Wow, I knew the physics but never realized the implication: where on earth hitting the brakes is easy and costs next to zero energy, in space breaking takes just as much energy as accelerating.
On my first attempt to get into orbit, I loaded up my rocket with as many engines and boosters as I could strap on and fired the rocket straight up. The rocket breached the atmosphere but to my surprise the rocket came straight back down. I literally thought that launching rockets into orbit meant shooting them really high until they were "in space" haha. I then quickly learned that being in orbit means having a horizontal momentum that is balanced with the gravitational pull of a given body. This balance produces a curved elliptical trajectory.
That's because if you fire your rockets to accelerate you end up in a higher orbit, i.e. your circle is much bigger, and vice versa, to overtake/catch up you want to fall to a lower orbit. That's also not intuitive to someone with "earth-experience".
What's the deal with that?
What's confusing about transfers is that elliptical orbits are orbits where your velocity varies based on where you are in the orbit. Your speed is highest at perigee and lowest at apogee. In fact, your speed at perigee will be higher than the speed of an object in a circular orbit at that altitude. The reverse is true at apogee; your speed will be slower than an object in a circular orbit at the same altitude.
So if you and your docking target are both in circular orbits, and you increase your speed in order to catch up, then you will simultaneously be raising the other side of your orbit, half a turn away. The difference at your current location will be minute, but will grow every continuously over that half a turn. If you're docking in the next few seconds it won't be a problem (the docking mechanism will handle a small offset, and it equalizes your speeds provided the difference isn't too much). If you're still several minutes away, however, you might find that the extra radial velocity means that you fly over the top of your target.
If you miss and continue to follow your new orbit you will indeed find that your overall time to orbit is now higher than your target's. It may be many orbits before you again have a close approach with your target, although if the difference in speeds is small then it may appear from your perspective that you are in fact orbiting your target.
The moon is going much, much faster than satellites in LEO. But it has so much farther a trip, it takes orders of magnitude longer to complete an orbit.
If you're behind a satellite in orbit and want to catch up, counterintuitively you have to decelerate. This slows you, but puts you in a smaller orbit. The effect of the smaller orbit greatly outweighs any loss in absolute velocity, so you catch up to your target.
(1) Your speed is higher; you cover the same distance in less time.
(2) The distance around the earth is larger than it would be if you were lower down.
> I literally thought that launching rockets into orbit meant shooting them really high until they were "in space"
It makes sense that having a high horizontal speed keeps you in orbit if orbit is your goal, but with enough firepower you should be able to make it to a point where you don't fall backwards anymore, right? Or at least where falling is negligible enough that you can stay in that spot for a while.
From the point of view of the game, several of my little green men can attest that coming home from a solar orbit is rather difficult (energy-intensive, time-intensive, or both)
If the plan is to end up in Earth's orbit, you don't want to shoot so far as to leave our sphere of influence. Earth's gravity drawing you towards it is a key component of orbit. But the other component is having enough horizontal (relative to the surface) to keep missing it - and that's what was missing from the parent's "what goes up, must come down" flights.
It could even have a smaller engine of its own; it'll take a lot less time to slow down something that's 10-1% the weight of the entire package.
Using a star a solar sail can change it's speed by ~60km/sec by accelerating much much slower than that for months. The problem is the further from a star get the lower your acceleration. To maximize final speed you basically want to get as close to the star as you can in a huge elliptical orbit and then on your final pass just keep accelerating. Or do the reverse on the way back.
Is that clear?
Further, the input power is fixed from the sun so the absolute best you could do would be 100% sail that's a single atom thick. However, that sail would be quickly destroyed by the sun and other things. Make an ultra thin sail with a tiny little cargo Say 98% weight of the sail and 2% cargo and dropping the cargo below that does not help much. You end up getting around 60km / second which is really really fast 1km/s = fast sniper bullet. But, we are comparing it to something going 4% of light speed which is insanely fast.
Another consideration is if rocket stops working after 10 years on a fly by you still eventually fly by the general area, but if you want to stop well that's just not happening.
Assume you want 1,000 years to Alpha Centauri and your ship boosts the entire way. (It'll take a few pictures as it flies through the system.)
s = 1/2 a t^2
a = 4.22 light years * 2 / ((1000 years)*(1000 years)) = 8 E-5 m/s/s
v = a * t = 4.22 light years * 2 / (1000 years) = 2.5E6 m/s = 0.8 c (!)
Then from the rocket equation you'll need a dry weight (payload, engines, etc.) to propellant ratio of
e^(2500000 / (9.8 * 8000) = 70,579,094,313,732 = 71 quadrillion
eg, 1 gram of spacecraft requires 70 kilotons of propellant
E = 1/2 * m * (v^2) = 2E20 Joule
Since E = m c^2
=> total annihilation of over a ton of antimatter
Note: I am not a rocket scientist and likely made mistakes. Even if I'm off by a few zeros, it's enough to show that an ion engine isn't going to get something to Alpha Centari within a few centuries.
