Putting the Solar System in Context

by Paul Gilster on December 3, 2013

Yesterday I mentioned that we don’t know yet where New Horizons will ultimately end up on a map of the night sky like the ones used in a recent IEEE Spectrum article to illustrate the journeys of the Voyagers and Pioneers. We’ll know more once future encounters with Kuiper Belt objects are taken into account. But the thought of New Horizons reminds me that Jon Lomberg will be talking about the New Horizons Message Initiative, as well as the Galaxy Garden he has created in Hawaii, today at the Arthur C. Clarke Center at UC San Diego. The talk will be streamed live at: http://calit2.net/webcasting/jwplayer/index.php, with the webcast slated to begin at approximately 2045 EST, or 0145 UTC.

While both the Voyagers and the Pioneers carried artifacts representing humanity, New Horizons may have its message uploaded to the spacecraft’s memory, its collected images and perhaps sounds ‘crowdsourced’ from people around the world after the spacecraft’s encounter with Pluto/Charon. That, at least, is the plan, but we need your signature on the New Horizons petition to make it happen. The first 10,000 to sign will have their names uploaded to the spacecraft, assuming all goes well and NASA approval is forthcoming. Please help by signing. In backing the New Horizons Message Initiative, principal investigator Alan Stern has said that it will “inspire and engage people to think about SETI and New Horizons in new ways.”

Artifacts, whether in computer memory or physical form like Voyager’s Golden Record, are really about how we see ourselves and our place in the universe. On that score, it’s heartening to see the kind of article I talked about yesterday in IEEE Spectrum, discussing where our probes are heading. When the Voyagers finished their planetary flybys, many people thought their missions were over. But even beyond their continued delivery of data as they cross the heliopause, the Voyagers are now awakening a larger interest in what lies beyond the Solar System. Even if they take tens of thousands of years to come remotely close to another star, the fact is that they are still traveling, and we’re seeing our system in this broader context.

The primary Alpha Centauri stars — Centauri A and B — are about 4.35 light years away. Proxima Centauri is actually a bit closer, at 4.22 light years. It’s easy enough to work out, using Voyager’s 17.3 kilometers per second velocity, that it would take over 73,000 years to travel the 4.22 light years that separate us from Proxima, but as we saw yesterday, we have to do more than take distance into account. Motion is significant, and the Alpha Centauri stars (I am assuming Proxima Centauri is gravitationally bound to A and B, which seems likely) are moving with a mean radial velocity of 25.1 ± 0.3 km/s towards the Solar System.

We’re talking about long time frames, to be sure. In about 28,000 years, having moved into the constellation Hydra as seen from Earth, Alpha Centauri will close to 3.26 light years of the Solar System before beginning to move away. So while we can say that Voyager 1 would take 73,000 years to cross the 4.22 light years that currently separate us from Proxima Centauri, the question of how long it would take Voyager 1 to get to Alpha Centauri given the relative motion of each remains to be solved. I leave this exercise to those more mathematically inclined than myself, but hope one or more readers will share their results in the comments.


Image: A Hubble image of Proxima Centauri taken with the observatory’s Wide Field and Planetary Camera 2. Centauri A and B are out of the frame. Credit: ESA/Hubble & NASA.

We saw yesterday that both Voyagers are moving toward stars that are moving in our direction, Voyager 1 toward Gliese 445 and Voyager 2 toward Ross 248. When travel times are in the tens of thousands of years, it helps to be moving toward something that is coming even faster towards you, which is why Voyager 1 closes to 1.6 light years of Gl 445 in 40,000 years. But these are hardly the only stars moving in our direction. Barnard’s Star, which shows the largest known proper motion of any star relative to the Solar System, is approaching at around 140 kilometers per second. Its closest approach should be around 9800 AD, when it will close to 3.75 light years. By then, of course, Alpha Centauri will have moved even closer to the Sun.

When we talk about interstellar probes, we’re obviously hoping to move a good deal faster, but it’s interesting to realize that our motion through the galaxy sets up a wide variety of stellar encounters. Epsilon Indi, currently some 11.8 light years away, is moving at about 90 kilometers per second relative to the Sun, and will close to 10.6 light years in about 17,000 years, a distance roughly similar to Tau Ceti’s as it will be in the sky of 43,000 years from now.

And as I learned from Erik Anderson’s splendid Vistas of Many Worlds, the star Gliese 710 is one of the most interesting in terms of close encounters. It’s currently 64 light years away in the constellation Serpens, but give it 1.4 million years and Gl 710 will move within 50,000 AU. That’s clearly in our wheelhouse, for 50,000 AU is the realm of the Oort Cloud comets, and we can only imagine what effects the passage of a star this close to the Sun will have on disturbing the cometary cloud. If humans are around this far in the future, GL 710 will give us an interstellar destination right on our doorstep as it swings by on its galactic journey.


{ 12 comments… read them below or add one }

stephen December 3, 2013 at 20:37

This web page describes the motion of Alpha Centauri through our skies during thousands of years in the past and into the future.


And somebody has compiled useful information for astrogators heading for Alpha Centauri, Barnard’s Star, Sirius, Procyon, Epsilon Eridani, 61 Cygni, Tau Ceti, and Gamma Virginis.

He has a bunch of other related information as well.


