11) Jool Probe-A-Thon

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Jool is an interesting planet in KSP. Based on Jupiter, it is a gas giant with several moons. One of them; Laythe, has oxygen, liquid water and an atmosphere similar to that of Kerbin (but thinner). The other moons I know little about at this point, I’ve tried to avoid ‘spoilers’ about what they’re like. I do know that there are 5 moons in total and that Laythe is the only one to have an atmosphere. I want to send manned missions to the area but knowing so little about Jool it seemed the right and space-programmy thing to do would be to send out some probes to study the planet and its moons first before sending hapless Kerbals.

I could have done 6 different missions but why do that when it would be more fun to try and do it all at once in one giant probe-a-thon!?


This will be a ‘high risk mission’. Basically that means that I have no idea what I’m doing. I haven’t tried something like this before and know very little about Jool so I expect at least some fails. But my main aim is to learn about Jool so hopefully my failures will teach me as much as the sucesses!


Flight 14 – Jool Probe-A-Thon

After designing a ship to carry all 6 probes (one for Jool and one for each moon), I plopped it on the launchpad for a final check-up.

And here it is!
And here it is!

The probes themselves are simple affairs, they are not meant to return to orbit so they only need enough equipment to succesfully land on the Jool bodies. They are all identical (so that the craft is balanced) with the exception of one that is destined for Laythe and so has it’s own parachutes. Other than that they consist of a small fuel tank, an engine, some scientific instruments, a solar panel and a probe body. They also had three landing legs which didn’t work out so well! But more on that later. The mid-stage that will take them to Jool is based on my standard interplanetary design, modified to be able to carry all the probes. The launcher was all SLS parts as the whole ship needs a lot of fuel and is a heavy beast!

After checking that all the stages made sense and Laythe parachutes weren’t going to deploy as I lifted off from Kerbin, the craft set off.

Blast off from Kerbin
Blast off from Kerbin
The interplanetary stage powers towards Jool
The interplanetary stage powers towards Jool
The probes have a looong flight ahead of them
The probes have a looong flight ahead of them ( I realise this isn’t the most efficient intercept but I really couldn’t be bothered waiting years for a decent window!)

On reaching the Jool system the first thing to do is perform an aerobrake around Jool. This is one of the main advantages of visiting Jool, establishing an orbit with other planets is often a fuel hungry business but using Jool’s atmosphere to slow down to more sane speeds saves a lot of that fuel. Looks cool too.

The probes were probably fiiiiiiine, can't be that hot right?
The probes were probably fiiiiiiine, can’t be that hot right?

Once in orbit around Jool, the craft adjusted itself to be ready to release it’s first two probes. The probes were released in pairs to keep the main ship balanced. So the first two to go were those destined for Jool and Laythe (the innermost moon). Neither of these would require much fuel (The Jool probe would simply be falling to it’s death and the Laythe probe would use it’s parachutes to land) so around 3/4 of their fuel was transferred to the main stage. Of course I could have just included less fuel from Kerbin launch but…. I didn’t think of it back then so…. shush.


Jool is a gas giant so this probe wouldn’t be landing as such. Much like my sun mission, this probe was meant to simply descend as far as it could while sending back information to the Kerbals back home.

The Jool probe approaches it's green doom
The Jool probe approaches it’s green doom

Jool looks very good (especially with the visual enchancements mod) and I managed to catch the greenest sunset I’ve ever seen whilst descending.

So..... green.... so.... pretty....
So….. green…. so…. pretty….

The atmosphere is crazy thick, at 5000m the probe was freefalling at a mere 50m/s!

After the probe was inevitably crushed in the Joolian atmosphere (At 300m on the altimeter), it was time to move on to the next probe.


The Laythe probe probably had the easiest job of all the probes. I intended it to parachute through the atmosphere for a nice splash down in one of the oceans that dominate Laythe. All that was needed was to burn a bit of fuel to point it on a collision course…

Like Kerbin but... Blue-er
Like Kerbin but… Blue-er
Laythe is another very good-looking planet
Laythe is another very good-looking planet
The Laythe probe drifts down through the atmosphere with Jool looming huge in the background
The Laythe probe drifts down through the atmosphere with Jool looming huge in the background

From orbit, the probe spotted bits and pieces of landed dotted around that would make great landing sites for future Kerballed missions.

It also noticed that nights on Laythe looked awesome.

Aurora Laythealis?
Aurora Laythealis?
I feel like those pointless landing legs are judging me
I feel like those pointless landing legs are judging me

Back at home Jeb is very happy. His last interplanetary trip to Dres left him a bit disgruntled. He worried that all the other solar system bodies would be as gray and boring but after seeing the amazing colours of Jool and Laythe he couldn’t wait to get out here! He expected that the other moons would be equally as…colourific. He was incorrect.

