04-FEB-99 2:00 AM PST

It is 2 AM PST on February 4, 1999, and we can confirm that Mars Global Surveyor's aerobraking is COMPLETE! and the spacecraft's orbit has been raised out of the atmosphere for good.

A 61 m/s bipropellant main engine burn on 04-FEB-99 at 12:11 AM PST did the job perfectly!

The Spacecraft Team at LMA in Denver confirmed the propulsion performance, as well as nominal performance by all of the spacecraft's other subsystems, during a small window of telemetry data after the spacecraft turned away from the burn attitude and telecommunications were reestablished over the high gain antenna to DSN station 63 at Madrid. A preliminary assessment of the doppler data by the Navigation Team at JPL also confirmed a good maneuver. It appears that all of the orbit parameters that we had targeted to have been achieved.

What an "exciting" last two days these have been - topped off by this perfectly executed event. We are now on the threshold of the real payoff of this mission - the beginning of mapping.

We could have never reached this point without dedicated leadership provided by Navigation Team and the Atmospheric Advisory Group. Their work was critical and is very much appreciated.

But, of course, the success of this long, arduous process was the result of the great team work that has characterized of our flight operations. Thank you to you all - the flight ops managers, Spacecraft, RTO, DSN, ground data system, TMOD, Nav, AAG, Mission Planning and Sequence, and science teams!

Glenn E. Cunningham

Mars Surveyor Operations Program

03-FEB-99 9:00 AM PST -- Based upon the orbit determination solution that came in 7 hours ago, the optimal orbit for the ABX burn has narrowed to either orbit number 1283 or 1284. The final determination will be made in 15 minutes.

03-FEB-99 9:45 AM PST -- Orbit number 1284 has been selected for the ABX burn that will end the aerobraking phase. This burn will occur on 04-FEB-99 at 08:01:31 UTC which is equivalent to approximately 12:01 AM PST. A number of events are associated with this burn.

03-FEB-99 10:30 AM PST -- Before the sequence builds can be prepared to perform the ABX burn, a new OPTG file had to be generated in order to calculate the correct timing. The file optg_i_990203_OD1271-1274_NoABX became available a half an hour ago. The ending NoABX means that the orbit solutions generated take into account the fact that the ABX burn has not yet occurred. A new OPTG file will be generated once the ABX burn has been completed. The current OPTG file is driving the Real Time Orbit Display which means that this display is current and as accurate as possible. The Detailed Orbit Display is using an older OPTG file, but is still fairly accurate. The next command conference meeting will be at 11:30 AM PST to discuss the P1281 build which sets the latch valves to the open position. After that there will be another meeting at 2:00 PM PST to discuss the A1284 STL sequence. Two more meetings will follow for the A1286 BSM sequence and the P1287 TMO sequence. The exact times of these meetings have not been determined yet. Check back at 12:30 PM PST for more details.

03-FEB-99 12:30 PM PST -- The P1281 sequence that controls the orientation of the spacecraft during the drag passes has been built and is approved for transmission. The A1284 sequence that will controls the firing of the main engine during the ABX maneuver has been built and is awaiting approval at the 2:00 PM command conference.

03-FEB-99 1:30 PM PST -- Nav Team Report

Lighter than anticipated atmosphere has reduced the MGS apoapsis decay rate. Therefore, the orbit at which the 450 km altitude will be reached has shifted from number 1278 to 1284. As a result, the ABX maneuver will occur at 12:15 am tonight, PST. The maneuver sequence is presently being built and tested at LMA. It is expected that the sequence will be up linked to the spacecraft this afternoon from DSS-45, at Canberra. The maneuver will execute while the spacecraft is being tracked by DSS-65, at Madrid. The main engine will fire for 70 seconds to increase the orbital velocity by 61 m/s and raise periapsis to 379 km. The post-burn orbit will have a period of 118 minutes, a local mean solar time at the descending node of 2:03:36 am, and an inclination of 93.0 deg. All orbital parameters are well within the required tolerances.

