Lowest altitude viable Mars orbit

Is it possible to be in Mars orbit and collide with Olympus Mons?

In article ,
Explorer8939 wrote:
Is it possible to be in Mars orbit and collide with Olympus Mons?

No, alas. (Not that anyone would want to collide with it, but an orbit
passing low over it would be quite the tourist ride...) Aerobraking and
reentry altitudes on Mars are actually similar to those on Earth -- the
atmosphere is much thinner, but the weaker gravitational field means
density drops off rather less rapidly with altitude. And Olympus Mons is
high, but it's not *that* high...
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Henry Spencer wrote:
In article ,
Explorer8939 wrote:
Is it possible to be in Mars orbit and collide with Olympus Mons?

No, alas. (Not that anyone would want to collide with it, but an orbit
passing low over it would be quite the tourist ride...) Aerobraking and
reentry altitudes on Mars are actually similar to those on Earth -- the
atmosphere is much thinner, but the weaker gravitational field means
density drops off rather less rapidly with altitude. And Olympus Mons is
high, but it's not *that* high...

But if you are doing touristy things on Mars, you could aerobrake to near it
then start a retrofire and fly past it on the other side to retrograde orbit.

--
Sander

+++ Out of cheese error +++

Henry Spencer wrote:
In article ,
Explorer8939 wrote:

Is it possible to be in Mars orbit and collide with Olympus Mons?


No, alas. (Not that anyone would want to collide with it, but an orbit
passing low over it would be quite the tourist ride...) Aerobraking and
reentry altitudes on Mars are actually similar to those on Earth -- the
atmosphere is much thinner, but the weaker gravitational field means
density drops off rather less rapidly with altitude. And Olympus Mons is
high, but it's not *that* high...

My remembrance of Martian aerobraking guidance studies in the late
1980's (single pass) was that we were using periapsis altitudes less
than the height of Mt. Olympus. That would depend on the mission of
course, and the atmospheric models have presumably been upgraded since
then.

- Matt

Managed to dig out a conference paper:
AIAA 87-2401, "Optimal Guidance for Future Space Applications,"
J.E.Bradt, M.V.Jessick, J.W.Hardtla. AIAA GN&C Conference 1987

The last example trajectory is a Martian aerocapture patterned
after the Mars Sample Return Mission studies of the day.
Periapsis altitude was 80000 ft, a bit less than 25km.
L/D 0.75 and 141 lbf/ft**2 ballistic coefficient

Of possible interest, it is my understanding that because of gravity, the
Martian atmosphere extends further from the ground then Earths, so any long
term orbit would necessarily need to be higher to avoid significant drag.
400 kilometers might not be a good place to park your crew return vehicle,
for example, unless you were confident that it could be reliably reboosted
as needed.

In article ,
Ben Hallert wrote:

Of possible interest, it is my understanding that because of gravity, the
Martian atmosphere extends further from the ground then Earths, so any long
term orbit would necessarily need to be higher to avoid significant drag.
400 kilometers might not be a good place to park your crew return vehicle,
for example, unless you were confident that it could be reliably reboosted
as needed.


On the flip side, how deep a hole would you have to dig on Mars so that
the atmospheric pressure at the bottom would approach 1 atmosphere? (And
how much thicker is the atmosphere at the bottom of Vallis Marineris
(say) than the "average" pressure on Mars?

--
Ben Weiss | Any technology distinguishable from magic
President & CTO | is insufficiently advanced.
Shell & Slate Software | - Clarke C. Arthur

"Ben Weiss" wrote in message
...
In article ,
Ben Hallert wrote:

Of possible interest, it is my understanding that because of gravity,
the
Martian atmosphere extends further from the ground then Earths, so any
long
term orbit would necessarily need to be higher to avoid significant
drag.
400 kilometers might not be a good place to park your crew return
vehicle,
for example, unless you were confident that it could be reliably
reboosted
as needed.


On the flip side, how deep a hole would you have to dig on Mars so that
the atmospheric pressure at the bottom would approach 1 atmosphere?

Would tossing Phobos at Mars be enough to dig that hole?

Ben Weiss wrote in message ...
In article ,
Ben Hallert wrote:

On the flip side, how deep a hole would you have to dig on Mars so that
the atmospheric pressure at the bottom would approach 1 atmosphere? (And
how much thicker is the atmosphere at the bottom of Vallis Marineris
(say) than the "average" pressure on Mars?

http://www.madsci.org/posts/archives...5313.As.r.html

You'll find your answer...

Basically, even 10 km down, at the bottom of Valles Marineris, you'd
only have 50 millibars, that's 5% of Earth's pressure, and 6 times
less than at the top of Mount Everest.

Ben Weiss wrote:

On the flip side, how deep a hole would you have to dig on Mars so
that the atmospheric pressure at the bottom would approach 1
atmosphere? (And how much thicker is the atmosphere at the bottom of
Vallis Marineris (say) than the "average" pressure on Mars?

Average pressure on Mars is 7 millibars and we want 1000 millibars,
so the pit needs to be log-n(1000/7) = about 5 scale heights deep.
Scale height is 10.8 km on Mars; therefore the pit will need to be
roughly 54 km deep.


;K

Ben Weiss wrote:
In article ,
Ben Hallert wrote:

Of possible interest, it is my understanding that because of gravity, the
Martian atmosphere extends further from the ground then Earths, so any long
term orbit would necessarily need to be higher to avoid significant drag.
400 kilometers might not be a good place to park your crew return vehicle,
for example, unless you were confident that it could be reliably reboosted
as needed.


On the flip side, how deep a hole would you have to dig on Mars so that
the atmospheric pressure at the bottom would approach 1 atmosphere? (And
how much thicker is the atmosphere at the bottom of Vallis Marineris
(say) than the "average" pressure on Mars?


As a very, very, very rough guess, the scale height of the martian
atmosphere is going to be some 10Km.
The atmospheric pressure is 1% of earth, so you need 7 scale heights, to
hit 1 bar.
70Km.