Suzi.

Is it in that a great word.

Good first gravel city in astronomy is a co alignment.

A lining up of three bodies in some sort of gravitational line.

So we're used to this all the time when we have eclipses.

We've got a city of the earth and the sun and the moon, different combinations, depending on which kind of eclipse it is.

And yesterday there was a city in the solar system because the planet Jupiter was in opposition.

And that means it is directly opposite the sun as seen from Earth.

So we're here on earth.

The sun is in one direction, Jupiter's directly opposite, which makes it great for viewing.

And we got a clear view.

It's up all nights.

It's really bright and very nice.

And so this made me think, How did we get from one planet to another?

If Jupiter is an opposition, which means it's pretty much just close is gonna get to us.

Wouldn't this be the best time to be sending a mission to Jupiter.

And the answer, of course, is no.

And so what we're gonna explore in this video today is the challenges of moving around the solar system when everything else is in motion and we've got gravity to contend with.

One of my favorite recent Siri's, has been the expanse.

Have you seen the expanse?

You would love the expanse.

It's, Ah, story that's entirely set in the solar system so that the solar system has been colonized.

The other planets Sini in the asteroid belt, have been colonized.

A riel prominent character in the stories is gravity is the challenges of getting around the solar system, the challenges of growing up and living in lower gravity environments what that means, and particularly the challenges of getting from one place to the other and everything that that involves.

So that's kind of what I wanted to talk about today because, you know, all the graphics that we see of the solar system have planets lined up.

You know, in a line you would see that obviously not to scale and Australia's upside.

How much I like crazy just for you Did that just for you And and even in science fiction that this happens a lot.

You think, right, you're gonna Jupiter and then you go a little further.

And yet to Saturn, there's no Pluto's.

Pluto's not a planet.

Wait.

So the point is the planets or not, we all know this.

The planets are not arranged in this nice line.

Just because we get to Jupiter doesn't mean that if we go a little further, we get to Saturn because these things are actually orbiting in circles around the sun or lips is.

And you know, Jupiter might be over here in one configuration, and Saturn might be way over here.

Have they ever been in a perfect line?

I'm sure we could cycle back through the solar system calculator and find lots of sister.

She's in the solar system.

I don't think it's very likely that everything has been in the line, but that would be anything to look for.

Maybe that's where they started really trolling me today.

Okay, so this is a view of where the solar system planets are today, at least the inner solar system starting from Jupiter, Jupiter with its spot there on, then Mars is over here and then because I said that Jupiter is an opposition.

That means that the Earth and the sun and Jupiter in a straight line and then further in you've got Venus and you've got mercury and the inner planets.

They're zipping around much faster.

The outer planets are moving much more slowly.

So let's imagine now that we just want to get from Earth to Jupiter.

So how would we do that?

We wouldn't send out a rocket pointed at Jupiter right now, because Jupiter's moving Jupiter's moving slowly, the earth is moving a little bit more quickly, and so what do we d'oh?

The most efficient way to get from one orbit to another is something called ah Hohmann Transfer orbit.

And this is beautiful and simple because all it needs are two impulses to changes of velocity, and that will get you from Earth to orbit eventually to Jupiter's orbit.

It's not the fastest way.

It's the most efficient way uses the least amount of fuel, which is obviously going to be a big consideration when you're traveling around the solar system.

That might not be the best way to do it in terms of human passengers and protecting them for radiation, keeping them fed, watered and everything but in terms of pure rocket efficiency.

Now I know from past experience that we've got many viewers out there who are participants in the curb all space program, which is a solar system, simulate.

And I know that when I talk about the home and transfer orbits, the hairs on the back of their neck will be going up and saying, OK, yes, but there's more complicated orbits and there's more things to think about, and that's absolutely true.

This, as many things are in our explanations of physics, is simplified.

This is the simplest orbits.

Assuming that the two bodies air perfectly in the same plane, it's assuming that they're starting on very circular orbits, and it's neglecting the gravitational effects from other bodies.

That being said, this is how you would get from Earth to Jupiter, using a home and transfer orbit.

All you have to dio is at the right point.

You want to launch your spacecraft from Earth orbit will neglect.

Getting off the earth will seem, were in the same orbit around the sun as the earth is and all you want to do is add velocity in a tangential direction.

So we want to give it an instantaneous boost that way.

What is that gonna d'oh We've known for hundreds of years since Kepler came up with his laws that beautifully described motions of the planets that the planets orbit the sun in elliptical orbits with the sun at one focus.

So if we're starting from a relatively circular orbit here and we add that impulse, we make this into bigger ellipse.

If we do it just right, will then follow an elliptical path.

And if we time it just right, that elliptical orbit will have its furthest distance from the sun.

Its appeal Ian right at the intersection with Jupiter's orbit.

So that gets us where we want to go.

Now we just have to get the timing right.

And so what we have to calculate is when to make that maneuver in order that we catch up with Jupiter and we crossed that orbit.

It's not Earth is going, and we crossed that orbit.

Justus Jupiter gets there.

So that's only half the story, though, because if that's all, we d'oh will fall back in on this orbit and will continue back into this nice, elliptical orbit that will cross Earth's orbit at one end and Jupiter's orbit at the other.

And when Space X launched the Tesla Roadster into orbit and said it was going to Mars, this is what it did.

It went into an orbit that took it across the orbit of Mars.

But then it's come back again, and it's gonna be in that Mars earth loop until something else changes it so you can track it.

I found a Web site, so it it is on its way back to Earth's orbit, not to earth.

Just like it didn't get to Mars because Mars wasn't there when it crossed Mars orbit.

It's gonna come back to Earth's orbit, but Earth is gonna be on the other side of the sun.

We're one day.