03-FEB-99 2:30 PM PST -- The A1284 sequence that commands the spacecraft to perform the ABX burn has been approved for transmission to the spacecraft. Although this sequence has already been tested three times, it will be tested one more time by the STL, Spacecraft Test Laboratory. In the unlikely event that any problems arise, the sequence will be modified and retransmitted to the spacecraft. The STL is a network of computers located at Lockheed Martin Aerospace in Denver Colorado that simulates the computers on board MGS was well as the sensors that feed data to the main flight computer. Sequences are first tested on the STL to verify they function as designed before being radiated to the spacecraft. The next meeting will occur at 4:00 PM PST to review the BSM and TMO sequences. The BSM is a backup sequence that will execute in the unlikely event the A1284 ABX sequence fails. The A1286 BSM sequence commands the spacecraft to perform a 12 meter per second thruster burn at apoapsis to raise the periapsis to 160 kilometers. This will take Mars Global Surveyor out of the atmosphere and give the Nav Team plenty of time to build another ABX maneuver. The BSM is a Backup Safety Maneuver to ensure that MGS is never in danger of impacting the surface of Mars. The TMO sequence is a Transfer to Mapping Orbit maneuver designed to freeze the argument of periapsis and lock in a sun synchronous orbit.

03-FEB-99 4:30 PM PST -- We are go for the ABX burn at on 04-FEB-99 at 12:15 AM PST. We will all be here when the main engine fires for the last time. Check back in about 7 hours around 11:30 PM PST.

04-FEB-99 12:20 AM PST -- The ABX burn has executed successfully!

The Nav Team is currently considering orbit number 1278 or orbit number 1282 as the two most likely candidates for the ABX maneuver. A number of factors are feeding into this decision, one of which is the most current orbit determination analysis which will be available around 2:00 AM PST on 03-FEB-99, or about 6 hours from now.

We will post the most current information as it becomes available.

The All Important Chart - Rev 4A

Drag Duration is Longer at the End of Aerobraking

An ABM Up maneuver of 0.4 m/s is required at Apoapsis1269.

To provide an additional margin of safety and not encroach at all into the 48 hour orbit lifetime constraint, the Nav Team has decided to perform four walkout ABM maneuvers as originally planned. As previously described, the 48 hour constraint provides a safety margin in the unlikely event that a command sequence fails to execute properly. In addition, the Nav Team factored into the decision the longer drag pass duration for this final phase of aerobraking.

On 27-JAN-99 there were five Up maneuvers scheduled for the walkout phase of aerobraking, but due to a 0.6 m/s glide slope trim maneuver that occurred on 28-JAN-99, one of the five planned walkout maneuvers was deleted. The timing of the glide slope trim maneuver made it similar to an Up walkout maneuver. This eliminated the need for one of the five Up maneuvers, which left four Up maneuvers for the walkout phase of aerobraking. As of today, three of the four Up maneuvers have been successfully executed. Throughout the walkout phase the spacecraft has experienced lower drag than what was predicted by the Mars-Gram Wave Model. The Nav Team has been continually comparing predicted apoapsis decay rates to actual apoapsis decay rates in determining when to perform the next ABM walkout maneuver. Given the low atmospheric densities encountered by the spacecraft, this morning the Nav Team debated whether or not the 4th ABM walkout maneuver was necessary.

In the last twelve hours the apoapsis height has been higher than predicted by the run outs for future orbits. At 8:51 AM this morning the actual verses predicted apoapsis heights for orbit 1271 became available. The actual apoapsis height is derived from information provided by the Deep Space Network. The predicted apoapsis height was 486.2 kilometers verses an expected actual height of 491 kilometers, which represents a difference of approximately 5 kilometers. Since the spacecraft had not experience as much aerobraking as predicted, this fact seemed to argued in favor of not performing the 4th ABM Up maneuver which is designed to slow down the rate of aerobraking. However, after reviewing past differences between actual and predicted apoapsis height values, the Nav Team determined that this slight difference could be attributed to "noise in the signal" and was not significant. Once again, maintaining the 48 hour orbit lifetime safety margin became the dominant consideration.

