, I get for the acceleration of Earth in its orbit a mere 0.006 m/s^2 (please correct me). The acceleration needed for the spacecraft to track a planet similar to Earth should scale with the same distance ratio we used above, meaning that we need to apply an acceleration of 1.2*10^-5 m/s^2. This ought to be quite feasible with the same propulsion system that can get us out there in the first place. Precision should not be a problem either, there are interferometers (planned or flying?) that rely on much more precise formation flying.

The thing that is so absolutely stunning about this is that it allows us to image exoplanets with a resolution similar to what we have in our own solar system. We can have high resolution images of exoplanets hanging on our classroom walls in a couple of decades. Surely there will be one among them that we would really want to go to, in the slightly longer term. This is much bigger than the cosmic microwave background or the galactic black hole, in my humble opinion.

Administrator December 19, 2009 at 9:07

Enzo wrote yesterday:

I have heard this many times, but there must be a limit to the distance you can use a star as gravitational lense, otherwise we could use nearby stars from Earth. What am I missing ?

Here’s Dr. Maccone’s response:

“You are missing the ATTENUATION of the power of the incoming radio signals across the vast interstellar distances. The power of the incoming radio
signals of course decreases with the inverse of the square of the distance. Since the nearest stars to us are at distances higher than 4.4 light years (Alpha Centauri system) it turns out that the attenuation caused by the distance BEATS the focusing effect of the nearby stars, and we see…
nothing magnified.

“In addition, the ALIGNEMENT between the radio source, the center of the
magnifying nearby star, and the surface of the Earth must be NEARLY PERFECT, i.e. within a few hundreds of meters compared to light-year stellar
distances. Since “everything moves” (i.e. the Earth moves around the Sun
etc., but also the magnifying star and the radio source move, and both are
many light-years away, the required alignment is extremely unlikely to
occur, and, if it does so, it is very short-lasting.”

Administrator December 19, 2009 at 9:08

Eniac, I’ll also forward your questions to Claudio and post his response here.

Eniac December 20, 2009 at 19:44

Correction: “an orbital velocity of 30 km/h” should, of course, be “30 km/s” instead.

Eniac December 20, 2009 at 20:37

With respect to Dr. Maccones’s response to Enzo’s question, I think it should be noted that what Enzo is proposing is actually being done, and is called “microlensing”.

I believe the first part of Dr. Maccone’s answer may need revision. Microlensing would not work the way it does if it was true. I think that what is going on is that, while there is, of course, attentuation with the square of distance, it is counteracted by an increase in aperture as the Einstein ring expands. I have not looked at the math of this, but it seems to me that in order to observe microlensing between two far-away stars as we do, the net attenuation of amplified light with distance can be at most linear, otherwise the attenuation would quickly beat the amplification, as Dr. Maccone says, and microlensing would not be observed.

Dr. Maccones second argument is of course correct, and the reason for the limited utility of microlensing. The events are rare, and any planets found are a one-time fleeting observation that can never be followed up upon.

Bounty December 21, 2009 at 15:44

Along the same lines, should we be sending up probes to hover in pre calculated microlensing locations?

Eniac December 21, 2009 at 23:42

I think to do that we would have to know the location of a substantial fraction of the stars in our galaxy to a precision that is far from realistically possible.

Eniac December 23, 2009 at 10:26

I have found two interesting papers, one old and one recent. The old one:

Liebes, Phys. Rev. 133, B835–B844, 1963
http://prola.aps.org/abstract/PR/v133/i3B/pB835_1

Has an interesting prediction that should be verifiable, although I have never heard of such phenomena: “As a result of their gravitational lens action upon the light from other stars, all stars are surrounded by radial spikes of concentrated light intensity. Such spikes might occasionally manifest themselves as luminous beams in cosmic dust or gas clouds.”

The recent one:

Koechlin et al, Exp Astron (2005) 20:307–315
http://www.springerlink.com/content/c1412861hu857254/

is available in full pdf and contains interesting and relevant analysis of the predicted imaging capability of the sun as a gravitational lens. As I skim it, it does not throw any wrenches into the idea of imaging nearby exoplanets, but it does not explicitly treat that application, either. The focus here is on X-ray and gamma astronomy, it seems.

Possible problems with imaging exoplanets may be low light levels and the inconvenient point spread function. If anyone knows an analysis of this, please let us know.

Eniac December 29, 2009 at 14:54

Any word from Dr. Maccone on the matter of observing Alpha Centauri?

Administrator December 29, 2009 at 15:39

Not yet, but I think he’s traveling over the holidays. Let me try again, Eniac.

Administrator January 1, 2010 at 22:43

Re Eniac’s question above, Dr. Maccone has now supplied an answer:

“My initial idea was that we should spiral up the trajectory of FOCAL in
order to cover a large field of view and thus DISCOVER possible PLANETS in
the Alpha Centauri system. However, my reader is right in suggesting that we
should rather concentrate on planetary disks, thus avoiding “observing
nothing” for most of the time. This planetary disk observation is feasible
because the tether length is reasonable (24 km), as he correctly points out.

“Equally valuable is his suggestion to watch by OPTICAL frequencies rather
than by radio frequencies. We must then, however, place an optical telescope
aboard the FOCAL spacecraft, and this both increases the spacecraft’s mass
(thus decreasing the spacecraft cruise speed, under equal launch conditions)
and complicates its design, rather than just for a “radio antenna
spacecraft” (inflatable, in my mind). But it could be done, of course. If I
never considered an OPTICAL spacecraft in detail, this is simply because
FOCAL was initially (< 1995) thought to be useful basically just for SETI.
The discovery of Hot Jupiters and more (since 1995), however, changed the
FOCAL goals by shifting them from SETI to Exoplanets Research! So, only now
it plainly appears that Optical would be better than SETI, at least for the
FOCAL mission intended to explore the Alpha Centauri system, rather than the
Galaxy bulge.

“At this point I would be grateful if my reader could WRITE A PAPER about his
“new” suggestions to exploit the FOCAL mission. He might present this paper
at the coming IAC in Prague. Later, if he agrees, his paper might also
become a further CHAPTER or an APPENDIX to the future, second edition of my
book “Deep Space Flight and Communications”. OK?

“Thanks very much to this reader and I wish him to become a Member of the
FOCAL Team in view of submitting a Proposal to NASA and/or other Space
Agencies.

With Best Regards and Wishes to all those contributing to the FOCAL space
mission.”

Eniac January 4, 2010 at 19:26

Thanks to Dr. Maccone for taking the time to address my suggestions. I am honored by his suggestion to write them up in more detail and join his team, but being busy with my day job in a biotechnology start-up I am not sure how far I could possibly take that.

One thing I would like to note is that according to a back of the envelope calculation I did not too long ago on this forum, an optical coronagraph with an aperture of around 10 cm should have sufficient resolution to resolve the Einstein ring from the sun, so the bulk/weight advantage might not be so clearly on the side of radio detectors as Dr. Maccone suggests.

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