The disappearance of Gaia, ESA spacecraft will be turned off on 27 March 2025 | Hacker News Reader

The disappearance of Gaia, ESA spacecraft will be turned off on 27 March 2025

cosmos.esa.int

126 pointscroes45 comments

Absent from that link but important context:

The reason why they are actively shutting it down, is that they are running low on fuel, and want to ensure that the spacecraft does not litter important orbital locations.

Gaia has spent its life at Earth's L2 Lagrange point, which is a valuable and limited area of space; they do not want to have to plan around dead spacecraft in future missions. Thus, before the spacecraft runs out of fuel, they have moved it away from L2 into its final graveyard orbit.

It is not capable of performing the science for which it was designed in its graveyard orbit.

Thanks. That page also helped me find one that sums up the mission so far, and explains what's to come in 2026 & 2023.

https://www.esa.int/Science_Exploration/Space_Science/Gaia/L...

> what's to come in 2026 & 2023

Weird, I checked the link but there's no mention of time travel.

The time travel discussion is on the 2028 page, but intentionally absent from the 2025 page to minimize risks of contamination.

I guess you didn't read to the line where it says

> two massive data releases are tabled for around 2026 and the end of this decade, respectively.

Oh?! I was always under the impression that Lagrange points were convenient calculation spots where everything sums out, but from what you’re saying, it’s a deep gravity well? Mind blown!

Edit: don’t mind me, I was completely off… thanks ChatGPT!

L4 and L5 are more stable. L2 isn't super stable, hence the need for fuel for station keeping. More polite to vacate on purpose rather than just erratically bounce around a nearby orbit after fuel runs out.

Idk if I'd describe any of them as "deep gravity well"

It's not a deep gravity well, it's a fairly weak gravity well, but with fuel one can orbit L2 reliably.

Without fuel, one can orbit L2 erratically.

People with the ability to send satellites places, don't like sending expensive satellites to places with large bodies in erratic orbits.

> Without fuel, one can orbit L2 erratically

It takes an infintessimal amount of energy to detatch something from L2.

Sure, you won't be at L2, but you may remain near L2 for some time.

Where everything sums out is a completely flat spot on a table, it won't hold a freely moving object. Lagrange points are more like shallow dents that tend to attract rolling marbles.

- "Earth's L2 Lagrange point, which is a valuable and limited area of space"

That's a strong exaggeration; it's millions of km across in three dimensions. Gaia's orbit alone is

- "263 000 x 707 000 x 370 000 km, 180 day-long orbit around L2"

https://www.esa.int/Science_Exploration/Space_Science/Gaia/G...

L2 is also said to be unstable, but I don't know how long it would take for Gaia to move far away enough that the collision risk would be negligible.

The issue with that is things that naturally escape from Lagrange points often crash into the Earth, which is one of the risks they're trying to mitigate.

If by “crash into earth” you mean “burn up harmlessly in the upper atmosphere” then ok - but it kinda sounds like that’s not what you’re claiming…

Pieces of spacecraft often survive atmospheric reentry.

I understood why they are shooting it into a graveyard orbit, but not why they're actively turning it off rather than trying to get whatever science they can out of it. (I'm sure that if I have a basic idea like this, the people at ESA also had it, but I haven't seen the reasoning explained anywhere.)

I think this part of the Wikipedia article answers it:

> In order to maintain the fine pointing to focus on stars many light years away, the only moving parts are actuators to align the mirrors and the valves to fire the thrusters. It has no reaction wheels or gyroscopes.

So without "fuel" (reaction mass/gas) for the cold gas thrusters, it likely couldn't hold a stable attitude, and a tumbling spacecraft is likely quite useless because you can't even properly communicate with it, let alone use instruments.

I wonder what considerations went into not attempting to use it as a low-value probe sending out telemetry using a low bandwidth omnidirectional antenna (which I would expect the spacecraft to have for recovery in case something goes wrong). Maybe we already have all the science that would be interesting from other probes, maybe management/budget decisions, maybe it wouldn't work, but it would be interesting to see the decision making.

Costs money to support the mission

The DSN is already strained

ESA is actively trying to reduce the amount of debris in orbit. To this end all missions have to actively clean them self's up at end of life. See https://www.esa.int/Space_Safety/Clean_Space/ESA_s_Zero_Debr... and https://www.esa.int/Space_Safety/Space_Debris/Mitigating_spa...

They also overwrote the EEPROM of the satellite computer before shutting it, because apparently they were concerned that the satellite would reawake, because it has multiple autonomous recovery modes.

It wants to live!

From elsewhere on ESA's site[0]:

> As part of its decommissioning, the names of around 1500 team members who contributed to its mission were used to overwrite some of the back-up software stored in Gaia’s onboard memory. Personal farewell messages were also written into the spacecraft’s memory, ensuring that Gaia will forever carry a piece of its team with it as it drifts through space.

[0]: https://www.esa.int/Enabling_Support/Operations/Farewell_Gai...