The 48 hour orbit lifetime constraint is based upon a 300 kilometer apoapsis height. The apoapsis height must not fall below 300 kilometers, or else the orbit will rapidly decay. This is especially true in the final stage of aerobraking when the drag pass time duration is long compared to the early stages of aerobraking. The Denver Spacecraft Team also played a role in making the decision to perform the 4th ABM. During each aerobraking meeting the MGS Spacecraft Team at Lockheed Martin Aerospace in Denver Colorado is in contact with the JPL Nav Team via the Teleconferencing Speaker Phone. At the end of this morning's meeting the controllers in Denver pointed out that the beginning of the 48 hour orbit lifetime safety margin is more valuable than the end of the safety period. This is due to the geometry of the final aerobraking orbits when the spacecraft is closer to the surface of Mars. Also, during this period the spacecraft spends more time behind Mars, making communications with Earth more difficult. For these reasons, the Nav Team and Spacecraft Team made the joint decision to maintain the 48 hour orbit lifetime safety margin and to perform an ABM walkout Up maneuver of 0.4 meters per second.

This may effect the timing of the ABX maneuver currently scheduled for orbit number 1278. This is the final aerobraking maneuver which will circularize the orbit and begin the transition to the mapping phase.

Nav Team Status as of 10:30 AM PST

The MGS aerobraking walkout continues on schedule. The spacecraft continues flawless performance. The fourth orbit raise maneuver will be performed tonight at 8:00 pm PST. This 0.4 m/s maneuver will ensure that the 48-hour orbit lifetime is preserved up to the time of the aerobraking exit maneuver (ABX). The ABX maneuver will most likely occur at about 1:00 pm tomorrow at apoapsis on orbit number 1278 when the orbit apoapsis decays to 450 km. The selection of the orbit for ABX will be made tonight at 8:00 pm at the Aerobraking Planning Meeting.

The ABX maneuver will utilize the bi-prop main engine which has not been used since the walk-in to Phase 2 aerobraking last September. This maneuver will change the velocity by 61 m/s and will raise the orbit periapsis well out of the atmosphere, from 115 km to 380 km. MGS will remain in this orbit for two weeks for the Gravity Calibration. After this calibration, the TMO maneuver will make another orbit periapsis adjustment to prepare for mapping operations.

No additional ABM is required at the present time.

An ABM Up maneuver of 1.0 m/s is required at Apoapsis1256.

The Nav Team weighted the benefits of two possible maneuvers at Apoapsis 1256. A burn of 0.8 m/s or 1.0 m/s were the possible candidates. The Nav Team had to consider a number of factors in arriving at this decision. One is the ever present 48 hour orbit lifetime constraint which mandates that the spacecraft always has 48 hours after a burn before the apoapsis altitude drops to 300 kilometers. Another factor was the desire to arrive at the ABX burn close to orbit number 1277 when the argument of periapsis will be 241.4 degrees. Since the wave model has been over predicting the atmospheric density, at first the 0.8 m/s burn looked like the better option. In this case another small burn would most likely be required within the next 24 hours to maintain the 48 hour orbit lifetime constraint. However, there will not be sufficient orbital data available this evening to make an accurate decision for an additional burn. For this reason, the decision was made to go with the 1.0 m/s burn which carries the risk of lofting past the desired ABX point at orbit number 1277. If we do loft past orbit 1277 for the ABX burn it will only slightly change the targeted argument of periapsis. Therefore, the 1.0 m/s burn has the lower risk of the two options.

No ABM is required at the present time.

The burn that was schedule for orbit 1250 has been pushed back to orbit 1256, due to lower than predicted dynamic pressures. The ABX burn that will circularize the orbit and officially end aerobraking is currently scheduled for orbit 1274 on 03-FEB-99.

No ABM is required at the present time.

An ABM Up maneuver will be performed on orbit number 1238.

