Date: October 9, 1997
Featuring: Dan Johnston
Trajectory and Aerobraking Design Analyst
Mars Global Surveyor
Jet Propulsion Laboratory, Pasadena, California
[ mars - 7 - 07:20:19 ]
Hello Hopkins is here
[ mars - 8 - 08:52:21 ]
Good morning from Coquille Oregon's 9th
grade Earth Science class. Joe Sterrett, teacher.
[ Sandy/NASAChatHost - 10 - 08:56:05
]
Good Morning Hopkins and Coquille Oregon's 9th grade Earth Science
Classs! So glad you could make it today! We will begin in about 10 minutes
(about 9:05)!
[ Sandy/NASAChatHost - 17 - 09:06:48
]
Good Morning Everyone! Dan Johnston, our Mars expert for today is
here! Dan is the trajectory and aerobraking design analyst on the Mars
Global Surveyor Operations Project, part of the Navigation Team. Dan will
fill us in on the spacecraft's aerobraking techniques and procedures and
if they're working.
Dan is writing a 3-4 paragraph intro for you to read before the chat.
[ DanJohnston/JPL - 18 - 09:07:02
]
After a 10 month interplanetary transit, the Mars Global Surveyor
(MGS) spacecraft successfully established orbit around Mars on September
12, 1997. The near polar, highly elliptical capture orbit had an initial
periapsis altitude (Hp) (closest approach to the planet) of 263 km, an
apoapsis altitude (farthest distance to the planet) of 54002.5 km, an
orbital period of 45 Hrs, and an inclination of 93.3 deg. The maneuver
(Mars Orbit Insertion (MOI)) that put us into orbit around Mars exhausted
75% of our total DV (or propellant) capability. During the cruise to Mars
we used another 2.5% of our total DV capability. To reach our desired
mapping orbit which is near circular with a periapsis altitude of 375 km,
an apoapsis alitude of 405 km, and a 2:00 PM descending node orientation
with respect to the sun, the spacecraft must aerobrake for 4.5 months. By
aerobraking we save a lot of propellant -- in fact we launched with a
propellant load around 50% of that actually required to establish our
mapping orbit propulsively. (cont)
[ DanJohnston/JPL - 19 - 09:07:37
]
MGS aerobraking operations began on Orbit Number 4 when the periapsis
of the orbit was lowered to the upper reaches of the Martian atmosphere
(150 km). As aerobraking progresses, the apoapsis of the orbit will decay
(be reduced) from 54000 km to 450 km. To control our aerobraking, we
lower and raise the periapsis altitude of the orbit by performing small
burns at apoapsis. The table below gives our progress to date (10/9/97).
(cont)
[ DanJohnston/JPL - 20 - 09:08:02
]
Currently we are on Orbit Number 16 around Mars. On Orbit Number 15
we saw our first large change in the Martian atmosphere and we moved to an
altitude of 121 km. During our closest approach to the planet, the
spacecraft tracks (travels) from the North Pole region to the South Pole
region over Mars. The periapsis altitude of the spacecraft is currently
located near 33 deg North latitude progressing North at a very slow
rate.(cont)
[ DanJohnston/JPL - 21 - 09:09:10 ]
| Orbit # | Hp (km) | Ha (km) | Orbit Period (Hr) | Mnvr (Burns) Magnitude |
|---|---|---|---|---|
[ DanJohnston/JPL - 22 - 09:10:27
]
Good morning everyone -- I am ready to take questions regarding Mars
Global Surveryor...
[ TimMcCollum/CharlestonMiddleSchool-mars - 23 - 09:10:39
]
Good Morning Sandy & Dan - Charleston Middle School signing on from
here and in Tokyo
[ Sandy/NASAChatHost - 25 - 09:13:15
]
Hello Tim and Charleston Middle School and Megan McCallister in
Japan! So glad you could join us today!
[ mars - 12 - 08:57:27 ]
How does the Global Suveyor get
information from Earth and where does it come from?
[ DanJohnston/JPL - 27 - 09:15:27
]
Mars: The MGS spacecraft recieves commands from tracking stations
located
around the world that are a part of the Deep Space Network -- Basically
radio signals are sent back and forth to the spacecraft. Currently it
takes 15 minutes to send a signal one-way to Mars.
[ DanJohnston/JPL - 31 - 09:19:07
]
RE: [Debi/Robinson-mars] What is the main
objective of the Mars Pathfinder?
Debi -- Mars Pathfinder is a lander mission looking a very
local region -- This week the focus is Mars Global Surveyor - an orbiter
that will map the complete surface of Mars.
[ DanJohnston/JPL - 40 - 09:23:30
]
RE:
[TimMcCollum/CharlestonMiddleSchool-mars] Dan - In your biosketch
you mentioned your passion for space exploration and interest in studying
past exploration of our own country. You are currently involved in the
unmanned MGS project. Where do you see the future balance between manned
and unmanned space exploration?
The unmanned (or robotic)
exploration missions seek to understand fundamental science objectives as
well as serve as precursor missions for future manned missions. The
programs are intimately linked.
