[X] My Assistant

[djellison]

What would be the point in using fuel now? I'm not sure what you're proposing. The spacecraft will land from the left in that diagram.

This is one of the 'not obvious at first' things about flying to Mars. Current - Phoenix is nearly ahead, and flying slower than Mars. Using the Sun as a reference Atmospheric Entry will actually speed up Phoenix

Doug

[SpaceListener]

Thanks Doug for the answer.

What would be the point in using fuel now? I'm not sure what you're proposing.

Previously I tought that Phoenix will throttle the engines to reduce its speed during its approach to Mars but it is discarded.

By the way, I have found the info which states:

Phoenix won't use any thrusters for breaking its speed toward Mars instead Phoenix will probably use the thrusters for short duration during two TCMs which are scheduled to be performed within the last three days. Just before entry, flight path data is sent to Phoenix that is used by the onboard computers during the descent and landing to guide the spacecraft to its landing site.

Hence, it is evident, up to now, the flight path for landing is still unconfirmed.

[slinted]

The latest MARCI Weather Report mentions a dust storm blowing around the north pole last week. It's somewhat hard to see in the rotating globe movie, but I think the edge of that system may have obscured the Phoenix landing site before moving on. I've been trying to get a sense of what the daily weather will be like once Phoenix lands. Is this what we might expect for this time of year? Localized dust storms? Pole wide storms? Clouds? I haven't seen much mention of this elsewhere.

[elakdawalla]

Thanks for the reminder about that site, slinted; it's an excellent idea to be checking the weekly weather reports to try and figure out what Phoenix is going to encounter on its way in. The penultimate report has some general information on what happens near the pole when spring arrives:

For those of us living in the Northern hemisphere here on the Earth, many of the telltale signs of spring have finally arrived. The northern spring on Mars officially began in December 2007, and over the past few months, we've observed many uniquely martian springtime weather phenomena. A few common springtime observations include dust storm activity and water ice clouds near the seasonal north polar cap edge, clouds and dust activity in the southern mid-latitudes, and the early development of the aphelion cloud belt. The weather this past week included all three of these springtime phenomena, with localized dust storm activity west of Argyre and in north Tempe near the seasonal north polar cap edge. Water ice clouds also persisted over the Tharsis volcanoes, with a notable "split" cloud at Ascraeus Mons, and mid-afternoon cloud formation centers apparent near the summits of Olympus and Arsia Mons.

--Emily

[dmuller]

When Phoenix meets Mars, the spacecraft will have a lower orbital speed around the Sun than Mars. Why? because the Phoenix orbit would take it all the way back to where it came from ... Earth (though Earth wouldnt be there at that particular time). So, in theory, Phoenix would have to speed up to match Mars' orbital speed and "stay" with it.

BUT

Phoenix (or any other craft going there) will get very close to Mars, and you want to stay there (land or orbit). So you cant fly "along" Mars because it's gravity would pull you in if you just match its orbital speed at close distance. The issue now is that you are too fast for Mars to capture you in its orbit, or land at a decent speed, so you have to slow down your orbital speed around MARS (not the Sun). Close to Mars, the gravitational force of Mars exceeds that of the Sun anyway, so you dont really care whether your orbital speed around the Sun goes up and down as you enter an orbit around Mars or land on it (it's a whole different story though if you use Mars for a swing-by).

Phoenix will reach the point from where the gravitational force to Mars exceeds the one to the Sun on 25-May-2008 11:25am SCET UTC, 12 hours before landing, at a distance from Mars of about 139,000 km

Phoenix will slow down its orbital speed around Mars as follows:
1. friction (glowing as it streaks through the atmosphere)
2. parachute
3. retro rockets for the last 500m or so only
4. ground impact

The remaining engine firings, TCMs, are not meant to slow the craft down to Mars, only to make sure it hits the atmosphere where it is supposed to.

Phoenix landing times are as follows:
Entry interface expected on 25 May 2008 23:31:12 UTC, landing expected on 25 May 2008 23:38:32 UTC Spacecraft event time
Entry interface expected on 25 May 2008 23:46:32 UTC, landing expected on 25 May 2008 23:53:52 UTC Earth received time

[Stu]

We REALLY don't want to come down in the middle of this lot, do we..?

[ElkGroveDan]

Unless we were to land just below that ridge right on top of that little crack that runs along the base of it.

[SpaceListener]

We REALLY don't want to come down in the middle of this lot, do we..?

Stu, I don't have the idea about the sizes of the stones. It can be interpreted as 1 cm or 1 meter since this picture has no referential size.