We'll get to one of them one day.

Well, one of them happened to be there when it will either get back to work, presumably, if it lasts long enough and you forward this infant infinitely.

But what's more likely is that it will be broken up or disturbed or or something.

The chances of it actually hitting.

Neither is very right.

So we sent our spacecraft on a transfer orbit to Jupiter.

When we get there, if we don't do anything else to the spacecraft, it's just gonna follow this elliptical orbit.

We need to change its velocity again so that we match the orbital speed of Jupiter on.

So then it apply one more impulse.

So that's all you need is to impulses to changes in velocity, one of Earth's orbit, one of Jupiter's orbit tangential to the direction of the planets.

Travel.

It's called Circular Rising.

The orbit.

You're gonna change from that lip to Corbett into a nice circular orbit matching what is the impulse when we get to Jupiter's over, When we get to do better, we have to apply another tangential kick in velocity to speed up to match Jupiter's orbital speed.

Now, if you wanted to get into Jupiter's orbit around Jupiter or if you want the land on Jupiter, that would require even more velocity change in terms of rocket trajectories.

We talked about a Delta V budget, so delta is the Greek symbol that we use for change, and Delta V is just the total amount of velocity changes you need to make your maneuver happen.

Forget Jupiter for a minute for trying to land on Mars.

Some of that Delta V could be acquired through aero braking.

If you want to land on a planet, you need to slow down or you're just gonna slam right into it.

So there's some atmosphere on Mars not very much, but some of that velocity you can lose by aero braking.

This would take 33 months to get from Earth to Jupiter by this orbital transfer, and that might not be what you want.

So, for example, the New Horizons spacecraft was sent on a mission to explore Ploo toe and a transfer orbit.

To get from Earth's orbits to Ploo toe would take 46 years, which isn't so good.

If you're trying to plan your career and solve scientific problems, that's a lot of waiting around in actual flag.

New Horizons got ploo toe in 8.5 years.

How did that work?

Well, two things.

First of all, it wasn't trying to insert itself into orbits at Pluto's distance from the sun or around Ploo toe.

It just want screaming by really, really fast.

And so there you don't have to worry about these beautiful lips is you just apply as much rocket fuel as you can in the right direction at the right time, and off you go.

And so when New Horizons launched, it was the fastest object made by humans launched from the Earth.

Hey, so what's screaming away from the earth at about 16 kilometers per second?

And that's enough to escape the solar system.

It was helped on the way by passing by Jupiter.

So, as as many of the outer system probes have, it got a little kick, got a gravity boost by flying by Jupiter, and by falling towards Jupiter, it slowed down Jupiter, a teeny tiny amounts, and some of that momentum was transferred to the spacecraft and sort of swing it out even further into the outer solar system.

This was used famously by the Voyager two spacecraft when the NASA scientists planning the mission realized there was a very fortuitous alignment of planets, which meant that they could visit not just Jupiter and Saturn, but then changed the trajectory to swing around and see Uranus and Neptune as well.

It always seems funny to me that this doesn't slow them down.

Surely I would have thought there were ways in which going by something like Jupiter would slow down, hurtling out of the solar system.

Yeah, absolutely.

It all depends on the direction in which you approach it.

And so these gravity slingshots can be used to slope spacecraft down just a CZ they can to speed them up.

And that's in fact, how we get to the inner solar system.

You can imagine the solar system is.

Is this big?

Well, this gravity well, And to get out, you have to apply energy and escape.

And so you might think, Well, it must be easy, then to just roll down the hill and drop down into the inner solar system.

But that neglects the fact that we're traveling really fast in a tangential direction around the sun.

That's why we don't fall into the sun.

It's because we're traveling really, really, really fast in a sideways direction.

That's why when we try to send things into orbit around the earth, we don't just send things straight up, and then they don't come down again.

We send things up, getting through the atmosphere, and then we send them sideways really, really fast.

And what happens then is that they're constantly falling towards the earth.

But they're falling so fast that they missing it continuously, falling round and round the other side just as we're continuously missing the sun as we fall around the solar system.

And so what do we need to do to get into the inner solar system?

We need to slow down some of that sideways motion.

So we do the same thing.

Apply a tangential Delta V to our spacecraft velocity.

But this time we're not trying to add velocity to it.

We're trying to take away velocity, and when we take away some of that sideways earth motion, we start to fall towards the inner solar system.

And this is how we've got spacecraft Venus to Mercury and, most recently, how we've sent the Parker Solar probe towards the sun lift off of the mighty Delta four heavy rocket with NASA's Parker Solar probe a daring mission to shed light on the mysteries of our closest star, the Sun.

This is a spacecraft that is designed to get as close as we've ever been to the sun.

It will actually pass within 10 times the radius of the sun.

It'll fly through the solar corona, the outer atmosphere of the sun, to try to learn about it.

It's an incredible mission.

It's not a straightforward maneuver to just cancel out all of this Earth's orbital motion again.

They are using this time Venus for a fly by.

And this time we're gonna do exactly what you suggested is come at it from the other direction, which will spook Venus up a tiny bit and slow the Parker solar probe down enough to gradually change its orbit into a nice, tight elliptical orbit that's going to send it, eventually screaming around the sun at 0.6% the speed of light when it reaches its maximum orbital velocity in 2024 and it's gonna take seven.

Fly buys a Venus to make that happen.

So how did like current Mars probes get there?

Pretty much.

This is the way that we are restricted to at the moment until we build a much bigger propulsion systems or much more efficient propulsion systems were simply stuck with the fact that doing anything else would require so much fuel both on the beginning end.

But also at the other end that it's just not practical to do anything but the most efficient maneuvers.

Now there are obviously some subtleties.

There are some extra things that you can do here.