Updating the Gravitational Focus Mission

by Paul Gilster on July 24, 2009

If you’ll examine the cover of Claudio Maccone’s new book carefully, you’ll see an interesting object at the lower right. It’s a spacecraft with two deployed antennae connected by a tether. The book is Maccone’s Deep Space Flight and Communications, whose subtitle — ‘Exploiting the Sun as a Gravitational Lens’ — tells us much about the author’s view of how early interstellar missions should proceed. And Maccone devoted a session at the recent conference in Aosta to these matters, making the case for taking advantage of this natural phenomenon.

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Uses of Gravitational Lensing

We’ve looked at the Sun’s gravitational lens, and the FOCAL mission Maccone champions to exploit it, many times here on Centauri Dreams. But for newcomers, gravitational focusing has been an active astronomical tool since 1978, when a ‘twin’ image of a quasar was found by the British astronomer Dennis Walsh. The gravitational field of a galaxy between the Earth and the quasar had bent the light from the more distant object, yielding the double image. And it was back in 1979 that Von Eshleman studied the Sun’s own gravitational focus at 550 AU, pondering how we might send a spacecraft there to study its effects.

The Sun at that distance becomes a huge celestial magnifier, and Dr. Maccone has been arguing that before we send a mission to any star, we should send a probe to this much closer target in the exactly opposite direction. Diffracting effects from the Sun’s corona may distort the image at the minimum 550 AU distance, so we may have to go farther, but we’re still talking about getting a spacecraft to well less than 1000 AU to begin making its observations. In fact, the further the probe travels, the less distortion from the solar corona, and the focal line extends to infinity.

A Tethered Solution to the Antenna Problem

Below is an enlarged image of the FOCAL spacecraft with tether clearly visible. The key point is that sending such a mission would avail us little if we couldn’t take advantage of its trajectory to get an adequate image of the focused light from the other side of the Sun. Maccone told the assembled scientists in Aosta that working with a gravitationally-lensed image would involve more than a single antenna, but noted that tether technology could be used to solve the problem. Hence the twin antennae of the image.

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An email from Dr. Maccone this afternoon clarifies the idea of using what radio astronomers call ‘aperture synthesis’:

While the spacecraft is moving away from the Sun, the tether is gradually released, and so two Archimedean SPIRALS are covered by the two antennae, yielding a full radio picture of the radio source, with a FIELD of view MUCH LARGER than the one provided by a single antenna.

Tethers have numerous space applications but they’re tricky to test on the ground because we can’t simulate zero gravity conditions sufficiently to study their characteristics. That has made for numerous space missions using tethers, dating all the way back to the latter days of the Gemini program. But note this: Maccone is talking about tethers of no more than several kilometers in length. The largest tether ever deployed successfully was on the YES-2 experiment conducted by the European Space Agency in 2007. YES-2 deployed a 31.7 kilometer tether in space, and robust work continues on the tether concept.

Maccone is convinced that a tethered system of antennae can resolve the daunting imaging issues posed by a probe at 550 AU, with stunning capabilities at magnifying the object being imaged. Thus the possibility of getting high quality images of any planets of interest in a target solar system, down to small details on the planetary surface. It’s worth remembering, too, that the distance a FOCAL probe would need to reach to study the Centauri stars is 278 times smaller than the actual distance of those stars. And the view that is potentially achievable from that distance far surpasses anything available through other kinds of observatories.

An Idea with a History

Deep Space Flight and Communications (Springer, 2009) represents Maccone’s current thinking on FOCAL as it has evolved over the years. Be aware that he was discussing the idea as far back as a 1992 conference in Turin, and in 1993 submitted a formal proposal to the European Space Agency to fund the mission design. Many things have changed in the time since, all summarized in the new book. As he did in the book, Maccone developed his ideas on the Karhunen Loeve Transform (KLT) and its uses for signal processing and analysis at the Aosta conference, showing how the KLT becomes a tool for improving the signal-to-noise ratio in SETI work as well as for communications with a fast-moving FOCAL probe.

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Image: Claudio Maccone (right) explains the uses of the Karhunen Loeve Transform (KLT) in signal processing to me at a fine lunch we enjoyed in the Italian Alps (note the cheese plate that sits between us — fabulous!). This photo was snapped by Roman Kezerashvili (City University of New York).

