OK, premature, premature, but still... After a succesful landing and thouroughly having dug holes and trenches in the original working volume of the arm: how about moving about a tiny bit? - I was wondering if the rocket engines could possibly be used to shift position just a few decimeters at a time. Small blips, which should be so weak that they wouldn´t overturn the lander.

It could also come in handy when the snows come in later in the year and threaten to bury the lander.

A few things would be necessary: no post-landing venting of possible excess fuel (who knows about that?). No permanent disabling of the rockets after EDL. A possibility to stow and unstow the solar panels (ok, that is probably a show-stopper, but just humour me here...). The last would only be an issue if it was thought that small blips of the rockets would raise sufficient dust to degrade the solar panels´efficiency.

Just thinking out of the box here... - And I know it very probably won´t ever happen. But if!

- Well, let us just get this baby down in one piece for now.

5 seconds post landing, the pressurizing helium gas of the prop system is vented, so it's a no from the start. If you look at the realtime EDL movie at the Phoenix website, this is shown.

Doug

One of the Surveyors (unmanned lunar landers) did a "hop" using its rocket engines. I think that's the only time such a maneuver has been planned or attempted.

There was some talk of doing this with NEAR after its touchdown on Eros, but nothing came of it.

As noted before, once the helium vents, Phoenix is where it will be when the next expedition will find it. Besides the helium, once the panels are deployed (one-way) the balance is thrown off.

But, does it make any sense to have a capability like this? I can see some lander landing, then using its cameras to see a spot 20m away where there is ice on the surface or a tree growing, then hopping over there. Phoenix is predicted to only use 40-45kg of the 67kg of propellant loaded. It should be possible to fly 20m or so with the fuel left. Once it's on the ground, if it were to fly again its landing ellipse would be meters or centimeters across instead of kilometers.

And why is the helium vented so soon after landing? It must be really high priority to be that early.

It makes sense, I guess, to get rid of any pressure as soon as possible, so that fuel line heaters etc can be turned off and not bothered with again. While the system is still pressurized, you can't let the fuel freeze, which it would do quite quickly, I would have thought, with -60degC outside or whatever it will be. Just guessing, but that's my take.

I would rate the chance of seing a tree growing as somewhat slim (particularly given that such a thing would be easily visible from HiRISE)

The point with Phoenix is that you really don't need mobility. The science is right under your feet wherever you end up. If you need mobility - rockets are a dangerous, heavy complex and unpredictable means of doing it. For missions that need mobility ( MER, MSL ) then they've got it.

Doug

My guess too is that the electric power budget overrides the slim possibilities of having the chance of a hop that saves the mission. Phoenix will be landing just before night fall, and they even switch off the lander radio one minute after touchdown, arguably to save energy until the solar panels are deployed and the sun rises again. And heating up the fuel at a later stage may just consume too much energy

Perhaps future landers might have a more volatile fuel (under Mars conditions) and that could be bled off instead, and saving the helium for blasting the dust off the solar panels every year or two.

I dunno, man. Correct me anyone if I'm wrong here, but the helium is a pre-load to provide sufficient pressure to assure delivery of the hydrazine to the thrusters (where regulators/restrictors/whatever step it down to the correct throughput). This is a pretty mechanically complex and obviously mission-critical system.

I can't see anybody signing off on additional plumbing & complexity to reutilize the helium after landing; too many scary potential failure modes. Blowing N2H4 all over the panels during flight accidentally is one off the top of my head...ugly enough, but worst case is a dead-open leak that either bleeds out all the fuel or reduces the tank pressure below levels needed for safe landing...splat.

Any sort of solar-array cleaning system has to be independent.

I think that is just plain wrong, the argument about not needing mobility. We can be in the very fortunate situation that everything is right where we want it, but it also might possibly not be the case.

The rovers have shown us that mobility is the only way to go. Those little wheels have enhanced the science return immeasurably. Even Phoenix would be able to to do ten times more science with just limited mobility. It is a left-over from another, more limited, era of space exploration.

Doesn't Phoenix land at the time of year when the sun is up nearly the entire Martian day?

