NASA's Mars Reconnaissance Craft Begins Adjusting Orbit

http://www.jpl.nasa.gov/news/news.cfm?release=2006-048

Guy Webster (818) 354-6278
Jet Propulsion Laboratory, Pasadena, Calif.

Erica Hupp (202) 358-1237
NASA Headquarters, Washington

2006-048

NASA's Mars Reconnaissance Craft Begins Adjusting Orbit
March 31, 2006

NASA's Mars Reconnaissance Orbiter yesterday began a crucial six-month
campaign to gradually shrink its orbit into the best geometry for the
mission's science work.

Three weeks after successfully entering orbit around Mars, the
spacecraft is in a phase called "aerobraking." This process uses
friction with the tenuous upper atmosphere to transform a very
elongated
35-hour orbit to the nearly circular two-hour orbit needed for the
mission's science observations.

The orbiter has been flying about 426 kilometers (265 miles) above
Mars'
surface at the nearest point of each loop since March 10, then swinging
more than 43,000 kilometers (27,000 miles) away before heading in
again.
While preparing for aerobraking, the flight team tested several
instruments, obtaining the orbiter's first Mars pictures and
demonstrating the ability of its Mars Climate Sounder instrument to
track the atmosphere's dust, water vapor and temperatures.

On Thursday, Mars Reconnaissance Orbiter fired its intermediate
thrusters for 58 seconds at the far point of the orbit. That maneuver
lowered its altitude to 333 kilometers (207 miles) when the spacecraft
next passed the near point of its orbit, at 6:46 a.m. Pacific time
today
(9:46 a.m. Eastern Time).

"We're not low enough to touch Mars' atmosphere yet, but we'll get to
that point next week," said Dr. Daniel Kubitschek of NASA's Jet
Propulsion Laboratory, Pasadena, Calif., deputy leader for the
aerobraking phase of the mission.

The phase includes about 550 dips into the atmosphere, each carefully
planned for the desired amount of braking. At first, the dips will be
more than 30 hours apart. By August, there will be four per day.

"We have to be sure we don't dive too deep, because that could overheat
parts of the orbiter," Kubitschek said. "The biggest challenge is the
variability of the atmosphere."

Readings from accelerometers during the passes through the atmosphere
are one way the spacecraft can provide information about upward
swelling
of the atmosphere due to heating.

The Mars Climate Sounder instrument also has the capability to monitor
changes in temperature that would affect the atmosphere's thickness.
"We
demonstrated that we're ready to support aerobraking, should we be
needed," JPL's Dr. Daniel McCleese, principal investigator for the Mars
Climate Sounder, said of new test observations.

Infrared-sensing instruments and cameras on two other Mars orbiters are
expected to be the main sources of information to the advisory team of
atmospheric scientists providing day-to-day assistance to the
aerobraking navigators and engineers. "There is risk every time we
enter
the atmosphere, and we are fortunate to have Mars Global Surveyor and
Mars Odyssey with their daily global coverage helping us watch for
changes that could increase the risk," said JPL's Jim Graf, project
manager for the Mars Reconnaissance Orbiter.

Using aerobraking to get the spacecraft's orbit to the desired shape,
instead of doing the whole job with thruster firings, reduces how much
fuel a spacecraft needs to carry when launched from Earth. "It allows
you to fly more science payload to Mars instead of more fuel,"
Kubitschek said.

Once in its science orbit, Mars Reconnaissance Orbiter will return more
data about the planet than all previous Mars missions combined. The
data
will help researchers decipher the processes of change on the planet.
It
will also aid future missions to the surface of Mars by examining
potential landing sites and providing a high-data-rate communications
relay.

Test observations from the Mars Climate Sounder, other images and
additional information about Mars Reconnaissance Orbiter are available
online at http://www.nasa.gov/mro and at
http://marsprogram.jpl.nasa.gov/mro
javascriptpenNASAWindow('http://marsprogram.jpl.nasa.gov/mro') .

For information about NASA and agency programs on the Web, visit
http://www.nasa.gov .

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate, Washington. Lockheed Martin Space Systems, Denver, is the
prime contractor for the project and built the spacecraft.

Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced by
the cool-off between "plunges".
Is NASA hung up on the ballistic missile model that the rocket business
started with?

wrote in message
oups.com...
http://www.jpl.nasa.gov/news/news.cfm?release=2006-048

Guy Webster (818) 354-6278
Jet Propulsion Laboratory, Pasadena, Calif.