It would be very pity if the spacecraft loose the balance after the touchdown. The case landing MER is safer than the ones of Phoenix / Viking way of landing. This is because the ball will most probably be stopped whenever the surface is on the flat. Hope that MSL won't take so much risks as Phoenix does since MSL will cost many times more than Phoenix.

[nprev]

Oh, good Lord, no!!!! (SL, I'm gonna bet that those boulders are <1m each, since I think that the maximum resolution of HiRISE is 0.25m).

Sure looks like frost upheaval at work there, churning up the substrate; hopefully, it's very localized, and well away from the landing site. Scary as all hell, though.

[dmuller]

Someone asked whether Phoenix will land heading NW (diagonally up) or SE (diagonally down) on the landing ellipse. I assumed the formed but after some research seem to have been wrong! Here's the reasoning I figured out and gave that person, but did I miss something?:

Mars, on 25 May, is about 7.4 million km above the eclipitc. Phoenix, when it launched, was right on it (by definition). So at the moment it is approaching Mars from the South, will cross it's equator and head North. Mars' gravity will pull it back South just a little. So it will land from the North West and head South East (from higher to lower) in the landing elipse. Compare the following 3 images (load them, then use back / forward in your browser):

http://space.jpl.nasa.gov/cgi-bin/wspace?t...=1&showsc=1
http://space.jpl.nasa.gov/cgi-bin/wspace?t...=1&showsc=1
http://space.jpl.nasa.gov/cgi-bin/wspace?t...=1&showsc=1

That's Mars seen from above (in 1 minute intervals), and you can see Phoenix heading "down" again as it starts descending through the atmosphere
That's what I can figure out, at least

[infocat13]

[quote name='dmuller' date='Apr 26 2008, 04:44 PM' post='112865']
Someone asked whether Phoenix will land heading NW (diagonally up) or SE (diagonally down) on the landing ellipse. I assumed the formed but after some research seem to have been wrong! Here's the reasoning I figured out and gave that person, but did I miss something?:


what we need is a friendly JPL mission design or better yet aerodynamics folks to give us a glimpse of some of the AIAA aerodynamics papers on Phoenix.Unfortunately AIAA is very jealous about its copyright even through all of there papers are stamped
"paid by nasa grant number........"

This is a prelaunch astrodynamics/mission design paper for Phoenix scroll down to page 14 and are good images of approach and entry geometry.

http://trs-new.jpl.nasa.gov/dspace/bitstre...1/1/07-0267.pdf

[Stu]

Stu, I don't have the idea about the sizes of the stones. It can be interpreted as 1 cm or 1 meter since this picture has no referential size.

Wow, if HiRISE can suddenly spot rocks 1cm across it's had a heck of an upgrade!

As HiRISE's best resolution is typically - correct me if I'm wrong, someone - "around a metre" then those rocks are each bigger than Phoenix. Again, I stand ready to be corrected here guys...

... but I'm going to have nightmares about "The Hump Of Horror" on PSP_002104_2485...!!!









Having said that, imagine the view if Phoenix lands smack bang in the middle of that Hump... so much for "few rocks and boulders visible..."

Actually, being serious for a moment, why would these be so many rocks here and not elsewhere in this region? Why have they collected - or gathered - around the edge of this feature? There's another similar mound further "down" the strip, and it too has rocks arranged around its edge. What gives..?

[dvandorn]

As HiRISE's best resolution is typically - correct me if I'm wrong, someone - "around a metre" then those rocks are each bigger than Phoenix. Again, I stand ready to be corrected here guys...

HiRISE's best resolution is actually roughly 30cm (or very roughly one foot, for those of us who still use Imperial units). That said, though, I'd think you're still correct, each of those rocks is likely as larger as, or larger than, Phoenix itself.

-the other Doug

[dmuller]

This is a prelaunch astrodynamics/mission design paper for Phoenix scroll down to page 14 and are good images of approach and entry geometry.

Thanks infocat ... I used that document, and duly cited it, for some of the information used on my real-time Phoenix simulation script. Slipped my mind to look at it again to answer that question. Looks to me like we are going with "Page 14, Figure 13 EDL Communications Geometry, Open Launch" as shown in that document.

But yes it is a premission document, some things changed quite a bit. More information would definitively be a great plus. Emily mentioned the script on her Planetary Organization Blog, and I've had 250 site views on Saturday alone.

Daniel

[edstrick]

"...in the middle of this lot..."

It would be perfectly nice if it came down in the MIDDLE... between boulders.... ON TOP OF... that's different!

Another lower probability nightmare is if it came down nicely on a nice BIG flat-rock.... nice and level and undamaged... and with the sample scoop able to reach down to 1 centimeter ABOVE the ground below the edge of the rock. aaaAAAAUUURRGGGG!