It’s interesting to see the tether concept being applied here. And I also want to note that Michel van Pelt’s new book Space Tethers and Space Elevators (Copernicus, 2009) goes into considerable detail on the history of tether missions in space — the number is far higher than I had realized. I’ll have more to say about tethers and their uses once I’ve finished the book, but it’s clear that refining the tether concept is going to take time, and that its potential uses are so beneficial as to make it a high priority.

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{ 23 comments }

Thomas July 24, 2009 at 10:31

Reading this wonderful blog, I often see references to using the sun in such a way, however, I’ve never read about just what the ability of a telescope at 550 AU could actually see. So let’s say we’ve got a good telescope way out there at 550 AU, HST or JWST quality. What could such a telescope see in the Alpha Centauri system?

Administrator July 24, 2009 at 13:39

Thomas, Dr. Maccone at one point in his talk pointed out that with proper instrumentation, the magnification would be so extreme that we could see vehicles moving on the streets of a habitable planet around one of the Centauri stars, assuming such vehicles and streets exist. This is at visible wavelengths, and the lensing capabilities are even higher at microwave wavelengths, particularly the 1420 MHz hydrogen line, of great interest in SETI research.

Of course, all of this assumes a perfectly functioning system operating at the optimum distance from the Sun. Many objections have been raised that a probe like this would not be able to take advantage of these effects, which is one reason why Maccone has been working with tether concepts and multiple antennae to answer the critics. Right now he remains the chief advocate for FOCAL although there are many scientists who are skeptical.

Pat Galea July 24, 2009 at 13:56

I can confirm that Maccone’s book is an excellent guide to this topic. I have read it through once, and am now looking at the KLT information in greater detail.

I’m trying to get a full understanding of the tweaks that Maccone has made to the KLT (which improve the computation speed). Once I’ve done that, I hope to code it up in Mathematica so I can play with it in various scenarios.

The KLT looks like an extremely powerful technique for pulling almost non-existent signals out of the noise. It is, unfortunately, computationally expensive and not really parallelizable. That’s where the fun begins!

BTW One of Travis Taylor’s scifi stories uses the gravitational lensing effect to image distant worlds. I think it’s either in Warp Speed or The Quantum Connection (which is the sequel).

NS July 24, 2009 at 14:29

Could the basic concepts be tested on the ground or in near-Earth orbit, using something besides the sun as a gravity lens?

Mark July 24, 2009 at 14:58

Wow, seeing vehicles on the streets of Centauri system planets? This has intrigued me like no other subject in astronomy today. A few questions though:

1) How do you aim the telescope? For example, once out to 550 AU, how do you move the focus from one Centauri planet’s street to the next? I’d assume by moving the spacecraft side to side or up and down? If so, how far would you have to move the craft to focus on one planet or another?

2) How do you focus? Is everything in focus on the other side of the sun out to infinity? Or do you have to move back and forth (toward the sun and away) to pull the focus onto the desired object?

3) once out at 550 AU, do you “stop” in one place or do you have to orbit something? Can you just stop or would you be pulled back toward the sun? If you would be pulled back in, how much fuel would it take to counter this effect?

4) 550 AU is just over 3 light days one-way. If cost were no object, how fast can we get into observing position there using the highest speed spacecraft delivery system that we have on the shelf today?

5) If there are little green men in the Centauri system observing the earth, should we be able to focus 550 AU beyond the Centauri system and see their telescopes?

Administrator July 24, 2009 at 16:01

Mark, I’ll take a shot at your questions 3 and 4 — the others are beyond my expertise but I’ll see if Dr. Maccone is available for an answer.

3) once out at 550 AU, do you “stop” in one place or do you have to orbit something? Can you just stop or would you be pulled back toward the sun? If you would be pulled back in, how much fuel would it take to counter this effect?

No, you don’t stop. The focal line extends to infinity, which for practical purposes means you just keep going (the way the Voyager probes, for example, are still leaving the Solar System).

4) 550 AU is just over 3 light days one-way. If cost were no object, how fast can we get into observing position there using the highest speed spacecraft delivery system that we have on the shelf today?

