Dawn of the dead: NASA space probe runs out of gas in asteroid belt after 6.4 billion-mile trip NASA’s Dawn space probe, our visitor to the Solar System’s protoplanets Vesta and Ceres, is cold and dead. The spacecraft finally ran out of fuel whilst it was orbiting Ceres, the largest object in the asteroid belt. NASA engineers noticed that it had missed two opportunities to check in with ground control on 31 October and 1 …
Presumably it will eventually crash on Ceres (possibly contaminating it) following orbit trajectory disturbance by the ever-changing combination of gravitational forces from other passing (even at great distance) bodies like Jupiter. Just guessing (wish article had explained).
Yes, it does seem to be a rather strange thing to say but perhaps it was just poorly worded.
Dawn is in a highly elliptical orbit orbit around Ceres, bringing in as close as 35km and taking it out as far as 4000km, where it could be subject to perturbations by other asteroids in the belt.
Saying that it'll probably remain in orbit around Ceres for at least 20 years suggests that no other large asteroids are going to approach Ceres closely enough to significantly perturb Dawn's orbit until then. It also suggests that another large asteroid is approaching Ceres and will approach closely enough to have an affect around that time, and which will perturb Dawn's orbit.
We can't predict exactly how Dawn's orbit will be changed but considering its close proximity to Ceres, which is a ~1000km wide target, the possibility of Dawn hitting it [Ceres] seem quite high.
Orbits are impossible to calculate precisely, except if there are only two bodies in the entire universe. For shorter periods you can simply calculations so that you only consider the object you are orbiting. However, for longer timespans you need to factor in everything else in the solar system. And you cannot describe it as a pure solvable mathematical system. You have solve it numerically where you enter all the coordinates for all objects or at least the major ones and their velocity vectors, step ahead a small amount of time, recalculate velocity and coordinates and repeat. The smaller the steps the more accurate you get. Mistakes accumulate though and in time you can get discrepancies.
For Dawn the orbit is so low that they risk a collision if it gets tugged into a lower orbit but for instance Jupiter. They have a very low chance within the next twenty years, and with fifty it is less than a percentage.
Maybe not.
Ion drives are probably heavier, and they do give much lower thrust. The mass of the drive would cut into the mass budget for fuel, and the low thrust might not be sufficiently responsive for the required activities. It is also would up the power requirements, which would also cause changes to the power sytems that might adversely affect the mass budget for the probe.
In fact, the design already used the ion propulsion system to help with attitude control, so clearly the probe's designers did not think ion engines were a complete answer to their needs.
Dawn had ion engine for trajectory changes.
What I'm curious about is whether it also has reaction wheels or similar for attitude control, whether those had also failed.
It would also be interesting to know what kind of spool-up spool-down delays, if any, ion drives have, and how much efficiency is lost during that time.. and if they could be made small enough to be used for attitude control. You'd need 8-16 of them.
I discussed this kind of application (at a space conference) with one of the major ISS designers (prior to ISS happening). He agreed it was a good application in principle, e.g. avoiding the “structural rattle” associated with (more impulsive) traditional gas-expansion attitude-control options.
But the aerospace industry is renowned for its conservatism. Who needs wheels when there is so much accumulated experience with skids? And risk assessors love accumulated experience.
One reasonable question (addressable) regards possibility of ionisation and hence structural charge.
Ion drives unfortunately are pretty useless for the quick thrust you need for orientation and positioning purposes. Ion drives are fantastic for slow but steady acceleration - movement between planets (or Protoplanets in this case) is a perfect case and that's what was used on Dawn. But for orientation it's too little thrust to achieve in any sort of useful time frame the desired results.
To offer an example, your currently doing science pointing instruments at ceres. But you need to change orientation to point the antennas at earth to deliver that science data. With hydrazine thrusters that's work of maybe 15 minutes. Say the transmission takes another 15,and another 15 to get back into science orientation. You've lost at most 45 mins of science time. With ion thrusters you would need at least 2 hour to change orientation in one direction (probably more) and so for the same action you've lost 4 hours 15 mins science days. If you have to do this every day that's 3,5 hours of science time per day lost. The mission will need to be massively longer to account for all of that missing science data. And that means you need more fuel for the ion thrusters, etc, etc. And in the end it just works out way more difficult to do it with ion thrusters. Not to mention the orbital calculations to work out how long you need to apply the ion thrusters for to get into the correct orientation position when you're talking reaction time frames measured in hours. If you under or overshoot with hydrazine, it's a quick fix, just squirt some more with ion you've got another couple of hours to wait.