Erica Hupp (202) 358-1237
NASA Headquarters, Washington

2006-048

NASA's Mars Reconnaissance Craft Begins Adjusting Orbit
March 31, 2006

NASA's Mars Reconnaissance Orbiter yesterday began a crucial six-month
campaign to gradually shrink its orbit into the best geometry for the
mission's science work.

Three weeks after successfully entering orbit around Mars, the
spacecraft is in a phase called "aerobraking." This process uses
friction with the tenuous upper atmosphere to transform a very
elongated
35-hour orbit to the nearly circular two-hour orbit needed for the
mission's science observations.

The orbiter has been flying about 426 kilometers (265 miles) above
Mars'
surface at the nearest point of each loop since March 10, then swinging
more than 43,000 kilometers (27,000 miles) away before heading in
again.
While preparing for aerobraking, the flight team tested several
instruments, obtaining the orbiter's first Mars pictures and
demonstrating the ability of its Mars Climate Sounder instrument to
track the atmosphere's dust, water vapor and temperatures.

On Thursday, Mars Reconnaissance Orbiter fired its intermediate
thrusters for 58 seconds at the far point of the orbit. That maneuver
lowered its altitude to 333 kilometers (207 miles) when the spacecraft
next passed the near point of its orbit, at 6:46 a.m. Pacific time
today
(9:46 a.m. Eastern Time).

"We're not low enough to touch Mars' atmosphere yet, but we'll get to
that point next week," said Dr. Daniel Kubitschek of NASA's Jet
Propulsion Laboratory, Pasadena, Calif., deputy leader for the
aerobraking phase of the mission.

The phase includes about 550 dips into the atmosphere, each carefully
planned for the desired amount of braking. At first, the dips will be
more than 30 hours apart. By August, there will be four per day.

"We have to be sure we don't dive too deep, because that could overheat
parts of the orbiter," Kubitschek said. "The biggest challenge is the
variability of the atmosphere."

Readings from accelerometers during the passes through the atmosphere
are one way the spacecraft can provide information about upward
swelling
of the atmosphere due to heating.

The Mars Climate Sounder instrument also has the capability to monitor
changes in temperature that would affect the atmosphere's thickness.
"We
demonstrated that we're ready to support aerobraking, should we be
needed," JPL's Dr. Daniel McCleese, principal investigator for the Mars
Climate Sounder, said of new test observations.

Infrared-sensing instruments and cameras on two other Mars orbiters are
expected to be the main sources of information to the advisory team of
atmospheric scientists providing day-to-day assistance to the
aerobraking navigators and engineers. "There is risk every time we
enter
the atmosphere, and we are fortunate to have Mars Global Surveyor and
Mars Odyssey with their daily global coverage helping us watch for
changes that could increase the risk," said JPL's Jim Graf, project
manager for the Mars Reconnaissance Orbiter.

Using aerobraking to get the spacecraft's orbit to the desired shape,
instead of doing the whole job with thruster firings, reduces how much
fuel a spacecraft needs to carry when launched from Earth. "It allows
you to fly more science payload to Mars instead of more fuel,"
Kubitschek said.

Once in its science orbit, Mars Reconnaissance Orbiter will return more
data about the planet than all previous Mars missions combined. The
data
will help researchers decipher the processes of change on the planet.
It
will also aid future missions to the surface of Mars by examining
potential landing sites and providing a high-data-rate communications
relay.

Test observations from the Mars Climate Sounder, other images and
additional information about Mars Reconnaissance Orbiter are available
online at http://www.nasa.gov/mro and at
http://marsprogram.jpl.nasa.gov/mro
javascriptpenNASAWindow('http://marsprogram.jpl.nasa.gov/mro') .

For information about NASA and agency programs on the Web, visit
http://www.nasa.gov .

JPL, a division of the California Institute of Technology in Pasadena,
manages the Mars Reconnaissance Orbiter for NASA's Science Mission
Directorate, Washington. Lockheed Martin Space Systems, Denver, is the
prime contractor for the project and built the spacecraft.

Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced by
the cool-off between "plunges".

Note that MRO will still be going at orbital velocity after these
maneuvers. This "stone skipping across a pond" method is meant to
*adjust* MRO's orbit. You can't use it to slow down to less than
orbital velocity before re-entry. (If you tried, you'd just re-enter.)

What I meant was initially use thrusters to kill orbital velocity (but not
by much), which would cause the spacecraft to sink "slowly" then use the
aerodynamic drag (which causes heating) to provide some lift into thinner
atmosphere. I know you can't de-orbit "on command" without thrust (although
near earth orbits decay eventually because of drag).

"addams013" wrote in message
oups.com...
Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced
by
the cool-off between "plunges".