This is quite tricky, because cost is such a variable. Ion propulsion is one possibility as are variously configured solar sails. Even so, we’re talking, realistically, at least fifty years at this point, and even that is quite a stretch. But that’s with very near-term tech — obviously we hope to find faster designs along the way (see the Haumea studies I reported on not so long ago, which are looking at missions using a scaled-up VASIMR engine, for example).

asj July 24, 2009 at 16:43

Did Dr. Maccone mention anything about the Sun’s internal focus at 24 AU?

Of course, the detection systems for gravitational waves and neutrinos would be completely different.

Pat Galea July 24, 2009 at 17:17

1. This is the very question I asked Maccone at the UK space conference. It is indeed the main difficulty in this kind of mission. We’re talking about maintaining the position of the craft within a matter of meters. We don’t have a solution to this problem yet. That’s why Maccone talks about having an initial mission to perform an extremely detailed survey of background radiation. This would not require the craft to focus on any specific point.

3. Right; the fact that the focus extends all the way to infinity is rather counter-intuitive when you’re accustomed to the concept of an optical focus. The idea is to send the craft out on a hyperbolic path, where the asymptotic ‘leg’ is in line with the target that you want to look at.

5. Yes. In fact, this is one of the ideas that Maccone talks about in his book (well, sort of). If we set up a colony at another star, a great way to communicate with Earth would be to have one craft at the Sun’s gravitational focus, and another craft at the colony star’s focus. This would make signals much easier to pass back and forth. (We’d still need to solve all the other problems about keeping both craft in line, of course.)

Another note: 550AU is the figure for our Sun. Other stars with different masses have different distances for their foci. (And as noted in the post, 550AU is the idealized figure; taking the corona into account, the actual usable focus is further out.)

Adam July 24, 2009 at 18:46

Stephen Baxter locates the alien teleport Gates at the Focal point of the Sun with Alpha Centauri in his book “Space” – a novel that teaches all about the Fermi Paradox’s various ‘solutions’ without the reader quite realising. Reid Malenfant travels to the Focal point in a nuclear pulse drive taking 6 years to do so.

D. Bay July 24, 2009 at 20:12

Relating the FOCAL mission to an earlier post re. climate engineering:

Currently we can’t verify our supercomputer climate models so that they are reliable predictors of changes due to intentional or accidental climate engineering, or rather, the verification would be after the fact and thus too late.

However a high resolution FOCAL survey of dozens or hundreds of Earth-like worlds would yield quit a bit of data about the effects of ocean, landmass distribution, cloud cover, atmospheric composition etc. on climate, perhaps sufficient to make our climate models robust enough to give us confidence about their predictions.

It seems to me that FOCAL is worth doing for that reason alone, and might persuade those who usually think of astronomy as being irrelevant to interests here on Earth to fund such a venture.

Administrator July 25, 2009 at 8:28

NS writes:

Could the basic concepts be tested on the ground or in near-Earth orbit, using something besides the sun as a gravity lens?

We could certainly run through the entire tether assembly with rigorous tests in near-Earth space, and would want to, and we can study gravitational lensing to some extent already, though on a much different scale. But as far as actually resolving the kind of images we want at 550 AU, I’m not sure how we could test the system on other objects. Interesting idea, and I imagine some of the readers may have further speculations.

Administrator July 25, 2009 at 8:40

asj writes:

Did Dr. Maccone mention anything about the Sun’s internal focus at 24 AU?

The Aosta presentation didn’t really get into that, but it’s quite interesting in terms of just the topics you mention, gravitational waves and neutrinos. For newcomers, the point is that while light rays cannot pass through the Sun’s interior, gravitational waves and neutrinos can, which yields the much closer foci, specifically at 22.45 AU (gravitational waves) and 29.59 AU (neutrinos). I’ve loaned my copy of Dr. Maccone’s new book to a friend, so I’m drawing these numbers from his older book on the same subject (the new one substantially revises it, so I’ll have to re-check the numbers). But these foci could indeed be useful in future detection attempts.

Wayne Farmer July 26, 2009 at 2:48

This design doesn’t seem to be among those mentioned in the Wikipedia article on gravitational lensing: http://en.wikipedia.org/wiki/Gravitational_lens . Adding a description there and a reference to the proposal could help stimulate discussion of the proposal.

