February 1st 06, 04:46 PM
http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_current.html
The PI's Perspective
Our Aim Is True
Alan Stern
January 31, 2006
New Horizons is nearing completion of its second week in flight, and
all
continues to go well. As Project Manger Glen Fountain is proud of
saying, New Horizons is now safely away from Earth, in the cold vacuum
she was born to thrive in.
In the past week, our spacecraft team, led by Alice Bowman and Nick
Pinkine at the Johns Hopkins Applied Physics Laboratory, continued
their
checkouts of spacecraft subsystems, and conducted Trajectory Correction
Maneuver (TCM) 1 with great success. As a result, the error in New
Horizons' trajectory, which was already small, has been reduced by a
factor of almost 20!
The purpose of TCM-1 was both to commission our propulsion system for
trajectory changes, and to null out launch injection errors.
Fortunately, our launch was so accurate that only about 40 miles per
hour of trajectory change needed to be made; this is less than one
quarter of our post-launch trajectory correction budget. Compare that
40
miles/hour number to our 36,254 mile-per-hour exit from Earth, and
you'll see just how fantastic a job our Atlas V/STAR-48 launcher did.
This allows us to bank the difference in fuel as savings for future
Jupiter, Pluto and Kuiper Belt Object encounters.
TCM-1 was split into two parts, called 1A and 1B. TCM-1A was a 5
meter/second test and calibration firing conducted on Saturday, January
28; TCM-1B was a 13.3 meter/second maneuver conducted on Monday,
January
30. Both maneuvers were successful. We plan to trim out the small
(about
4%) residuals from the two TCM-1 burns, and to correct to the much
better orbit solution we will have from another couple of weeks of
tracking by the Deep Space Network (DSN) in TCM-2. This burn, which is
likely to be the smallest of the three post-launch maneuvers, is
scheduled for Wednesday, February 15.
TCM 1A and 1B were conduced "open loop," by pointing the spacecraft in
the correct direction for the burn, stabilizing it like a spinning top,
and making a timed burn. In contrast, TCM-2 will be conducted in
"closed
loop" fashion, with the spacecraft three-axis stabilized and using its
onboard gyros in the loop to cut the burn off when the precise targeted
velocity change (Delta V) is achieved. As such, TCM-2 should produce an
even closer-to-spec burn than TCMs 1A and 1B. As you can see, we wanted
to "walk before we ran" in terms of TCM complexity, which is why TCM-1
used the simpler but less accurate technique described above.
As we gain flight experience with our spacecraft, we are coming up the
learning curve and seeing some of its idiosyncrasies. This is something
all spacecraft teams benefit from in early flight. For example, careful
tracking has shown that New Horizons is still slowly outgassing some
absorbed water it took on during its construction and testing on Earth.
This outgassing produces tiny puffs of gas from time to time,
particularly when a spacecraft surface that has only been in shadow is
exposed to sunlight, which vaporizes the water in it. Although the
forces at work due to water vaporization events are incredibly tiny,
less than 10-7 Gs, we are able to detect them from tracking data.
Another idiosyncrasy we are seeing is a few radiation-induced
single-bit
upsets in the spacecraft memory each day. Although these are occurring
at a somewhat higher than predicted rate, they are no problem and are
corrected automatically onboard the spacecraft when it does its
once-per-minute memory scrubs.
For those of you interested in the question "Where is New Horizons?",
our Web site at http://pluto.jhuapl.edu now has a feature that gives
you
both graphical location and trajectory displays, some distances to
Earth, Jupiter, and Pluto, and other information too. As I write these
words, the spacecraft has just passed the 12-million kilometer range
from Earth. Enjoy!
For those of you interested in our now-derelict third stage, we decided
not to include it in the "Where is New Horizons?" feature, but I can
tell you that orbit extrapolations tell us that our third stage is now
about 15,000 kilometers from New Horizons. By the time it reaches
Jupiter, the defunct stage will be about 400,000 kilometers away from
our spacecraft. Owing to it missing the Pluto aim point at Jupiter by
this amount, the third stage will miss Pluto by about 200 million
kilometers - which is about as far as the average distance from the
Sun to Mars.
Finally, for this week, I'll remind you that this coming Saturday
(February 4) will be the 100th anniversary of Clyde Tombaugh's birth.
I'll be helping Clyde's home state, Kansas , celebrate that occasion
with some events in Lawrence , Kansas , this weekend. If you have a
moment to reflect on this when Saturday comes, give some thought to the
fact that Clyde's hard work resulted in the discovery of not just a new
planet, but also the Kuiper Belt, and with it, a huge and unexpected
new
piece of the geography of our home solar system.