The main stage left near-Jool orbit and aimed for an orbit in between it’s next two targets; Vall and Tylo.


Vall looked pretty small from orbit and sure enough only a small amount of fuel was needed to get into orbit and start a burn for the surface. The probe had almost a full tank available for a powered landing and Vall’s gravity wasn’t pulling it down too vigorously.

Even from this distance... you can almost hear Jeb crying over the lack of colour.
Even from this distance… you can almost hear Jeb crying over the lack of colour.

It was only after landing on Vall that I noticed a major flaw in my probe design. The landing legs which had looked like a good tripod formation from the Kerbin launchpad were in fact…. lopsided. They couldn’t keep the probe upright and I realised I was going to have the same problem in all my probe landings. Oops! Oh well, a bit late to send an extra leg or two from Kerbin, they would just have to rest on their sides. It didn’t look good but the legs were enough to cushion the impact so the probes could at least serve their function of giving me practise landing on and information about Jool’s moons.

Landing legs fail
Landing legs fail

The probe landed with about 1/4 of it’s fuel still left, as long as the remaining moons weren’t too much more massive there should be no problems.

Next up… Tylo.


Hard to tell from this angle but…. Tylo does indeed look significantly more massive. Uh oh 😦

Big Momma of Jool
Big Momma of Jool

Sure enough no matter how hard I tried I couldn’t seem to get the probe to land. Establishing an orbit itself took quite  a chunk of fuel and little guy simply didn’t have enough left over to land on Tylo. The gravity was high enough that my LV-909 engine barely had enough thrust to fight it. It couldn’t operate at full blast for long and so inevitably, the probe disappeared and became part of the dust of Tylo. Weirdly, the GRAVMAX did report that Tylo had ‘lower gravity than expected’. How much frikkin gravity were they expecting!

RIP Tylo probe
RIP Tylo probe

Well, that didn’t go well but at least I learnt a thing or two about Tylo and no Kerbals had to die to do it!


The main stage powers to it’s next orbit, between Bop and Pol.


Pol approach
Pol approach

Pol is the outermost of Jool’s moons and turned out to be a bit weird. Landing wasn’t too difficult and turned out to be about the same as on Vall. Plenty of fuel was left over. The surface of Pol wasn’t colourful… but it was interesting.


The ground was covered in spiky rocks. There was no atmosphere so I have no idea how such things would have been formed. Pol might be an interesting place to explore with a rover. Unfortnately I couldn’t keep an eye on it using the probe. The probe landed without issues. I had some science equipment strapped to it but not enough solar panels to run the antenna constantly so I used time dilation to speed up the transmission of data back to Kerbin. Unfortunately, as soon as I returned to normal time the probe violently detonated!


Only the probe body and a landing leg survived the blast but of course with no rocket they didn’t survive the fall back down to Pol’ surface. So I had lost my eyes on Pol but  no matter, at least I had landed on it!


Bop was the final destination for the probe-a-thon and represented an unusual challenge. The problem is summarised in these two pics:

The Jool system from above... All looks normal
The Jool system from above… All looks normal
A side on pic of the Jool system. Guess which one Bop is?
A side on pic of the Jool system. Guess which one Bop is?

All of Jool moons are on pretty much the same plane. Except for Bop. It’s the ‘special’ moon of the family and isn’t anywhere near being on the same plane as the others. The probe has a small supply of it’s own fuel but I was worried that the chunk of fuel needed to correct it’s inclination wasn’t going to leave enough left for a landing (especially after my experience with Tylo). Luckily, the interplanetary stage still had a little fuel left. I used this to correct the inclination while carrying the last remaining probe. Unfortunately, only having one probe made the ship horribly unbalanced and a lot of RSS fuel and gentle thrusting (eww) was needed to keep it pointed in the right direction. Eventually though, the inclination matched that of Bop and the final probe was released as the main stage gave up the last of it’s fuel.

Looks like a giant asteroid... kida lumpy
Looks like a giant asteroid… kida lumpy

Bop has to be the most lumpy thing in space. I mean just look at this:


Despite how weird it looked from space, Bop wasn’t much to look at from the ground.

No spikes here
No spikes here

Still, perhaps one day some enterprising space-racer will set up an awesome dirtbike course and make use of dem epic rampz. Until then though…


So that’s it! All the bodies of Jool visited with varying degrees of success. I like that in KSP even failures make you feel like you’ve achieved something. I could have found much of this information on forums and wikis but nothing beats learning for yourself what the different moons are like and what would be involved in future missions to them.

Already Jeb is preparing for his triumphant return to a colourful destination!




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