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 currently proposed 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.)

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 – went into orbit around Mercury on the 18th March 2011. It was the first space craft ever to do so. So far its found significant water in the planet’s exosphere, evidence of past volanic activity and evidence for a liquid planetary core. To get some idea of the crazy orbital scheme required to get it there, here is a diagram of its journey since launch.

Messenger 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?

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