[ DanJohnston/JPL - 42 - 09:25:21
]
RE:
[ElizabethM/CharlestonMiddleSchool-mars] Is this the first time
aerobraking has been used to obtain the final mapping orbit?
Yes
-- this is the first time that aerobraking is performed before the desired
mapping orbit is reached. On the Magellan mission, aerobraking was
demonstrated at the end of thier mapping mission at Venus.
[ DanJohnston/JPL - 45 - 09:27:33
]
RE: [DorothyW/Hannah/CMS-mars] Hi how are
you
today Mr.Johnston? i was wondering how long did you go to school before
you started this job?
Dorothy & Hannah - I went to school for 6
years and
received both Bachelor and Master's degrees from the Univ. of Texas
[ mars - 34 - 09:22:17 ]
How high of temperature can the Solar
Pannels withstand and what is the range of temperature would you want to
have it to work in?
[ DanJohnston/JPL - 49 - 09:28:15
]
Mars: The solar panels can stand around 160 deg C.
[ SaraF/CMS-mars - 35 - 09:22:29 ]
How does it feel to have one
of your projects out in space?Is it hard to navigate the Global
Surveyor?
[ DanJohnston/JPL - 52 - 09:29:29
]
SaraF - It is very gratiyfing to see the spacecraft make it to Mars.
Navigating the spacecraft is at times a difficult proposition and requires
the team work of a lot of people.
[ mars - 37 - 09:23:05 ]
What special functions does the Global
Surveyor have?
[ DanJohnston/JPL - 54 - 09:30:26
]
Mars -- A good description of the functions are on the MGS Web
Site (marsweb.jpl.nasa.gov)
[ Debi/Robinson-mars - 38 - 09:23:25 ]
What direction should
high school curriculum take for a student interested in a space science
career?
[ DanJohnston/JPL - 58 - 09:32:15
]
Debi - Space science career objectives are grounded most in math,
physics, chemistry, etc. Communictation skills are also important so
english, etc can not be neglected.
[ DanJohnston/JPL - 60 - 09:33:13
]
RE: [Kristen/CMS-mars] When approaching
Mars, what exactly causes the spacecraft to slow
down?
The spacecraft is slowed by firing its main engine.
[ DanJohnston/JPL - 62 - 09:34:18
]
RE: [Cara/CMS-mars] What is it like
knowing that something you helped create is up in space?
It is
very gratifying to see the spacecraft actually operate and fly in
space.
[ DanJohnston/JPL - 66 - 09:35:52
]
Satchmo - We are in near continious contact with the spacecraft at
this time. All three DSN stations are (in their turn) tracking our
spacecraft as we perform the aerobraking operations.
[ DanJohnston/JPL - 70 - 09:36:56
]
RE: [Megan-mars] You know there are blue
prints for a house is this sort of what you do to make up the Mars Global
Surveyor?
The spacecraft is drawn out in the desgin phase -- so
like blue prints for a house, engineering drawing are made before the
spacecraft is manufactured.
[ DanJohnston/JPL - 72 - 09:37:59
]
RE: [BigFoot/CMS-mars] What is the most
facinating thing about your job?
Exploring the unknown and the
ability to fly a vehicle where only a few have flown before.
[ DanJohnston/JPL - 73 - 09:38:23
]
RE: [Dorothy/CMS-mars] How old were you
when you got your B.S. degree?
21
[ DanJohnston/JPL - 76 - 09:39:41
]
RE: [Grizz/CMS-mars] Do you think we will
ever make it to another planet? If so which one will be
first?
Humans will eventually move out to other planets -- Mars
is the first natural choice because it has a lot of similiarities to
Earth.
[ DanJohnston/JPL - 78 - 09:40:49
]
RE: [Megan/Japan-mars] If the Mars Global
Surveyor didn't come out of its I guess its case what would you do about
it?
If the spacecraft would have not deployed properly at
launch the mission would have been over with a failure.
[ DanJohnston/JPL - 80 - 09:42:28
]
RE: [Hopkins-mars] What is your job
about?
See my bio on the Live From Mars Page -- Basically I have
strategic and tactical responsibilites for the aerobraking flight
operations. I also define the flight path the spacecraft takes as we
journey to the mapping orbit.
[ DanJohnston/JPL - 81 - 09:44:01
]
RE: [Debi/Robinson-mars] Is it a future
objective of NASA to send man to Mars? If so, approximately how far in
the future?
The answer is yes but I am not certain of the
timetable.
[ DanJohnston/JPL - 84 - 09:46:36
]
RE: [Steph/CMS-mars] Is your famoly
interested in what you do and are they supportive?
Yes -- The
whole family supports the work that I do and even the company that my wife
works for is very supportive of the unusual hours that I work and the
demands that that places on her. My daughter Katie prefers Jupiter,
however, because it is a bigger planet.
[ DanJohnston/JPL - 86 - 09:47:05
]
RE: [Tearleft/CMS-mars] What planet would
you like to see Americans visit next? Mercury, Venus, Neptune, Pluto,
Jupiter, Uranus, or Saturn. Please tell me!!!!