Note that MRO will still be going at orbital velocity after these
maneuvers. This "stone skipping across a pond" method is meant to
*adjust* MRO's orbit. You can't use it to slow down to less than
orbital velocity before re-entry. (If you tried, you'd just re-enter.)

wrote:
What I meant was initially use thrusters to kill orbital velocity (but not
by much), which would cause the spacecraft to sink "slowly" then use the
aerodynamic drag (which causes heating) to provide some lift into thinner
atmosphere. I know you can't de-orbit "on command" without thrust (although
near earth orbits decay eventually because of drag).

"addams013" wrote in message
oups.com...

Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced

by

the cool-off between "plunges".

Note that MRO will still be going at orbital velocity after these
maneuvers. This "stone skipping across a pond" method is meant to
*adjust* MRO's orbit. You can't use it to slow down to less than
orbital velocity before re-entry. (If you tried, you'd just re-enter.)

Let's try this again. Drag reduces the altitude on the *other* side of
the orbit. It produces a delta V opposite to the velocity, slowing down
the spacecraft, reducing its angular momentum and energy. This produces
a more circular, smaller orbit. If drag were impulsive rather than
continuous, the resulting orbit would be tangent to the first one at
perigee. In a highly elliptical orbit as for MRO, drag acts over a
small range of true anomaly centered at periapse. The periapse altitude
will decrease very slightly, but the bulk of the orbital change is still
at apoapse.

Once the orbit gets to be nearly circular, drag acts continuously,
constantly lowering the other side of the orbit so that the trajectory
becomes a spiral.

The bottom line: in order to de-orbit, you need to get rid of about
50 km^2/sec^2 of specific energy. That can happen by thrusting or by
aerodynamics. Given the thermal and structural problems which can
result from aerobraking, is it any wonder that thrusting is to be
preferred whenever possible?

-- Bill Owen

What I meant was initially use thrusters to kill orbital velocity (but not
by much), which would cause the spacecraft to sink "slowly" then use the
aerodynamic drag (which causes heating) to provide some lift into thinner
atmosphere.

Drag != lift. You lose some energy, which means that you can't "skip"
back up to your original height. And above a certain (very low,
compared to low Earth orbit) altitude, you can't generate lift anyway.

In article t,
writes:
Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced by
the cool-off between "plunges".

[Newsgroups trimmed]

This question has been answered in the space newsgroups. The basic
answer is that a slow reentry decreases the _peak heating rate_ but
increases the _total heat input_ to the spacecraft because the
heating lasts longer. You would indeed get some radiative cooling
between "plunges," but when the tradeoffs were done, the quick
reentry has turned out to be better.

Quick reentry also means small path errors have no time to build up
into big ones.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

In article t,
wrote:
Why don't manned spacecraft use this "stone skipping across a pond"
technique to return from earth orbit? The heat buildup would be reduced by
the cool-off between "plunges".

Because it doesn't work for reentry.

People fantasized about such approaches in the days when space travel was
just speculation, but when it came time to actually do the numbers, those
ideas simply didn't work out. There is no way to *stay up* long enough to
greatly reduce the overall heating problem. You have to stay up with
aerodynamic lift, and lift always comes bundled with a certain amount of
drag -- quite a bit of it, at hypersonic speeds, especially since the sort
of thin pointy shape that maximizes the lift/drag ratio tends to have a
prohibitive heat concentration at the pointy end. So you *can't* brake
very slowly while still staying up. It's simply not possible.

Skipping doesn't actually help much. You need the same amount of lift,
averaged over the whole trajectory, so you incur the same amount of drag,
averaged similarly. The benefit from cooling off between plunges is
almost completely canceled out by the plunges themselves being harder
and hotter.

MRO is using brief dips into the atmosphere to lower its orbit toward a
circular one, not to enter the atmosphere and descend from a near-circular
orbit. Those are two very different problems. In the former, orbital
mechanics handles the problem of staying up. In the latter, it can't.

Is NASA hung up on the ballistic missile model that the rocket business
started with?

All manned reentry vehicles since Gemini use aerodynamic lift to stay up
in thinner air as long as possible. It does help; it just doesn't help
all that much.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

In article , Henry Spencer wrote:
Is NASA hung up on the ballistic missile model that the rocket business
started with?

All manned reentry vehicles since Gemini use aerodynamic lift to stay up
in thinner air as long as possible. It does help; it just doesn't help
all that much.

How did ... Eddington? ... put it ? "A beautiful hypothesis laid
low by an ugly fact."

--
Aidan Karley, FGS
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233