Pedal to the Metal

by Paul Gilster on March 24, 2011

We have a long way to go before we can get a probe to another star in the space of a human lifetime. The figure always cited here is the heliocentric speed of Voyager 1, some 17.05 kilometers per second, which is faster than any of our outward bound spacecraft but would take well over 70,000 years to reach Alpha Centauri, assuming Voyager 1 were pointed in that direction. New Horizons is currently making 15.73 kilometers per second on its way to a Pluto/Charon flyby in July of 2015, impressive but not the kind of speed that would get us to interstellar probe territory.

Interestingly, the fastest spacecraft ever built weren’t headed out of the Solar System at all, but in toward the Sun. The Helios probes were West German vehicles launched by NASA, one in 1974, the other in 1976, producing successful missions to study conditions close to the Sun for a period of over ten years. The orbits of these two craft were highly elliptical, and at closest approach to the Sun, each reached speeds in the range of 70 kilometers per second. Helios II, marginally faster, lays claim to being the fastest man-made object in history.

It’s fun to juggle these numbers even as we think about how far we have to go before an interstellar probe becomes a possibility. If the goal is to reach Alpha Centauri with a mission lasting, say, forty years, then we need a tenth of lightspeed, or roughly 30,000 kilometers per second. That makes .10c a figure of distinction, because it creates a mission that can be built, flown and studied to completion by the same team. I’ve long argued that the goal of missions that could be completed within the lifetime of a researcher is one we’ll ultimately have to ignore, because for a time our deep probes beyond the system are going to take a much longer time than that. But get us to a tenth of c and that goal becomes possible, at least in terms of Centauri A and B.

Image: Smoke and steam fill the launch pad in January of 2006 as New Horizons roars into the blue sky aboard an Atlas V rocket. No craft ever had a faster Earth escape velocity, but the fastest spacecraft now exiting our Solar System is Voyager 1. Credit: NASA.

With the recent success of the MESSENGER probe in reaching Mercury orbit, it’s worth pointing out that another mission design from the Johns Hopkins University Applied Physics Laboratory (JHU/APL) would create a spacecraft even faster than the Helios probes. Like Helios, the Solar Probe Plus is headed for the Sun, with a goal of studying the outer corona. In its approach to within 8.5 solar radii (that’s just 0.04 AU), Solar Probe Plus would achieve a velocity of close to 200 km/sec, three times faster than Helios II. In interstellar terms, that works out to 6450 years to reach Centauri A and B, better than Voyager 1 but a numbingly long voyage even for a generation ship. Thus the difference between where we are now and where we would like to be.

New Horizons just passed the orbit of Uranus on March 18, described by an update on the mission site as “the fastest spacecraft ever launched.” And in a sense, that’s also true — New Horizons left Earth orbit traveling faster than any previous vehicle launched into interplanetary space. But the expected speed at Pluto/Charon encounter is about 14 kilometers per second, and it’s unlikely that any conceivable gravitational assist in the outer system could boost its speed to surpass Voyager 1’s. In any case, a recent tweet from the New Horizons team says there will be no Pluto gravity assist because it would impinge upon the scientific investigation of Pluto and its moons.

The good news, though, is that New Horizons should have enough fuel after Pluto encounter for one or even two Kuiper Belt Object flybys, targets that will not be selected until 2015. We thus keep our eyes on the coming flyby in the outer Solar System, with every indication that all is well aboard the spacecraft. A successful New Horizons should highlight yet another JHU/APL initiative, the Innovative Interstellar Explorer, a design intended to push out to 200 AU. Slowly, but with ever increasing steps, we’re learning our way around this Solar System and sketching out the regions beyond it. The propulsion challenges ahead are clearly defined, and energizing.

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Scott G March 24, 2011 at 11:06

Aside from better propulsion systems, we clearly need to reduce the weight of our spacecraft if we ever hope to achieve the high velocity required by an interstellar probe. For example…

At 722 kg and traveling at a heliocentric velocity of 17.05 km/s (17,050 m/s), Voyager 1 has a kinetic energy of KE = 0.5*722*(17,050^2) = 1.0494*10^11 Joules (that’s ~100 billion Joules).

A spacecraft with the same kinetic energy, but with a mass of only 1 kg would have a corresponding velocity of v = ((1.0494*10^11)/(0.5*1))^0.5 = 4.581*10^5 m/s.