The decision as to when to perform the next ABM is being driven by the 2 Day Orbit Lifetime Altitude constraint. In the above chart the red line represents a 2 Day Orbit Lifetime Altitude of 300 km. If an ABM maneuver fails to execute as planned, the spacecraft has two days before it reaches an apoapsis altitude of 300 kilometers. At this point the orbit will very rapidly decay and the spacecraft will impact the surface of Mars within a small number of orbits. For the Up1 there are two orbits below the red line, for the Up2 there are four orbits below the red line. This basically means that the Nav Team has two days to get another Up maneuver command to the spacecraft before they would be in danger of loosing Mars Global Surveyor. This is why there is a two day orbit lifetime constraint. Although it is rare for a command sequence to fail, there needs to be a margin for error. If the first attempt to send an Up ABM command to the spacecraft fails, the Nav Team still has a 48 hour safety margin.

The next ABM opportunity will be on orbit 1250.

No ABM is required at the present time.

By analyzing periapsis acceleration plots and other orbital data, the Nav Team has determined that the Mars-Gram Wave Model is over predicting the amount of drag the spacecraft will experience in the next series of aerobraking drag passes. For this reason, an ABM maneuver will not be performed on orbit 1232. The next possible opportunity for an ABM burn will be on orbit 1238. Once every 12 hours the Nav Team must now reset the parameters that feed the programs which predict the next required ABM maneuver. In the past, this process took place once a week. The new parameters were reviewed in the weekly Reset Meeting. However, the margin for error is so small now that the analytic models must be recalibrated before the next ABM can be determined.

The decision of whether or not to perform an ABM on orbit 1238 will be made at 8:00 PM this evening.

29-JAN-99 8:00 PM PST

The first ABM walkout burn on orbit 1214 executed successfully.

Now the times of all of the succeeding ABM maneuvers have changed once again. The spacecraft is now experiencing lower than expected dynamic pressures. The next most likely ABM will be on orbit 1232 at approximately 4:20 AM UTC on 31-JAN-99. However, we will not know for sure until 12 hours from now. The only real concern at this point is that we must not drop below a self imposed 2 day orbit lifetime constraint. This is the time it takes for the apoapsis height to decay to 300 km at which point the spacecraft will impact the surface of Mars within 3-4 more orbits. Since the atmosphere is now less dense and yielding lower dynamic pressures, in the words of one of the chief navigators: "We are just going to float on over to the next burn opportunity and proceed from there".

No ABM is required at the present time. Check in tomorrow morning for the next 12 hour update.

The first walkout ABM burn has been executed; the walkout phase has begun!

We are waiting to received the most recent telemetry to determine the effect of the1.1 m/s burn that that took place 40 minutes ago. The one way light time (OWLT) from Mars is currently 9 minutes and 59 seconds. There are other short delays in receiving the data from the Deep Space Network. We will know shortly.

The Nav Team has decided upon the orbit for the first ABM walkout maneuver burn. It will be on orbit 1214 at approximately 4:20 PM UTC on 29-JAN-99 which is equivalent to 8:20 AM PST tomorrow morning. This first ABM walkout maneuver will be an "up maneuver" of 1.1 m/s. Because of the 0.60 m/s glide slope trim maneuver that occurred this morning, one of the original 5 ABM walkout maneuvers was deleted. Below is the updated schedule for the walkout phase.

The MGS team at Lockheed Martin Aerospace in Denver Colorado is currently building the sequence which will execute the ABM burn at Apoapsis 1214. It will be uploaded by the Deep Space Network shortly.

Nav Team Update

The first of four aerobraking walkout maneuvers is predicted to occur on 1/29/99 Friday at about 8:00 am PST (9:00 am MST). The ABX maneuver is predicted to occur on 2/3/99 Wednesday at about 12:00 pm PST (1:00 pm MST). The ABX time can still change by 24 hours depending on the atmosphere densities encountered.

Because of higher than anticipated atmospheric densities, a special "glide slope" trim maneuver of 0.6 m/s (up in altitude) was performed this morning at 9:30 am PST. Because of this maneuver, the first walkout maneuver was delayed 12 hours.

