Thursday, January 20, 2011

Slingshots and Cosmic Coincidences

It's easy to get dizzy thinking of humanity reaching out into the Universe. Especially if you have a head full of space opera with FTL drives, wormholes and other 'standard' transportation techniques.

No wonder it feels disappointing in the extreme to watch the progress of us Earth-dwellers into the local space.

Our first steps into the solar system will be much along the lines of what has gone before. Chemical propulsion will likely remain the standard for planetary lift-off for some time to come. If we are lucky, perhaps we can satisfy the EPA and use Nuclear Thermal Rockets, which might double the available impulse. Reentry will remain much more economical with technology used since the space race - i.e. heat shields and parachutes - while space vehicles of the like of the Shuttle will retire to museums.

When it comes to getting around the solar system, again we will probably be relying on chemical propulsion - all the current manned Mars missions are based on this. Again NTR rockets may offer potential in the future if the safety concerns can be addressed. For missions where time is not no much of an issue, electric rockets (ion drives) with their highly respectable exit velocity of 30 kilometers per second are capable of bringing spacecraft to high interplanetary velocities (but their low thrust means they will never get us to orbit).

Here's where the slingshot manoeuvres come in. These have been used to great success in the early interplanetary probes such as the Voyagers. Basically, these rely on the principle of conservation of momentum. The same principle as slamming a cue ball into a billiard ball to get the billiard ball in the pocket. The momentum is transferred from the cue ball into the billiard ball but the momentum of the whole system stays the same. In this case it is the angular momentum of the two bodies movement around the sun that is conserved. The tiny spaceship takes a low trajectory over a big planet like Jupiter, and is shot out of the planet's gravitational field at ninety degrees to its original direction of travel. The spaceship is now on a new trajectory that does not centre on the sun, and its angular velocity has increased by the same amount as Jupiter's Sun-orbital velocity of 13 kilometers per second (while Jupiter's angular momentum decreases by a minuscule amount).

Voyager 1 used a gravity slingshot manoeuvre to get to its present velocity of 17 kilometers per second. (Try not to think about the fact that it would take 70,000 years at this speed to reach our closest stellar neighbours.)

But surfing gravity wells can be taken further than this. If a spaceship can apply thrust during a slingshot manoeuvre it can capitalise on the orbital mechanics even further. If a probe approaches the sun within 1.5 million kilometers along a parabolic solar orbit, then increases its velocity by 2 kilometers per second, it will leave the Solar System at an impressive 41 kilometers per second.

NASA's Mariner 10, which performed flybys of Mercury in 1974 and 1975 relied on Venus gravity-assist manoeuvres to get it into position. The Messenger probe to Mercury - the first probe to visit the planet in 30 years - will be going into orbit around Mercury later this year. It will be the first space craft ever to do so. To get some idea of the crazy orbital scheme required to get it there, here is a diagram of its journey since launch.

Talking about cosmic coincidences, has anyone noticed that the gravity of both Mars and Mercury is 38% of Earth standard? I had to check that twice. Given the fact that Mercury may have water in its dark side craters (its tidally locked), it makes you think of the possibilities.

The other one that always gets me is that both the Moon and the Sun are exactly the same angular size in the sky.

What other cosmic coincidences have you noticed?


MataPam said...

There may be thresholds of sizes of bodies that resist break up. Beside Mars and Mercury, there are six other bodies in the 3-7k diameter class, all moons of various planets.

All the giant planets are believed to have rocky or metallic cores about the size of Earth. Much compressed, due to the pressure, their mass is up to 15 earths, in the case of Jupiter. How close are we to the threshold where a planet begins to accumulate hydrogen, and bulks up into a Gas Giant? It's probably all a matter of timing, with the Sun igniting and the solar wind sweeping the hydrogen out of the inner solar systen before Earth and Venus accumulated enough to hold onto it.

It would be interesting to see what happens to Gas Giants when their stars mature. Do all those elderly red giant stars have the Earth-size-range rocky cores of former gas giants still orbiting them? What odd minerals may have formed, where the metallic hydrogen met the higher elements? And will we name one of them "Unobtainium?"

Chris McMahon said...

Hi, Matapam. An interesting idea, that for sure. To think that in some far flung future there may be a habitable world orbing a red giant:)

Interestingly many of the worlds discovered are large gas giants very near stars.

I think the theory is that there were many Mars and Mercury size worlds in the early years of the solar system, but some of these collided with others. Our current Earth formed after a collision with another solar system body of smaller or similar size - The Giant Impact Hypothesis - the body is called Theia after a greek Titan who was the mother of the Moon.

The theory is that the two early bodies collided and their cores combined, explaining out strong magnetic field, while a whole lot of the outer material 'sloshed' off into space to form the Moon.

The Asteroid belt is supposed to be the result of a collison of two verge large bodies.

MataPam said...

I suspect that last collision is the reason we have such a thin atmosphere, compared to Venus's thick one.

The conditions for the formation of gas giants is interesting. If I were a astrophysicist instead of a dilitant, I'd check and seeif perhaps those systems with close giants have noticibly higher angular momentum than systems without.

Higher angular momentum in the original cloud would delay the infall of material into the protostar, and give close-in planets time to bulk up.

I tend to be skeptical of theories about Jupiter forming close in and then moving out, somehow leaving the inner system with tidy well spaced planets in near circular orbits.

I figure at the end of the Sun's main sequence life, if human's are still just a local species, we can hide out inside Jupiter's magnetic field, and hope to find habitable planets further out, once the Sun has settled down to a nice quite retirement.

Chris McMahon said...

Hi, Matapam. I suspect you may be right about the angular momentum, I seem to recall that the systems with close-in gas giants have amazing fast orbits, which would probably result from system with a lot of spin to begin with.

Venus really intrigues me. It should be the next Earth, given its size, potential atmosphere and its near-Earth gravity, but its totally in the too-hard basket. Frustrating!

Kate Paulk said...

Venus has a lot of potential for some really cool bioengineering experiments.

Suppose you developed bacteria that ate sulfur and carbon dioxide, and excreted nitrogen and oxygen, then seeded the upper Venusian atmosphere with them?

Given a good enough start and monitoring, you could gradually reshape Venus's atmosphere to something more suitable for us.

Of course, who knows what we'd find when we eventually got down to the surface? It's rather difficult to sort out what you've got in an atmosphere that melts lead.

Chris McMahon said...

Hi, Kate. They reckon that the surface of Venus may actually go molten every so often in a cycle - a damn lot of heat trapped on that planet. It needs some serious cooling down.