Administrator July 26, 2009 at 8:20

Good idea, Wayne. I’ll see if we can get someone on this.

hiro July 27, 2009 at 18:49

If we can put LISA at 22.45 AU from Sol, then it’ll be quiet interesting to observe what is going to happen in near term future. The major problem is that we have to wait at least 5 years for spacecrafts traveling to that distance. Anyway, I think we can do this with our current technologies and it can be started sometime around 2020-30. The 550 AU mission heavily depends on ultra-modern propulsion technologies like fusion/antimatter rocket, so the mission period should be less than 25-30 years. Otherwise, we might have new technology to develop methods making faster spacecrafts.

Blade O'Grass July 29, 2009 at 4:50

Why not develop a four-bomb mini Orion/probe? Two blasts up, and two blasts to stop…or whatever floats your boat. Then all that’s needed is a VASMIR or ion drive for station keeping. We have the technology; It tests the theory with a model, and it could be done within everybodies life time.

I wanna see cute, fuzzy and blue colored Alpha Centaurian beings goin at it freestyle in the streets of their vesion of a New York city traffic snarl (the charnage)! Oh, and the astronomy would be good too. :)

Ronald August 6, 2009 at 13:45

(Just back from my vacation in northern Italy; I got jealous of Paul so I went there myself with my family ;-) , and a lot of reading to catch up with, lots of interesting stuff again).

Ref. Thomas and others with regard to Alpha Centauri: of course, open door, it would only be worthwhile to aim such a telescope at AC once it was confirmed by less ambitious instruments that there was something really (REALLY) interesting to look at, and those other instruments may actually come up with other even more fascinating targets.
However, I think I understood that such a gravitational focus telescope would have to be moved around quite a bit in order to aim it at some other target, depending of course on the (angular) difference in position. This might make it a rather cumbersome instrument to use for relatively nearby target stars.
Wouldn’t it be much more feasible and worthwhile to use such a gravitational focus telescope for a relatively distant and compact group of targets, such as e.g. (terrestrial) planet searches in the Andromeda galaxy, or even the Virgo cluster?

Administrator August 6, 2009 at 13:58

Ronald, I hope your northern Italian experience was as positive as mine. I’m sure it was.

Changing targets from star to star is obviously a major problem for a single FOCAL probe, so we have to assume a future where the cost of building such probes comes down enough for us to assign a probe to each system we want to investigate closely. Dr. Maccone favors the idea as a precursor study before any probe being sent to such a star, but as you point out, FOCAL could also be used for distant targets like Andromeda.

Ric Capucho November 11, 2009 at 13:04

So what’s the gravitational focal point of Jupiter? I can well imagine there’s a minimum mass necessary to actually reach focus, but one wonders if there’s somewhere a bit closer than that 550 AU…

Ric Capucho November 13, 2009 at 10:05

Erm, don’t bother answering that one. After a bit of a rethink it’s obvious that the focal point of a *less* massive object would be further away, not closer… and Jupiter’s focal point would be a looooooooooong way away.

Tiago March 15, 2010 at 16:49

Since 550 AU is already three days and four hours out from the Sun, perhaps letting the probe go farther and farther away is not such a good idea. However, if the probe itself is sufficiently light, perhaps designing it as a statite (as in Dr. Forward’s concept) would be enough for station-keeping without expending any fuel, just energy to vary the angle/reflectivity of the sail in parts. Then the station-keeping problem becomes just a computational problem.

Administrator March 15, 2010 at 17:56

Tiago, there is no need for station-keeping. The FOCAL probe continues to make observations beyond 550 AU because the focal line extends to infinity, which is one reason why a gravitational lens is so useful. As we go further out, we also have less trouble with coronal fluctuations. Stopping the probe would require additional propellant and would not result in any scientific gain.

Nick Stevens March 2, 2012 at 6:21

A bit late to the party, a fascinating concept!

I have a thought though. If we are aiming the scope at a nearby star, and the light is amplified by a factor of 10^8, how bright is that star at the FOCAL probe?

My naive calculation would be that 10^2 is five magnitudes, so 10^8 is 20 magnitudes brighter. Wikipedia has a full moon at magnitude -13, and the Sun at -27 (from Earth). So Sirius would maybe come in at -21.5?

That’s about the sun as seen from Saturn, but a rather rough calculation. A bit more and solar panels could become feasible!

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