That is, a 1 kg spacecraft (with the same KE as Voyager 1) would be traveling at roughly 458 km/s instead of 17 km/s. So that leaves us with only 2 orders of magnitude left to reach a velocity on the order of 0.10 c.

Scott Monaghan March 24, 2011 at 14:17

Scott G,

I like the way you’re thinking. Strap a high powered laser sail to an iPhone and we’ll be good to go. There’s probably already an app for that.

- Scott M

The Cosmist March 24, 2011 at 15:24

I can’t help thinking that basing our calculations on current human lifespans and physiology is going to look pretty silly once interstellar flight becomes possible. I’d be very surprised if the dominant species on this planet in another century or two has the same limitations as we do. If we want Earth-based intelligent life to reach the stars, we need to work on ways to move beyond these extremely limiting primate substrates at least as much as on space propulsion systems!

David March 24, 2011 at 19:46

I had some dumb questions.
How fast is the IIE supposed to be?
How fast could a 1 kg probe get blasted by a Saturn V off the Moon or better yet an asteroid?
How much of a laser would a take to accelerate a sail with the probe in the sail structure to 10% light? I assume the whole mass might be less than a kg here with nanotechnology

ant6n March 24, 2011 at 21:56

Just build the whole damn probe on one wafer!

JohnHunt March 24, 2011 at 23:27

I think that the other commenters are making the same points that I would make. Reduce mass, use beamed energy, consider longer travel times, and consider a different form of humanity. One additional thing that I would add would be to consider launching multiple modules separately and then let them merge together in flight in order to create a larger, more capable craft. Until we get beyond the conceptual limit of large craft, short journey, and scientific purpose, interstellar flight will seem nearly impossible.

NS March 25, 2011 at 18:32

The IIE is supposed to attain a speed of 7.9 AU/year, getting out 200 AU from the sun 30 years after launch. The proposed mission uses a gravity assist from Jupiter so the launch windows are in 12-year intervals from 2014 on.

Personally, even if the IIE was launched in 2014 I would be 88 years old when it arrives at 200 AU, so I’m hopeful some faster way to interstellar space will be found in my lifetime. There’s been discussion here of “sun diver” solar sails that might fill the bill.

Martin Alfredsson March 25, 2011 at 20:47

The crafts sent out from earth has not been designed to be high speed.
High speed have problems like energy consumption structural integrity (dont know how important in space) and flyby times.
Crafts sent from earth use gravity assist to speed up are extremly energy efficiant and are supposed to study the objects they pass or orbit.
Create a nuclear probe with the sole purpurse to go as fast as possible then lets se how fast it will go…

Adam March 26, 2011 at 5:06

For just 30 GW of in-space laser power, Jordin Kare’s Sail-Beam can push a 1 ton probe to 0.1c. Whether we can give a 1 ton probe enough power to do anything useful at Alpha Centauri when it gets there is a whole other question.

Alternatively the “probe on a chip” concept might allow a string of billions of micro-probes to be launched towards Alpha Centauri and relay their data back to us for much less power. The concept requires 0.1 c to be achieved via Jupiter’s magnetosphere and some tricky electrodynamics. Plus billions of chip-probes.

stephen March 26, 2011 at 11:22

How often would we have to use a gravitational slingshot to accelerate a probe to anywhere near .1c? Should we accelerate the probe first and then use the slingshot x number of times? Or should we use the slingshot first x number of times, and then accelerate using fuel? Or would it matter?

How close could a white dwarf or neutron star be, for us not to have detected it by now? We could use that for the gravitational slingshot.

I’ve read about negative mass; a probe which is exactly half negative mass, would have a total mass of zero…I don’t know if that would help any; we don’t know how to make negative mass yet.

Is that helpful?

Thanks.

Interstellar Bill March 26, 2011 at 15:47

A flyby at 0.1c could terminate spectacularly with a deliberate collision with a planet, so that the one ton causes a million and a half megaton explosion. If 10% of the energy becomes a 5-second visible light pulse that is emitted into a hemisphere, then at 6 light years it would appear as 24th magnitude, just barely visible to our biggest telescope.

