It seems that Voyager 1 has recently (or is about to, depending on your source) left the solar system. It’s taken 35.5 years to travel 123 AU. If we were to send out another deep space probe, could today’s technology propel it out to 123 AU significantly faster? How many years could we cut off the travel time?
Voyager owes most of its speed to gravity assist from their flybys of Jupiter and Saturn. New Horizons, launched in 2006, left earth at a faster speed and did a Jupiter gravity assist, but with only one gravity assist, it will never reach the speeds of the Voyagers.
As far as I know, there is no new proven technology that allows for much faster probes. But it’s certainly possible to scale up existing technology, i.e. putting a very small probe on the largest rocket available, and doing multiple gravity assist.
A solar sail craft can achieve 30 AU/year according to Wiki.
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Well, let us put that in the actual context from which it was cited:A sailcraft with σ = 1 g/m² could achieve over 30 AU/yr (0.000474 c) in cruise (by keeping the perihelion at 0.2 AU),…
So, a sailcraft massing about a third of a ping pong ball per square meter of sail area can make good time inside the orbit of Mercury. Given that solar flux–and therefore, presumably, speed–scales as to a square of the distance from the Sun, we would be looking at ~1.2 AU/y at Earth orbit, -~0.5 AU/y at Mars, a paltry 0.04 AU/y at Jupiter, 0.01 AU/y at Saturn, and after that it is arguable that the level of thrust–less than 1% of what it would see at Earth orbit, or 0.04% of the thrust level at 0.2 AU–would be enough to overcome the drag of the interplanetary medium. Solar sails, therefore, are only useful inside the asteroid belt.
scr4 is correct that there are no demonstated propulsion systems which can provide high or sustained theust levels (although several missions including NASA Deep Space 1, JAXA Hayabusa, and ESA Smart 1, have demonstrated low continuous thrust ion engines) but there are a number of concepts including nuclear pulse propulsion (ORION), nuclear thermal rocket (NERVA, Rover, Timberwind), radioisotope rocket (Poodle), nuclear electric (Prometheus), fission fragment, and nuclear liquid salt rocket which have either demonstrated significant proof of concept or have been evaluated as having feasibility compared to the existing state-of-the-art for interplanetary propulsion at useful thrust levels that could provide total delta-V comparable to or greater than gravity assist maneuvers, and also allow for a wider range of missions that are not dependant upon particular planetary configurations. The reasons these have not been further developed is a lack of “suitable” (i.e. military or immediate commercial) applications, but they certainly aren’t fantastical or require fundamental breakthroughs in physics or material science.
Stranger
NASA’s webpage on ion thrusters states that they could accelerate a space craft to 200,000 MPH. Based on some pretty quick and inaccurate calculations on my part I think that translates to 19 AU per year.
But how long would it take them to get to that speed?
That would be true if the spacecraft speed is an equilibrium point where drag equals thrust. But is that a reasonable approximation for a solar sail in the solar system? I thought photon pressure was much larger than any collisional effect.
I suppose a solar sail would reach that point eventually, where sunlight is so weak that thrust is equal to drag. But the spacecraft could jettison its sail at that point, and from then on, only be slowed by gravity (i.e. speed would decrease as inverse of distance to the sun, not inverse of square of distance).
Correct. For future reference - Google actually does this type of unit conversion automatically. You can just type in “200,000 mph in AU/year” and it gives you the answer.
Sure, you’ll have some degree of net positive thrust out to heliopause. But if you are talking about millinewtons per metric ton, who cares. At that point, you’ll get more mileage out of staring hard.
Stranger
The slow solar wind flows outward at around 84 AU/Y, so I kind of doubt aerodynamic drag would be much of an issue until reaching the heliopause.
My point was that in the inner solar system, I think it’s more accurate to approximate the spacecraft speed as inverse of distance from the sun, rather than inverse of square of distance.
Interesting (if very complicated) points. Voyager I made much of its speed gains via gravity assistance (which I should have accounted for in the OP). How optimized were those assists, and if not, how much could they be improved upon?
In other words, if you were tasked with sending a probe outside the heliosphere in the quickest (and economically-feasible) way possible with today’s tech, how would you do it?
You would certainly use gravity assists, but precisely which gravity assists, and in which order, would depend on your precise time of launch. As I understand it, the two Voyagers were able to take advantage of an unusually favorable arrangement of planets, and even with our better technology, we’d probably have to wait at least a few years for a similarly-favorable arrangement.
Both light pressure on the sail and acceleration due to the Sun’s gravity decrease as a factor of 1/r[SUP]2[/SUP]. Now, orbital speed decreases by a factor of 1/√r, but in terms of outward velocity we only care about the radial component which increases by the net acceleration times the interval.
The trajectory of the Voyager probes were optimized for the Grand Tour mission (visiting Jupiter, Saturn, Uranus, and Neptune), not for absolute maximum outward speed, but you are not going to see anything like an order of magnitude increase by just changing the trajectory. The only way to obtain a significant increase in net outward velocity is by carrying a propulsive stage into the gravity well of one of the heavier planets and performing an impulse maneuver during the swing-by pass (the so-called “Oberth maneuver”).
Stranger
if the solar sail, or any craft really, used perigee kicks it would increase its apoapsis - farthest distance from the sun - much faster and in theory be able to get onto a escape ‘orbit’ pretty quickly
I’m not really sure what point you are trying to make, but a solar sail doesn’t provide a “kick” (impulse); the entire point is that it provides continuous thrust without having to expend propellant mass. (The photons that transfer momentum to it are effectively the propellant.)
Of course, if you have a trajectory that heads down close to the Sun where the photon flux is higher, you will get more impulse, but there is no real advantage to doing so, especially since the change in momentum required to make a near solar pass (from Earth orbit) is much much greater than any benefit you would receive. The main advantage of swing-by maneuvers isn’t the absolute gain in speed (which at most is twice the orbital speed of the body–obviously zero for the Sun) but the ability to get “free” momentum transfer and resultant change in direction. Propulsive periapsis transits gain the advantage of the Oberth effect discussed above, but this doesn’t apply to solar sails since they are not carrying the “propellant” with them.
Stranger
How much mechanical pressure would the solar wind impart to a sail?
About 0.2 x 10[SUP]-6[/SUP] lbf/ft[SUP]2[/SUP].
Stranger