The PI's Perspective
Our Aim Is True
Alan Stern
January 31, 2006
New Horizons is nearing completion of its second week in flight, and
all
continues to go well. As Project Manger Glen Fountain is proud of
saying, New Horizons is now safely away from Earth, in the cold vacuum
she was born to thrive in.
In the past week, our spacecraft team, led by Alice Bowman and Nick
Pinkine at the Johns Hopkins Applied Physics Laboratory, continued
their
checkouts of spacecraft subsystems, and conducted Trajectory Correction
Maneuver (TCM) 1 with great success. As a result, the error in New
Horizons' trajectory, which was already small, has been reduced by a
factor of almost 20!
The purpose of TCM-1 was both to commission our propulsion system for
trajectory changes, and to null out launch injection errors.
Fortunately, our launch was so accurate that only about 40 miles per
hour of trajectory change needed to be made; this is less than one
quarter of our post-launch trajectory correction budget. Compare that
40
miles/hour number to our 36,254 mile-per-hour exit from Earth, and
you'll see just how fantastic a job our Atlas V/STAR-48 launcher did.
This allows us to bank the difference in fuel as savings for future
Jupiter, Pluto and Kuiper Belt Object encounters.
TCM-1 was split into two parts, called 1A and 1B. TCM-1A was a 5
meter/second test and calibration firing conducted on Saturday, January
28; TCM-1B was a 13.3 meter/second maneuver conducted on Monday,
January
30. Both maneuvers were successful. We plan to trim out the small
(about
4%) residuals from the two TCM-1 burns, and to correct to the much
better orbit solution we will have from another couple of weeks of
tracking by the Deep Space Network (DSN) in TCM-2. This burn, which is
likely to be the smallest of the three post-launch maneuvers, is
scheduled for Wednesday, February 15.
TCM 1A and 1B were conduced "open loop," by pointing the spacecraft in
the correct direction for the burn, stabilizing it like a spinning top,
and making a timed burn. In contrast, TCM-2 will be conducted in
"closed
loop" fashion, with the spacecraft three-axis stabilized and using its
onboard gyros in the loop to cut the burn off when the precise targeted
velocity change (Delta V) is achieved. As such, TCM-2 should produce an
even closer-to-spec burn than TCMs 1A and 1B. As you can see, we wanted
to "walk before we ran" in terms of TCM complexity, which is why TCM-1
used the simpler but less accurate technique described above.
As we gain flight experience with our spacecraft, we are coming up the
learning curve and seeing some of its idiosyncrasies. This is something
all spacecraft teams benefit from in early flight. For example, careful
tracking has shown that New Horizons is still slowly outgassing some
absorbed water it took on during its construction and testing on Earth.
This outgassing produces tiny puffs of gas from time to time,
particularly when a spacecraft surface that has only been in shadow is
exposed to sunlight, which vaporizes the water in it. Although the
forces at work due to water vaporization events are incredibly tiny,
less than 10-7 Gs, we are able to detect them from tracking data.
Another idiosyncrasy we are seeing is a few radiation-induced
single-bit
upsets in the spacecraft memory each day. Although these are occurring
at a somewhat higher than predicted rate, they are no problem and are
corrected automatically onboard the spacecraft when it does its
once-per-minute memory scrubs.
For those of you interested in the question "Where is New Horizons?",
our Web site at http://pluto.jhuapl.edu now has a feature that gives
you
both graphical location and trajectory displays, some distances to
Earth, Jupiter, and Pluto, and other information too. As I write these
words, the spacecraft has just passed the 12-million kilometer range
from Earth. Enjoy!
For those of you interested in our now-derelict third stage, we decided
not to include it in the "Where is New Horizons?" feature, but I can
tell you that orbit extrapolations tell us that our third stage is now
about 15,000 kilometers from New Horizons. By the time it reaches
Jupiter, the defunct stage will be about 400,000 kilometers away from
our spacecraft. Owing to it missing the Pluto aim point at Jupiter by
this amount, the third stage will miss Pluto by about 200 million
kilometers - which is about as far as the average distance from the
Sun to Mars.
Finally, for this week, I'll remind you that this coming Saturday
(February 4) will be the 100th anniversary of Clyde Tombaugh's birth.
I'll be helping Clyde's home state, Kansas , celebrate that occasion
with some events in Lawrence , Kansas , this weekend. If you have a
moment to reflect on this when Saturday comes, give some thought to the
fact that Clyde's hard work resulted in the discovery of not just a new
planet, but also the Kuiper Belt, and with it, a huge and unexpected
new
piece of the geography of our home solar system.