Really? With only 90 sols, 8 TEGA ovens and 4 MECA suites? At a site where all the obital data says the science we are after is essentially homgenous across the ellipse. Ten times more science? Please do explain how, exactly.

I would argue the EXACT opposite of what you say. There would be ZERO scientific benefit for Phoenix to be mobile. Would a rover in the polar plains be interesting and exciting? Yes. For most science investigations on Mars is mobility a benefit? Yes, clearly, MERA and MERB have shown this to be the case. Would a rover offer benefits for the science that Phoenix is attempting? No. Phoenix has a full schedule to conduct as thorough a characterization as it can manage within the time of it's expected life span. Phoenix is niche, it's arguably one of the few scientific missions to Mars that doesn't need mobility ( a second being the long overdue net-lander type mission).

But to claim that Phoenix would be able to do 'ten times more science with just limited mobility' is wrong - very very very wrong.

Doug

At the time of landing, it will be up all day. I guess human nature just means we call 5, 6, 7pm 'evening' even though, when it's still daylight at midnight, it isn't.

I thought at one point the mission did have a rover, called Murie Currie, like MPF when it was originally scheduled to launch in 2001 but was removed to due the budget.

The point of this mission is not to see pretty scenery but to examine the soil and search for water. Although having mobility would make the mission more exciting to the public, the landing area will be flat and homogeneous for as far as any mobile robot could travel. You would essentially be seeing more of the same everyday.

You really can’t underestimate the ‘vertical mobility’ that Phoenix has. Most of the scientific discoveries made by the MERs have been due to looking under the dust covered surface of Mars. Opportunity’s evidence for a past damp Mars is through rock stratigraphy exposed by craters and Spirit has used the meagre trenching ability of its wheels to expose sulphates.

Phoenix is the first lander that will be able to see virgin material that has not been directly exposed to the Martian atmosphere. It’s going to be able to dig down to up to 0.5m! If Opportunity had been able to do that and look at samples as it dug down it would have made most if not all of its discoveries without the need to drive.

Back when it was the 2001 Lander, it had the beginnings of the MER Athena payload on board. Mini-TES, Pancam, Mossbauer Spectrometer on an Arm. It also had the spare of Sojourner - Marie Curie - which would have been picked up by the arm and dropped down onto the surface.

And the landing site would have been Meridiani Planum..I wonder what that looks like

Doug

Hmm well memory failed me again and I seem to have mixed up the Earth and Sun terminator lines on that map :-( Anyway, at midnight local Phoenix time it's not gonna be all that bright compared to midday. Any unfortunate tilt in the lander attitude may make it even worse. Does Phoenix ground ops get affected by that? Do they schedule certain activities (other than recharging batteries) only in the day for power reasons?

Fleshing out my idea a little more:

There apparently is an existing valve that vents the helium, so a piece of pipe on the discharge of that valve leading strategically towards the deployed positions of the panels would not introduce another leak path into the helium plumbing, so there is no impact to the overall reliability re helium pressurization. A failure of that valve post landing leaves us no worse off than we are now.

The existing hydrazine is unsuitable for long term storage onboard due to freezing effects.

However, if a subsequent lander had suitable hypergolic bipropellents (methane, nitrogen tetroxide ??) they could be vented safely though the combustion chamber through the already existing valving, separately, with no impact to the mechanical reliability of the propellent system. There would be a software fault possible that would cause the system to fail, but there are existing similar software failure modes to the existing control system we manage to live with now.

True enough. Maybe something like that would be workable for a long-lived fixed lander (seismic/meteorological monitoring stations, maybe? Sure would rather put a small RTG on such critters instead, though (sigh).)

Rovers would still need independent systems, though; can't see them backtracking to a powered landing stage for a quick dust-off without sacrificing innumerable mission goals.

A few thoughts of my own...

First off, after you vent the helium, there is no pressure in the tanks. You'd have to add pumps to pump out anything from the unpressurized tanks, and those tend to be heavy enough to preclude their use. (Hence the use of helium to pressurize the tanks so the fuel delivery systems are pressure-fed and pumpless.)