Hope that explains a bit why we still use chemical thrusters for orientation instead of ion thrusters. :)
What I'm curious about is whether it also has reaction wheels or similar for attitude control, whether those had also failed.
Yes, and yes.
The third of four reaction wheels failed in 2017. Dawn has had reaction wheel failures throughout its mission. At Vesta, wheel failures led to "hybrid" thruster/wheel operations, and the approach to Ceres was an odd overshoot-and-return because Dawn couldn't perform a typical spiraling ion engine capture orbit with (then) two functional reaction wheels.
Fun Fact: Most every thing we toss out of the gravity well exceeds it's mission with aplomb. Oh, the screwups are spactacular to be sure, but that stuff tends to be well built by people who "believe" in what they are doing. It's super good kit, and all of it is compromises besides.
Also, I assume that in order to have a reasonable certainty of a minimum lifetime of "x" all of it needs to be so massively over-engineered that under not particularly adverse circumstances the actual lifetime invariably ends up being a large multiple of the minimum (not considering necessarily depleting things like fuel).
A 390+ square meter piece of directionless junk flowing blind about here and there until eventually it breaks down into smaller pieces.....flowing blindly about, here and there..until eventually is joined by other small pieces of space junk blowing blindly about in space. intergalactic plastic beach on your marks, get set....go
Surely, a sign that the NASA explorations & others around the world may need to re-investigate or indeed reinvest in small nuclear powered engines. Nuclear powered rocket motors could surely, now, in this age of extreme Health and Safety margins be built which will pass the tests of endurance, resilience and safety. The USA had a huge R&D program in the very early 1960's thru early 70's; which endured the Kennedy Presidency.
Project Nerva and the Phoebus nuclear-rocket motors, just by example.
There was much done, with several motors from a handful of major engineering players at that time,
I would revisit it, without hesitancy.
We are unnecessarily, now, holding ourselves back from the very urgent business of exploration of space, both near-field and deep.
The long & short of it is, intelligently & ethically motivated humans must succeed in space and stop tinkering. Start taking bold-steps. And stop talking a strong space-program down. NOW!
Read the History.
https://www.lanl.gov/science/NSS/issue1_2011/story4full.shtml
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https://www.lanl.gov/science/NSS/issue1_2011/story4a.shtml
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https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899.pdf
Nuclear rockets are all well and good, but what actually matters is exhaust velocity. For a hydrogen/oxygen rocket it's about 4,400 meters per second at the nozzle. For an ion engine it's about 29,000 M/S. So ion engines have far more total impulse than chem rockets ever will.
Nuclear rockets may have a larger flow rate, but I doubt the exhaust velocity will be any higher that with ion engines (if that), and they will have to accelerate much more dead mass in the form of fissionables.
Perhaps for large payloads already in space it could be acceptable if exhaust velocity is high enough to more than offset the dead mass. Kinda like a large-scale equivalent of the ion engine's efficiency.
> "But the fissionables aren't "dead mass" they are fuel mass."
No, the fissionables are used to heat and/or accelerate the reaction mass, in theory anyway. Jettisoning your power supply out the tailpipe is a really bad idea, particularly if it's radioactive and you're still in the atmosphere.
"Nuclear rockets are all well and good, but what actually matters is exhaust velocity."
Last I checked, due to the Laws of Motion, what REALLY matters is FORCE, which the equation states depends on mass. The catch with ion engines is that while they emit mass at high velocities, that mass is minuscule; thus, its ultimate thrust force is on the order of millinewtons. Thus it's only practical for use once you're already in orbit. OTOH, nuclear rockets are being tested with the idea of replacing chemical rockets (which means forces on the order of meganewtons).
Satellites could use a backup Electro-Magnetic drive to putter back to Earth orbit.
What I'd like to see is a solar sail attached to the likes of "Kepler II" that we could redirect *extra* solar radiation onto using a a big mirror in space. (for possible faster travel).
So that we could power those wonderful satellites back to earth orbit and reclaim/refuel/rejuvenate them.
Also would not have Earth's garbage left through the solar system.
I think that if we as a species ever create enough space-junk to significantly clutter up the solar system, we'll have done quite well for ourselves (unless the space junk was caused by unwanted planetary fragmentation following a war involving q-bombs or the like, I suppose).
Satellites could use a backup Electro-Magnetic drive to putter back to Earth orbit.
Nope, since the electromagnetic drive has been debunked.
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