28-JAN-99 8:00 AM PST

Late last night the Nav Team reviewed the most recent reconstructed nav data and determined that the spacecraft is experiencing higher than expected dynamic pressures. For this reason, a decision was rapidly made to perform an up ABM maneuver on orbit number 1203. This 0.60 m/s burn will speed up the spacecraft at apoapsis which will raise the periapsis and slow down the rate of apoapsis decay. This burn will also effect the timing of the first ABM associated with the walkout phase. This first walkout ABM will now occur on orbit 1216.

Please note that the 1:00 PM daily aerobraking meeting has been changed to 8:00 AM and 8:00 PM daily until the walkout phase is complete.

No Aerobraking Maneuver (ABM) is required at the present time.

The Nav Team focused on discussing the schedule for performing the walkout burn maneuvers. The first walkout burn is currently scheduled for orbit 1208 on January 28th at approximately 7:45 PM PST. The walkout is composed of a series of 5 small ABM maneuvers which will decrease the rate of apoapsis decay. The final maneuver is a large burn termed the ABX, which will raise the height of periapsis to 379 km. These maneuvers are analogous to a plane coming in for a landing; the rate of altitude loss is gradually decreased until the pilot executes a flare just before the plane touches down on the runway. The ABX burn has been termed a "flare maneuver" in which the rate of apoapsis altitude loss is reduced to zero. Just as landing an airplane is the most difficult part of flying, these 6 maneuvers are the most difficult part of aerobraking. Below is the current schedule:

Reset 21 Report

Charts from Reset Meeting

Aerobraking Orbit Plot

AAG Plots of Atmospheric Data

No Aerobraking Maneuver (ABM) is required at the present time.

The Nav Team is currently considering when to perform the ABM burn that will begin the walk out phase. The burn to end aerobarking and begin the walk out phase was scheduled for around orbit 1214 on January 30th. However, the final phase of aerobraking has proceeded slightly ahead of schedule and the burn will most likely take place around orbit 1207 on the 28th. This is the final and most critical ABM burn of the aerobraking phase. Since the orbital period is now quite short, the Spacecraft Team has less time to build the sequence files that command the spacecraft. In addition, the Nav Team must consider the effect of a missed opportunity to perform the ABM and determine the next possible ABM maneuver. A new OPTG file must be generated before the timing and magnitude of the burn can be determined. The maximum spacecraft life time also diminishes as the spacecraft spends a greater percentage of time in darkness on the backside of Mars. When the spacecraft is eclipsed by Mars it is forced to draw off the batteries which the diminishes the maximum spacecraft life time in the event of an anomaly. All of these factors combine to make this the most critical phase of aerobraking. For this reason, starting with tomorrow's Reset Meeting we will report spacecraft status twice daily until the walk out phase ABM is successfully executed.

26-JAN-99 5:30 PM PST -- Update from Nav Team

The first of four AB walkout maneuvers is predicted to occur on 1/28/99 Thursday at about 8:00 pm PST (9:00 pm MST). The ABX maneuver is predicted to occur on 2/2/99 Tuesday at about 7:00 pm PST (8:00 pm MST). These times can still change by 24 hours depending on the atmosphere densities encountered. The decision for the orbit for the first walkout maneuver will be made at either the 1/27/99 or 1/28/99 APG meeting.

No Aerobraking Maneuver (ABM) is required at the present time.

The Nav Team reviewed the decision to use a different method to scale the Mars-Gram atmospheric model. Until recently the Mars-Gram model was scaled by comparing the density at periapsis as predicted by Mars-Gram to the density as determined by navigation data. As long as the periapsis sweep angle was small, this method yielded good results. However, as the orbit became more circular the drag pass sweep angle increased with the result that the density at periapsis is no longer a good indication of the average density over the entire drag pass. If the acceleration curve is smooth the old method would still work, however, there is now a high degree of variation in the acceleration values along the drag pass. For this reason, the Nav Team is now using was has been termed an "Integrated Effect" to scale the Mars-Gram model. This is the delta V or the area under the drag pass acceleration curve. Mars-Gram is based on Viking data and must be scaled to produce the Mars-Gram Wave Model which is used to predict the decrease in apoapsis altitude.