Second, long-term storage of hypergols on Mars is probably not a great idea due to freezing, but only if you ever plan to use the remaining propellants. Otherwise, if you design your fuel tanks and plumbing properly, you can keep the remaining fuel (frozen or otherwise) in those tanks forever. Hypergols, particularly nitrazine and UDMH (unsymmetrical di-methyl hydrazine) were first developed as rocket fuels in the U.S. so you could maintain fully fueled missiles in their silos for months or even years and have them ready to go at literally a moment's notice. Only in later years were the hypergols traded out for solid fuels in American nuclear missiles.

In fact, there is probably significant hydrazine and nitrazine left in the tanks of the Viking landers to this day. AFAIK, no powered landers (including Surveyors, Lunar Modules, Viking landers, etc.) ever actually vented their propellants after landing. They all vented the pressurizing helium, but never the actual hypergols. (Indeed, if you vented your hypergols and they managed to mingle somewhere below your lander, you're going to get more than a "little hop" out of it...)

-the other Doug

Really? It may be "summer", but they are landing on permafrost. When asked Chris McKay earlier this year, he said maybe the first few centimeters...

WRT mobility, I have to disagree, Doug. The area the arm can reach is probably going to be pretty homogeneous, and a Surveyor-style hop may be enough to get to a different-looking place. Five TEGA runs of the same sample don't produce much more science than a single run.

Also, remember that Mars is a low-gravity environment (0.38g), so the fuel requirements are not as egregious as a terrestrial rocket...

Simon

"...Five TEGA runs of the same sample don't produce much more science than a single run..."

That's NOT what they're going to do... They'll likely skim the surface and get essentially only mobile dust, then a second sample of "near surface soil", then as they dig deeper and see signs of ice, "near-ice soil", then "dirty ice", saving one for whatever seems most useful.

It's not going to happen with Phoenix, remember that. 5 seconds after landing, the prop is depressurized. And if you want mobility on Mars, rockets are a very inaccurate, dangerous, complex and heavy way to do it.

And I'm still going to maintain - Phoenix has a full 90 sols planned investigating where it lands. There is no evidence to suggest there is science to be gained by moving horizontally. I am somewhat concerned that a few people here don't know what Phoenix is about at all.

Doug

Actually, I expect there would be some value in Phoenix being able to move... particularly as the once concieved Viking 3 lander <with mini-crawler treads instead of footpads>

The landing site is homogenous on a large scale, 100 meters or so, but heterogeneous on a scale of 10 meters or so. It also has topography on that scale.

The lander might land, for example, with a polygon-trench just out of reach of the arm... it would be of value to be able to move 5 meters and trench across the polygon, even if all onboard sample analysis ability was exhausted.

It would be of value to be able to move 8 meters to the south side of a nearby boulder and do a week's multispectral petrologic study of the boulder, both with mast camera and the arm camera.

Viking 2 saw well defined small drifts in the shallow troughs at it's site. It would be of value to be able to move to where it could trench across one if they are present at the Phoenix site and examine internal structure of the soil... deriving info on recent climate / meteorology history.

And it would be nice to be able to crawl so one foot is in a trough and the others are on the north side, tilting the solar panels to the south to get another month of autumn meteorology.

I thought Mars Surveyor Lander 2001 should have had such ability, I think Phoenix would benefit from it, but it wasn't in the budget or weight margins and it's not onboard.

Some landers, netlander type landers, really don't benefit from the added complexity of limited roving, but a landed "station" like Vikings or other rocket-and-landing-legs landers would.

Mmm, I have the feeling this topic will be quite active once we have the first images from the landing site...

No doubt. I just hope that everyone realizes that due to the added cost, a Phoenix that moved would never have been approved, would never have flown. The ONLY reason we are now enjoying the anticipation of this exiting mission is because it was put together from an existing, IMMOBILE craft.

It would, indeed, be of value. But that's not Phoenix's job. It's got a very specific very unique job to do and one that mobility is not required for.