David March 26, 2011 at 18:56

http://adsabs.harvard.edu/abs/1995SPIE.2525..161W

If we have these why arent we trying for speed ,
I think it would be seriously worthwhile to try to pedal to the metal an IPOD just to try it. What happened to the days when we thought it worthwhile to just try one thing instead of trying to do everything at once

Adam March 26, 2011 at 23:58

Hi All

Interstellar Bill, the kinetic energy of 1,000 kg at 0.1c is the equivalent of ~110 megatons, not 1.5 million. A big bang, but relatively minor compared to an asteroid or other giant impactors.

stephen, gravitational slingshots can’t happen twice with the same object and be hyperbolic orbits. Even Jupiter can only give a boost of ~13 km/s at most.

Single stars can’t give “gravitational slingshots”, but they can be used for an Oberth maneuver. A large brown dwarf would be ideal because the temperature experienced would be mild compared to the equivalent Solar “fryby” for the most effective Oberth boost. A 50 jupiter mass brown-dwarf would allow a ~100 km/s boost for a 10 km/s burn at closest approach.

Solar mass neutron stars or black holes would need to be about ~10,000 AU away for their gravitational influence to have escaped detection. Ideally the condensed mass needs to be a binary, to work as a Dyson Gravitational Machine – a pair of brown dwarfs would allow a boost to ~0.002 c (600 km/s), a pair of White dwarfs (which would be too bright to be so close) can boost a vehicle to about ~0.01 c, and a binary neutron star can boost a vehicle to ~0.27 c, though the tidal forces would be extreme.

Personally I think the binary brown dwarfs are the most exciting option, if they’re sufficiently close for a gravity machine.

Astronist March 27, 2011 at 18:42

Paul, you’re not comparing like with like! Yes, certainly in one sense Solar Probe Plus will become the fastest manmade object, but then so would anything dropped so deep into the Sun’s gravitational well. What matters from a propulsion point of view is a vehicle’s total energy, kinetic plus potential, and here Voyager 1 wins easily. The Helios and Solar probes don’t even have enough kinetic energy to get out to Voyager 1′s current distance of 116.6 AU, let alone escape the Sun’s gravity altogether!

Stephen
Oxford, UK

John March 27, 2011 at 21:45

Today we can accelerate subatomic particles to approximately 0.99c, which is a good start. Accelerating a massive object like, let’s say a human to such speed would be an enormous waste of energy and human capital. Fortunately, scientific knowledge and technological development will continue apace to the point where miniaturization brings us to the point of functional nano-scale devices.

I envision a fleet of thousands of nano-scale self-replicating probes sent to a promising star system. There they would seek out resources on asteroids to build a relay station to keep in contact with us (either on Earth or elsewhere) and also build robust probes to investigate the planets for possible inhabitants or potential inhabitation. If there is a candidate planet for human habitation then, and then would the slow-moving generation starships with humans on board leave the solar system.

Erik Anderson March 28, 2011 at 11:49

To consider just how pathetic Voyager’s 17 km/s speed is in the context of voyaging to Alpha Centauri, consider that Alpha Centauri itself has a heliocentric space velocity of 32.6 km/s and will come to within 3.2 light years of the sun in just 27,700 years. After that date, its distance from the sun will begin increasing. I’ll leave it as an exercise to the reader to figure out if 17 km/s is fast enough to intercept Alpha Centauri ~at all~.

Rob Henry March 28, 2011 at 16:40

Astronist, you raise the point of whether heliocentric hyperbolic excess is a truer record of manmade rocket speed. In my opinion its not that clear-cut, since a small extra boost to Helios at perihelion would transform much of its peak velocity into your suggested measure of velocity at infinity.

Eniac March 29, 2011 at 9:33

According to the formulas on the Oberth effect on Wikipedia, if we approach the sun as closely as Solar Probe Plus, and if there is enough time at perihelion to finish the entire boost, we get an amplification of delta-v of almost 7-fold. I believe a Saturn V or any similar rocket can provide 10-15 km/s to a normal sized payload, maybe 20 or so to a small payload with an extra stage. In total, then we should be able to reach over 100 km/s of escape with a chemical rocket, provided we can safely bring it to 0.04 AU of the sun and operate it there. It may be more feasible to use a sail, which can provide good thrust at that distance. In the end, anything substantially more than 100 km/s will certainly require a nuclear drive of some kind.

Steven Colyer March 29, 2011 at 11:12
Astronist March 29, 2011 at 15:06

Rob Henry and Eniac,

Suppose we have Solar Probe Plus at perihelion (velocity = 200 km/s, distance from Sun = 0.04 AU). Then the delta-V needed to get it from there onto a course for Jupiter (where it can benefit from a Voyager-style gravity assist) works out as 9.9 km/s, thus rather greater than the velocity change needed to reach LEO from Earth’s surface. I doubt whether Solar Probe Plus will have this delta-V capability at its disposal.