24-JAN-99 1:00 PM PST

No Aerobraking Maneuver (ABM) is required at the present time.

No Aerobraking Maneuver (ABM) is required at the present time.

The Nav Team continues to analyze predicted values derived from the Mars-Gram Wave model verses those observed. Mars-Gram is a model of the Martian atmosphere based on Viking data which characterizes variations in atmospheric density as a function of altitude, latitude, temperature, and other parameters. However, the model does not have any longitudinal dependency. In order to model the longitudinal dependency, the output from the Mars-Gram model is correlated with historical data by using regression to arrive at series of cosine terms. As part of this process a Mars-Gram scale factor is produced that adjusts the model to yield density values closer to those observed from historical data. This scale factor is label as "MG-fac actual" in the Mars-Gram Wave Model graph produced for the weekly Reset Meetings. The output from the Mars Gram Wave Model is a predicted density which is used to determine future apoapsis decay rates. However, the predicted rates of apoapsis decay often differ from the observed rates. The degree of variation has been termed the "effectivity" which is used to arrived at the actual dynamic pressured needed to obtain a desired apoapsis decay rate. In this manner, the Nav Team is continually "feeling their way" through the Martian atmosphere by using recent drag pass data to make reasonable predictions for the next series of orbits.

No Aerobraking Maneuver (ABM) is required at the present time. The Nav Team continues to compare the predictions from the Mars-Gram Wave model verses those reconstructed from know orbital data. It was determined that the effective dynamic pressure during the drag pass must be higher in order to achieve the desired apoapsis decay rate. This is attributed to the atmospheric profile in the south polar region of Mars where the drag passes are currently taking place. For this reason, the Mars-Gram scale factor was increased by 24% to yield values closer to those obtained from reconstructed drag pass data.

No Aerobraking Maneuver (ABM) is required at the present time. See Reset 20 Report below.

Reset 20 Report

No Aerobraking Maneuver (ABM) is required at the present time.

Over the last week the Nav Team has modified the criteria used to determine whether or not the spacecraft is aerobraking at the rate required to meet the targeting objectives. Until this week the rate of apoapsis decay was the primary concern, now Local Mean Solar Time (LMST) is also a consideration. Prior to the final phase of aerobraking, the decision to perform an ABM was based upon a corridor range of acceptable values. ABMs where only performed with the spacecraft was nearing the edge of the acceptable aerobraking corridor. In the final phase of aerobraking this was modified to a baseline approach in which the spacecraft is required to follow a particular path to within a tight margin. As the spacecraft locks into a sun synchronous orbit, the acceptable error in the degree of apoapsis decay can be relaxed. Currently the spacecraft is on target for achieving the correct orbit at the start of the walk out phase. This is the final transition to mapping in which propulsive maneuvers will raise the periapsis and lower the apoapsis until a circular orbit is achieved.

Also in the past week the atmosphere has begun to stabilize as the argument of periapsis moves to the center of the "polar vortex". As a result, the wave model now predicts more gradual variations in the constitution of the atmosphere.

No Aerobraking Maneuver (ABM) is required at the present time. The next possible ABM will be at Apoapsis 1122.

No Aerobraking Maneuver (ABM) is required at the present time. Over the last two days the Nav Team has had difficulty correlating expected density and dynamic pressure with the anticipated rate of apoapsis decay. There is currently a high degree of variability in the atmosphere and this is producing a statistical component to the deceleration experienced by the spacecraft during the drag passes. The Mars-Gram Wave Model that predicts the atmospheric constituency for future drag passes is yielding values that do not correlate with observed measurements. For this reason, the Nav Team is using the "Nav Predicts" derived from the last 10 drag passes. This morning it was determined that the spacecraft had achieved a higher than expected degree of apoapsis decay and therefore the schedule ABM maneuver for Apoapsis 1104 was canceled.