Astronist December 3, 2013 at 20:39

How long it would take a twin of Voyager 1 to reach Alpha Centauri? — a trick question, and I fell for it by trying to work it out! Am now kicking myself!

The star system is currently moving towards us at 25 km/s. In 28,000 years it will be at closest approach of 3.26 light-years, while a ship moving at the speed of Voyager 1 will only have covered half that distance (1.62 light-years). Therefore in 56,000 years the stars will, by symmetry, be moving away from us at 25 km/s, and the Voyager twin will still not yet have reached it. Therefore anything leaving the Solar System now at the speed of poor old Voyager 1 will just never catch up!

Stephen A.

Paul Gilster December 3, 2013 at 22:19

Well you have to admit, it was an interesting question… ;-)

Ron S December 3, 2013 at 22:46

Astronist: “Therefore anything leaving the Solar System now at the speed of poor old Voyager 1 will just never catch up!”

You just need to change trajectory a bit. Aim in the opposite direction and meet AC as it comes around again in its galactic orbit. It’ll just take a little longer. About 200 million years. Remember to keep some fuel around for a course-correction burn a million years or so before rendezvous.

Astronist December 4, 2013 at 6:53

Yes, it is interesting. We think of the stars as fixed and the spacecraft as moving, but Alpha Centauri is actually moving at about 31 km/s relative to the Sun, so nearly twice as fast as Voyager 1.

(By a “trick question”, I meant one whose solution, or lack thereof, would be obvious to someone with a good mathematical imagination without needing to start writing down equations.)

A related question: what would be the minimum speed required to get a spacecraft to Alpha Centauri, starting today? I think that it would need to arrive when the stars are at closest approach to the Sun (not sure at the moment how to prove this). Then in order to cover 3.26 light-years in 28,000 years the probe (or indeed worldship) would need to travel at 0.000116 c, taking 8589 years to travel each light-year, thus an interstellar cruising speed of 34.9 km/s, about twice that of Voyager 1. (But we cannot now build any kind of vehicle that would still be in working order after 28,000 years!)

Stephen A.

kzb December 4, 2013 at 8:53

I’ve been thinking for some time that this stellar diffusion is an argument against those who portray interstellar travel as impossible. It intensifies the Fermi paradox.

Imagine civilisations that are a few kpc nearer the galactic centre than us. The space density of stars is several times greater than at our galactic radius, and sub-light-year approaches are more common (at least on galactic, rather than human, timescales). So these civilisations have correspondingly easier star-hop, even if they are always limited to a few % of c.

ljk December 4, 2013 at 10:12

It is unfortunate that this wonderful project idea or something similar to it was not added aboard New Horizons before it left Earth. Despite what the mission team said, it could have been done by an outside group on their own time just as the Voyager Interstellar Records were accomplished.

Now we have to hope that NASA will go along with the NHMI. I also hope this will inspire others to make sure that every deep space mission carries information about us and our world for any potential finders. To send vessels into the Milky Way galaxy without some kind of identification is both short-sighted and a wasted opportunity to preserve a record of ourselves.

What is on the New Horizons probe now (so far):


Mephane December 5, 2013 at 3:33

“If humans are around this far in the future, GL 710 will give us an interstellar destination right on our doorstep as it swings by on its galactic journey.”

I don’t have any numbers, wouldn’t just the distance matter when travelling to another star system, but also the relative velocity between the sun and the destination star? Unless a probe or ship is going for a mere flyby, wouldn’t that also require matching galactic orbits? What amounts of delta V would be required, for example, to break into a solar orbit around GL 710 in that scenario?

Christopher Phoenix December 5, 2013 at 19:08

@Mephane In principle, yes, but since even fast-moving stars are only moving at a speed of a few tens of km/sec relative to us and the distances between the stars are so great, the ∆v requirements of the speeds required for a useful starship dwarf the ∆v requirements of solar escape and matching galactic orbits with the destination.

Even a small fraction of C is measured in thousands of km/sec., not tens. Clearly the ∆v requirements of reaching even 1% C (3,000 km/sec) will dwarf the solar escape velocity and orbit-matching requirements of the trip!

Andrew Palfreyman December 5, 2013 at 23:03

The speed of our technological development greatly outpaces considerations about close approaches measured in tens of thousands of years hence (assuming we can stay on track). We will know more physics and we will continue to have 10^26 Watts at our disposal.

Astronist December 6, 2013 at 9:01

Ron S, an interesting thought. But 200 million years is the period of the Sun’s orbit around the galactic centre, and the period of Alpha Centauri’s orbit is presumably very similar. Depending on the geometry of the two orbits, they may intersect again after half a galactic revolution, thus after only (!) 100 million years. But if the two periods are significantly different, which will be the case if the semi-major axes are different, then the Sun and Alpha Centauri may not make another close approach for billions of years (by which time all stellar orbits will begin to suffer disruption as M31, M32 and M33 merge with the Milky Way). Meanwhile the operational lifetime of any spacecraft we can conceive would be measured in at most thousands of years…

Stephen A.

Eniac December 10, 2013 at 0:28

kzb mentions a great point. Usually we think about us (or hypthetical ETI) spreading into the galaxy in a spherical type of expansion. In reality, any such spread will be more like the spread of a drop of milk in a stirred cup of tea: Much faster and more thorough than mere diffusion. The galaxy is more like a stirred brewing vat than a petri dish…

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