Mars
[ DanJohnston/JPL - 87 - 09:47:54
]
RE: [Meghan/CMS-mars] Is there any other
planet that we would be able to put a rover on?
Most likely, the
other terrestrial planets -- Mercury and Venus.
[ DanJohnston/JPL - 90 - 09:49:41
]
RE: [Megan/Japan-mars] On the average how
much time does it take to make a spacecraft or so going to another
planet?
That is difficult question -- Mars Global Surveror was
built in less than 3 years. That was extremely fast.
[ DanJohnston/JPL - 92 - 09:52:17
]
RE: [Elizabeth/CMS-mars] What difficulties
have you faced during your work on Mars Global Surveyor?
The most difficult challenge is the development of an
aerobraking flight profile that achieves the desired mapping orbit without
flying to deep in the Martian atmosphere while also flying deep enough to
make the time constraints for the mapping mission.
[ DanJohnston/JPL - 93 - 09:52:43
]
RE: [Sagar/CMS-mars] Can you land on gas
planets?
Don't know.
[ DanJohnston/JPL - 95 - 09:53:26
]
RE: [mars] Do you know about how much(
pounds or whatever unit is used to measure this) fuel at lift-off it takes
to get a pound of propellant to Mars?
Not right off -- It takes
a lot though.
[ DanJohnston/JPL - 96 - 09:54:47
]
RE: [mars] Is data posted somewhere
showing the characteristics of the orbit daily at the same point of the
orbit? We would like to plot say the apoapsis or some significant
characteristic as a function of time.
I belive the information
is on the MGS web site. Near daily statuses are provided.
[ Sandy/NASAChatHost - 97 - 09:54:51
]
RE: [TimMcCollum/CMS-mars] Sandy & Dan -
Thanks so much for your time and answers. Charleston Middle School
signing off. See you in the next session.
Goodbye Tim and
Charleston Middle School. Thanks for joining us today and thanks for your
questions!
[ Sandy/NASAChatHost - 99 - 09:57:13
]
RE: [Hopkins-mars] Thank you for your
answers. Hopkins Elementary signing off. Thanks again.
We're
really glad you could join us today! Thanks for your good questions! We
hope to schedule another chat in 2 wks--the week of Oct. 20. Further
details will be posted on the "Live From Mars" web site.
[ DanJohnston/JPL - 100 - 09:57:33
]
RE: [AstroKye-mars] As orbits become more
circular, does the craft have to move faster or slower to remain in
orbit?
The velocity of the spacecraft is not constant over the
orbit. The velocity of the spacecraft at periapsis will slow down as the
orbit becomes more circular.
[ Sandy/NASAChatHost - 102 - 10:00:30
]
EVERYONE: It's time for Dan to get back to what he does best! Thank
you so much for joining us today! We hope you had fun and look forward to
more of your great questions during the next WebChat (week of Oct. 20). A
special thank you to Dan for chatting with us today! Your answers were
most informative!
[ mars - 103 - 10:00:48 ]
Thank you for your time and answers,
Dan.
Coquille High School
Earth Science Class
Coquille, OR
[ AstroKye-mars - 105 - 10:02:10 ]
Thanks Dan and Sandy. Next
time I'll have the time differences straight! I got here an hour too late!
[ Sandy/NASAChatHost - 106 - 10:02:19
]
RE: [mars] Thank you for your time and
answers, Dan.
Coquille High School
Earth Science Class
Coquille, OR
Goodbye Coquille High School Earth Science Class!
Thanks for your questions! Chat with you again soon!
[ Sandy/NASAChatHost - 107 - 10:02:58
]
RE: [AstroKye-mars] Thanks Dan and Sandy.
Next time I'll have the time differences straight! I got here an hour too
late!
You're most welcome. Sorry about the time confusion. Look
forward to chatting with again!
[ mars - 101 - 09:57:46 ]
Why was the 2 o'clock mars position
chosen for mapping? Will that orientation be difficult to maintain?
[ DanJohnston/JPL - 108 - 10:03:28
]
Mars: The 2:00 PM orbit is a collective agreement reached by the MGS
science principal investigators for their respective science instruments.
E.G. The camera does not want to be too close to noon for imaging purposes
(shadows) -- The spectrometer wants to see a "hot" surface for a good
return signal. The mapping orbit is sun-synchronous so once we establish
the mapping orbit we should not drift too far from the 2:00 PM condition.
[ Sandy/NASAChatHost - 109 - 10:04:03
]
RE: [Debi/Robinson-mars] Dan - Robinson
Jr. High appreciates you answering our questions. They were very
informative.
Goodbye Robinson Jr. High! Thanks for your
questions! Chat with you in two weeks.
[ DanJohnston/JPL - 110 - 10:05:00
]
Unfortunately, time has expired here and I must return to normal work
now -- Thanks for all the questions and the interest. We will be talking
to you all at another time.
MDJ signing off...