Live From Mars WebChat

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
(-) negative are “down” maneuvers
(+) plus are “up” maneuvers

[ 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 (

[ 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?

[ 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!!!!

[ 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...