It was determined that an Aerobraking Maneuver (ABM ) is necessary and will be performed at Apoapsis 1104. The "Down Maneuver" burn will decrease the velocity at apoapsis by 0.36 meters per second. This will lower the periapsis which will increase the rate of aerobraking and apoapsis decay.

No Aerobraking Maneuver (ABM) is required at the present time. The Nav Team continues to analyze the effects of the last ABM performed at Apoapsis 1069.

No Aerobraking Maneuver (ABM) is required at the present time. It may be necessary to perform an ABM down maneuver at Apoapsis 1086 if the dynamic pressure during the upcoming drag passes falls lower than expected. Currently the rate of apoapsis decay is 0.3 km/rev under the planned baseline rate of 14.4 kilometers per revolution. The plan is to let the apoapsis decay rate stabilize, measure the new decay rate over a number of orbits, and then determine the next required ABM.

The spacecraft is on the verge of entering the region of steepest descent into the "polar vortex" at 70 degrees south latitude. This is the region know as the Terminator, or the point where the Polar Circle begins and no light reaches the surface. This is equivalent to the extreme northern and southern latitudes on Earth which during winter months experience an "Arctic Night". Currently the density gradient of the atmosphere is steeper on the inbound leg of the drag pass than on the outbound leg. This is due to the fact that as the spacecraft descends along the inner wall of the "polar vortex" it comes in contact with a colder atmosphere than when it ascends along the outer wall of the vortex. The colder atmosphere drops away from under the spacecraft more rapidly and creates a steeper density gradient.

Yesterday's ABM was successfully completed. The current inclination of the orbit is 93.008 degrees with a period of 2 hours and 55 minutes. Over the last 24 hours the average dynamic pressure during the drag pass has been 0.190 newtons per square meter. No additional ABM is planned for the immediate period. However, within 4 days the spacecraft will pass through the center of the "polar vortex", a region of the atmosphere that expends to a lower altitude. Depending upon on steepness vortex wall, it may be necessary to perform a small ABM down maneuver.

Reset 19 Report

It was determined that an Aerobraking Maneuver (ABM ) is necessary and will be performed at Apoapsis 1069. The "Up Maneuver" burn will increase the velocity at apoapsis by 0.19 meters per second. The burn will occur 5 minutes after apoapsis passage. It will raise the periapsis which will decrease the rate of aerobraking and slow down the rate of apoapsis decay. In addition to achieving the correct final orbital period, the Nav Team must now control the final rate of aerobraking to also achieve the correct orbital inclination for a 2 AM sun synchronous orbit.

A number of graphs were presented in today's Reset Meeting which indicate the reasons for the ABM maneuver. As shown in the Inclination Variation Graph, the rate of aerobraking is presently too great to meet the targeting objectives. In this graph the red pattern indicates the desired rate of aerobraking and the blue pattern indicates the actual rate of aerobraking. The black pattern shows the likely path if no ABM is performed. If the spacecraft were to continue along this path we would not achieve the desired 2 AM orbit as shown in the Apoapsis Altitude verses Local Mean Solar Time graph.

In addition, the scale factor of the atmospheric wave model was increased by 20% to accommodate for the fact that the model has been under predicting the Dynamic Pressure. The wave model is based on a series of successively higher order cosine functions that attempt to predict the cyclical variation in the atmosphere as a function of longitude.

No Aerobraking Maneuver (ABM) is required at the present time. Predictions show a very slight increase in atmospheric density even though the periapsis altitude continues to cycle lower. This is due to the fact that the drag pass is now occurring over the polar region. Because the temperature of the atmosphere is less in the polar region, it does not extend to as high an altitude as in the equatorial region of Mars. In addition, the variation in density as a function of altitude is not as uniform. Atmospheric models are now being run to determine the range of dynamic pressures that the spacecraft could potentially experience as it continues to aerobrake in the polar region.

It was decided that no Aerobraking Maneuver (ABM) is required at the present time. The spacecraft is on track for achieving a 2 am Local Solar Time (LST) orbit simultaneous with the desired 112 minutes orbital period.