Great images from the project here:

https://www.esa.int/Science_Exploration/Space_Science/Gaia

Also available as a 3D App for desktop/VR :

https://zah.uni-heidelberg.de/gaia/outreach/gaiasky

"While the Gaia spacecraft will enjoy its well-deserved retirement"

Don't anthropomorphize machines. They hate it when you do that.

I find it wild that we’ve been using triangular reckoning (parallax) for over a millennium, all the way back to early mariners. Measure two angles, draw a triangle, know where you are. That same method now maps galaxies. We’re still navigating the unknown with geometry.

Pretty amazing.

"Gaia is measuring their positions to an accuracy of 24 microarcseconds. This is comparable to measuring the diameter of a human hair at a distance of 1000 km."

https://www.esa.int/Science_Exploration/Space_Science/Gaia/G...

Thank you to everyone involved in the Gaia mission and to everyone whose contributions (including taxpayers) who made it possible. Gaia's data will shape our understanding of the universe for generations to come.

What is the advantage of keeping the passive in orbit instead of dropping it in an ocean?

If for any reason it fell from orbit, later, it could impact an important surface (city, nuclear plant), and definitely a larger area.

Generally getting things out of space also requires effort/fuel. A graveyard orbit is specifically easier to get to but uninteresting from a "hitting other things" perspective.

Folks don’t appreciate how much energy it actually takes to “fall” into the Earth or even the Sun.

It’s not really intuitive when most personal experience is how hard it is to keep things from not falling.

Both Kerbal Space Program and Outer wilds can sometimes do a good job of driving this point home (especially in OW if you're outside of the ship). It even humbled Thor from PirateSoftware during his stream, which I found hilarious. :D

The only way to understand is to play Kerbal.

Most of the attractiveness of the concept of "rods from god" relies on this misconception (if you haven't heard the term, it's a theoretical space weapon consisting of placing large tungsten rods in orbit, then dropping them on whatever needs to be destroyed in a purely kinetic strike with the energy of a massive bomb or tiny nuke).

The concept isn't entirely infeasible (you could deorbit the rods actively), but the advantages over just hitting the target with an ICBM quickly dwindle once you realize you can't just "drop" them.

Plus the amount of energy needed to place them there in the first place is also probably way more cost-prohibitive with much less stealth than said ICBM.

I also imagine you could probably knock these things off-course for much cheaper than it cost to put them there or to keep them there.

> Folks don’t appreciate how much energy it actually takes to “fall” into the Earth or even the Sun.

It's not that bad from the Lagrange points thanks to multi-body gravitational dynamics. The Lagrange points form part of the so-called 'interplanetary transit network' (https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...) that supports extremely low-energy transfers in the solar system, provided you're willing to wait a very long time.

That being said, there's no reason to send a deep-space probe back to Earth for reentry when it's even easier to send it into a deep-space graveyard orbit of some description with no risk of any potential debris-related problems.

> If for any reason it fell from orbit, later

You’d need 320+ m/s of delta-V to knock something from Sun-Earth L2 to Earth [1]. Not impossible. But akin to the energy required to go from Earth transfer to Mars or Venus transfer [2]. Less likely to fall to Earth than become an orbital pest.

(It's a good question.)

[1] https://www.researchgate.net/figure/Duration-and-delta-v-req...

[2] https://en.m.wikipedia.org/wiki/File:Solar_system_delta_v_ma...

You mean 3,200+ m/s of delta-V.

I actually made a comedy of errors when I went back and edited my first estimate ("about 300 m/s").

For debris to become a problem to us ground people, it really just needs to make it to LEO–the atmosphere will take care of it from there. So I was trying to estimate the Δv for L2 to LEO, which is about 200 m/s according to Table 3 on that source. So probably not 3 km/s. But 320 is too precise given the handwavy math I am doing.

That's the delta-V to get away from Earth, and 90% of it is spent shifting from a circular low orbit into a transfer/escape orbit.

When you're coming back, you don't need to spend that energy. Instead you slightly adjust your aim so your transfer orbit hits the atmosphere instead of missing it.

It's actually much less than 320 m/s—that's a figure for a fast transfer. If you have time for it, three-body dynamics let you get arbitrarily cheap transfers in the regions of the Sun/Earth/Moon Lagrange points,

https://nebula.esa.int/sites/default/files/neb_study/1167/C4...

(Table 1 disposal options: 88.56 m/s to crash Gaia into the Earth—or even just 6.18 m/s to crash Euclid, another SEL-2 dweller).

To make it less abstract than the argument that it costs "delta v": it's a bit like asking, what if this rock I placed on the floor impacts a nuclear plant one day? It requires energy to move there. This change in velocity, making something drop from orbit by itself, happens across months~years for only the lowest orbits around the earth, caused by stray particles from our atmosphere impacting the spacecraft/satellites there, but at L2 that's afaik not a factor

Gaia is a about 4 times farther out from Earth than the Moon, there's little reason to fly it back to Earth.

And insufficient fuel.

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