But suppose it had been launched away from the Sun in the first place? Its actual V-infinity relative to Earth will be around 2.8 km/s (it needs to position itself for the first of seven Venus encounters). The V-infinity relative to Earth to get to Jupiter is around 8.8 km/s, if my maths is correct, so it would not be capable of reaching Jupiter directly. Of course it might still get to Jupiter Cassini-style, with successive Venus and Earth encounters to pump up its orbit, and thence via Jupiter encounter to solar escape velocity. But it would not thereby greatly exceed (if at all) what Voyager has demonstrated in practice.

To my mind the figure of merit in discussions of interstellar probes is the hyperbolic excess velocity, or V-infinity. It is essentially the speed at which almost all of any interstellar cruise will be conducted. In the case of Voyager 1 this is 16.60 km/s, or 0.000055 c. This is the figure to beat!

It is presumably about as good as one can get using gravity assists from Jupiter and Saturn. I may be wrong about this, but perhaps not enormously wrong. Gravity assists are only really meaningful at interplanetary velocities in the tens of km/s. For interstellar velocities (thousands of km/s and upwards) only some kind of direct artificial propulsion will do, like the VASIMR engine being proposed for Icarus Pathfinder.

Thanks for a fascinating discussion.

Stephen
Oxford, UK

Eniac March 29, 2011 at 16:10

Astronist: I was suggesting putting a fully fueled launch rocket where Solar Probe is, not using Solar Probe’s limited propulsion capabilities. My calculation remains showing a V-infinity of ~100 km/s under these (admittedly farfetched) circumstances, which would beat Voyager by quite a bit. I doubt that a Jupiter flyby would add very much to that, as I believe high initial velocity reduces what can be gained from a flyby.

Rob Henry March 29, 2011 at 16:19

Astronist, your solar probe is mighty unusual. The lowest energy path to the Sun from the Earth is via Jovian gravitational assist. Thus I would expect my solar probe to have its original aphelion beyond Jupiter’s orbit. I suspect that you are using figures after your probe has already killed speed at a previous perihelion in order to have an orbit that spends longer in the vicinity of the sun to facilitate its further study.

Oh, and don’t you feel slightly uncomfortable with Helios having an imaginary velocity for the purposes of the “Astronist book of Records”.

Astronist March 29, 2011 at 20:26

Rob Henry and Eniac, thanks.

Solar Probe Plus is planned to make seven Venus flybys to pump down its perihelion, but not to go out to Jupiter (according to http://solarprobe.jhuapl.edu/). Helios 1 and 2 flew simple ellipses with aphelion at Earth’s orbit (Wikipedia article). Ulysses of course did the Jupiter manoeuvre, but I think that was more to change its orbital plane than to get particularly close in to the Sun.

Have any solar probes so far actually used a Jupiter flyby to lower their perihelion close in to the Sun? I can’t think of any right now (though this was at one point suggested for Solar Probe Plus, it doesn’t seem to be the current plan).

As for Helios having an imaginary velocity at infinity, all that means is that it has negative total energy, i.e. insufficient energy to escape the Sun. And if we’re thinking of going to another star, we need total energy (kinetic plus potential gravitational energy) greater than zero just to get started!

Stephen

Rob Henry March 30, 2011 at 20:06

Adam, what a great thought it is to speculate about the consequences of finding a very nearby and close pair of brown dwarfs. It had never struck me that they could inspire us to launch our first interstellar probe until you pointed out the potential. It almost seems a pity that we will not be able to so speculate for much longer.

As for the potential of white dwarfs to interstellar travel, surely there only practical use could be in helping change the trajectories of a interstellar shuttles as the run around huge circuits of stars within an established empire or federation?

Rob Henry March 30, 2011 at 20:44

Perhaps I should elaborate on my previous comment. I realise that a probe sent via the “gravity machine” sent by Adam would take a few thousand years, to get to its destination, but I believe that is not the reason that we currently do not make more of an attempt. To me the problem with starting this process is that the promise of slightly faster travel is always around the corner. With the finding of a pair of brown dwarfs of just the right characteristics we would have a method of delivery within a speed window that could not be beaten for a long time. Thus we might think of building the first probe right now.

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