Distance is irrelevant when it comes to space travel. Unless an object is within the vicinity of the Oort cloud, we stand very little chance of actually leaving this solar system in a controlled fashion.
There are ways, but they are expensive and not without risk. Nuclear pulse propulsion (orion) can yield vast acceleration, and fusion rockets, while unproven, are feasible. You then have less exotic options such as laser driven light-sails and nuclear powered ion drives.
If we were prepared to throw half a trillion bucks at the problem we could see a flyby in our lifetimes - but without a damn good reason, it won't happen.
Proxima Centauri has a luminosity of 0.0017 suns , so Earth-like conditions would occur at ~sqrt(0.0017) au = 0.04 au, an apparent angular separation of arcsin(0.04 au / 4.2 light years) = 0.03". JWST wants star-planet separations at least an order of magnitude larger , ideally with very hot, infrared-bright planets.
E-ELT should be able to resolve them, according to . Page 7 gives values for a very similar scenario: an Earth-like planet around an M dwarf (like Proxima), 6 pc away (5 times farther), at an angular separation of 0.015" (half as wide). In this scenario, E-ELT could image the star and planet as separate points, and take useful spectroscopic measurements of the planet's light. For it example it could detect a spectral line of oxygen (O2) in 4 hours of exposure time.
angle(in degrees) = size / distance / pi ⋅ 180°, take times 3600 for arc seconds
I didn't find a easy round number for James Webb, but Hubble's angular resolution is 1/10th of an arcsecond, at least a factor 2000 too low to even resolve the planet's disk as one pixel.
> The specification is that the telescope is diffraction limited at 2 μm, which means a Strehl ratio of 0.8 and a wavefront error of 150 nm rms. With a 6.5 m telescope, 1.22 λ/D = 0.077 arcsec at 2 μm. The smallest pixels (NIRCam 0.6-2.5 μm) are just 0.034 arcsec. But a lot of the wavefront error is due to imperfect alignment of the parts, and it's possible to do better for a small part of the field of view.
A telescope can still detect something smaller than its resolution. We can see stars with our eyes, after all. Even astronauts in space, where there is no atmosphere to move the light around. Here are some of the best attempts at direct imaging with a telescope: http://www.planetary.org/explore/space-topics/exoplanets/dir...
The problem is the planet is dim and close to a bright star. If this planet is 1 AU out, then there's a 0.77 arcsec between it and the the star, which would make it easier to distinguish the two.
That seems a hell of a lot more practical and cost effective
Getting a probe there in a reasonable time is entirely feasible if can figure out how to keep the darned thing accelerating without stupendous fuel requirements. I'm confident we'll figure that out sooner or later.
Stop thinking about outer space when we are in the middle of an anthropogenic mass extinction here on planet Earth.
Well, we don't know that it's the most habitable; it's just the most habitable we know of. And huge sections of it are completely unsuited for human life. 2/3rds of it will kill a human being in a matter of hours. Anything above or below, let's call it 50 degrees latitude is inhospitable without technology, and the equator isn't a great place to live, either.
OTOH I disagree with the notion that we have to have a full grasp on how to sustain us on this planet before we can make other planets inhabitable for us. I think there are things we will only learn if we try. I am not saying that trying right now is feasible, though.
Invasive species don't co-evolve to exploit the ecosystems they stumble upon. They happen upon a ecological space where there are incredibly low hanging fruit that they are better equipped at taking advantage of. Imagine space-faring humans stumbling upon a world where the oceans resemble that of the Earth's as they once existed in the 1500's--a world densely populated with an abundance of marine life, ripe for harvesting.
While there is some probability that biochemistry is not 100% universal (ie. alien metabolites could be toxic to humans or cause an immune response), I imagine there are scenarios where the same set of molecules and polymers are used. The principles of biochemistry are going to be similar in a lot of places in terms of how energy is exploited and harnessed.
I think it's incredibly more likely that humans will never be interstellar travelers en masse than to say that Earth is the optimal naturally occurring human habitat in the universe. As far as we know there is an infinite space full of limitless planets. Plenty of biological solution space with which to theoretically work / exploit.
In that case I think you should dial it back a bit. Humans are pretty adaptable. Even assuming the changes in temperature and sea-level are on the high end of predictions, we will adapt. People will move, infrastructure will be built, and so on. Mass extinction certainly isn't going to happen from global warming as we understand it today.
You might argue there there will be mass casualties, but that is also uncertain, difficult to quantify, and is quite a different thing than 'mass extinctions'. Of course, mass casualties due to weather has and is already a problem -- it isn't something that is uniquely associated with global warming.
I assume the parent is referring to the https://en.wikipedia.org/wiki/Holocene_extinction which is certainly happening, and has been for thousands of years.
Climate change is a rather recent contributor, albeit a very serious one of which we've only glimpsed the beginning.
Well, we'll see.