As I have said - would a rover been nice in the polar plains? Yes. Does Phoenix need mobility to do its job? No. Would Phoenix even have the time to use Mobility to great effect? At the time of selection, they had a full 90 day digging schedule with one massive trench. The analysis that suggests it is soil then concrete-like-ice will change that perhaps, but Phoenix wouldn't have had time to do the science it's sent for AND be mobile. Let me repeat this so people don't missinterpret me here. Mobility is good. Mobility is great. Mobility is sexy and very very usefull if you're trundling around doing geology on rocks somewhere like Meridiani or Gusev , and indeed a long lived rover up near the pole would be very cool. But - would mobility make Phoenix 10x more scientifically productive?

If Phoenix were mobile, it would not be 4 days from Mars. It would be on PPT's on hard drives on Earth as it wouldn't have gone under the scout budget, and MARVEL would be 4 days out from MOI.

The argument is moot anyway. Phoenix is Phoenix. It's got a job to do, it's going to do it. A fossilized martian 5 cm outside the arm range doesn't change the fact that for the job that Phoenix is going to do, mobility is not required.

The question ' Wouldn't it be nice if Phoenix could move ' is essentially ' Wouldn't it be nice if Phoenix never got selected '. Because that what mobility would have meant to Phoenix during the scout process.

Doug

I've understood that, in the eventuality of one sample showing something "unusual", one will be filled up with sterilized material coming from Earth in order to be compared with? Did I get it rigth?

PS : in French prononciation "Scoop" and "Scout" are very similar.

Unless the surface is very very even

PS : I'm wondering if some people are not joking about mobility just to drive Doug nervous as was the case for the Dust Devils cleaning up Sprit panels

When you stress the 90 sols argument, I think that is mainly to underscore your thesis that mobility would be of no use whatsoever to Phoenix. I agree , if the plug was pulled on Phoenix after those 90 days. With a 90 day mission, definitely one trenching operation is more than enough.

But the whole idea of this thread is to see what could be useful beyond the 90 days, if we ever get that far. And in that "extended mission" it would have been great to be able to nudge a bit of movement out of the lander. Well, it is not going to happen, obviously.

That the science would be "essentially homogenous across the ellipse"... well... I just don't believe that. Maybe on some very high scale, but - surely - on the scale that Phoenix operates, there are local differences. From one square/cubic meter patch to another! If we had two trenching operations within meters of each other and they turned out to be different, that would tell us so much more than one single trenching operation, the validity of which we can't say a lot about, because there is just this one operation.

Phoenix is legacy hardware, and it is certainly much better to have the present Phoenix than none at all. But how much does that argument advance things?

I guess the arm have "left" and "right" ability and not just "strait forward" axis.
If I'm right, we could have a second trench (1 m?) from the first during extended mission, don't we?

Whoa, people. Phoenix isn't even safely down yet and you're already talking about extended missions.

It advanced them from not having any mission, to having a mission that will do a LOT of science. It wouldn't do '10x' the science, as you claim, if it could move, because within 5 months or so, it's going to be dead. It's going to take most of it's life to fully understand and investigate what we land on. If it could move, it wouldn't have time to do the same to a second site and the data tells us that a second site would be the same as the first. Maybe we'll get a polygon edge within the work volume - that would be a nice bonus. But for what Phoenix is being sent to do, it's purpose, it's prime goal... mobility is not in any way justified, required or 'missing'. By Sol 90 after the full characterization of the landing site, the sun will begin to set. From that point on, it's going to be camping out on ever decreasing power, on an atmospheric and surface monitoring program. Phoenix isn't going to last 1500 Sols. This is a short lived tightly focused mission for which mobility is not required.

Would it be nice to have a rover. YES. Would there be a benefit to Phoenix to be mobile? No - because it wouldn't even be on Mars - it would be on PPT's getting turned down at the scout selection phase. A lack of mobility made Phoenix possible. Lamenting that fact is moot.


Indeed, refering back to your quote ' how much does that argument advance things?' - debating mobility for Phoenix advances nothing whatsoever. There isn't a debate to be had. It's not mobile. Argument finished. I struggle to see what your point is beyond that.

Doug

You would be much better off advocating for improvements to future missions.