Global Surveyor and the Face on Mars

Mark Carlotto, Ph. D.

 

The Internet provides unprecedented access to planetary imagery. Hundreds of megabytes of imagery sent back from the surface of Mars by the Pathfinder probe have been archived and made available on-line. As of the beginning of November, well over one hundred images of Mars have been acquired by the Mars Orbital Camera (MOC) aboard the Mars Global Surveyor (MGS). Yet only 6 images (about 5%) have been released to the public.

The MOC was developed and is operated by Malin Space Science Systems (MSSS), a private company under contract to the Jet Propulsion Laboratory (JPL). The trickle of imagery released by MSSS is in stark contrast to the steady stream of Pathfinder images posted over the course of the summer and early autumn. The lack of MGS imagery is troubling in light of the controversy surrounding the Face and other anomalous objects in Cydonia. MSSS and JPL have stated repeatedly that the MOC narrow angle (high resolution) camera is not capable of imaging these objects because of its limited field of view and pointing accuracy. However, examination of the few images released to date suggests that the camera may be more capable than we have been led to believe.

This article considers three questions:

I use the term MGS/MOC here because even though the camera itself cannot be steered, the platform to which it is bolted, namely the MGS, can. The two thus function as a system.

First the available images and data will be analyzed to ascertain the actual capabilities of the MGS/MOC system. Then a rough analysis of the orbit will be made to assess the likelihood that opportunities to image the Cydonia targets have existed up to this point, and might exist between now and the time the spacecraft reaches its final mapping orbit.

 

Analysis of MOC Images to Date

As of 1 November, MSSS has posted 4 images on its web site [Ref. 1]. (Actually there are really six: two are black and white, and two are 2 band color images.) The four images are listed in Table 1. Three were shot during aerobraking. The first two from orbits 5 and 6 were acquired by the narrow angle (high resolution) camera. The third was taken with the wide angle (low resolution) camera. All three were taken with the camera looking east of the terminator, which at the time was just below the spacecraft. Later, during orbit 24 after aerobraking had been suspended, an image of Olympus Mons was taken looking straight down using the wide angle camera. By this time the terminator had moved to the west so that the ground under the spacecraft was illuminated.

The attitude of MGS is controlled using a set of three reaction wheels, not thrusters. Changing the spin of a reaction wheel changes its angular momentum which in turn causes the spacecraft to rotate around the corresponding axis. Thus even though the MOC cannot be positioned independently of the spacecraft, the spacecraft itself can be moved with great ease and flexibility without having to use any fuel.

The above images reveal that, in fact, MGS is being steered to point the camera towards selected targets. Given this, statements by Michael Malin regarding the limited flexibility in pointing the MOC because it is bolted to the MGS; e.g., "The MOC is body-fixed to the spacecraft," and "The spacecraft has limited pointing control" [Ref. 2] are misleading.
 

Orbit/Image ID 

Target 

Distance 

Camera Angle 

Resolution 

5/P005_03

Labyrinthus Noctis (4.6 deg. S, 102.6 deg. W) 

1600 km 

25 deg. 

12 m 

6/P006_05

Nirgal Vallis (28.5 deg. S, 41.6 deg. W) 

800 km 

35 deg. 

9 m 

13/P013_01,02 

Valles Marineris (5-10 deg. S, 73-86 deg. W) 

600-1000 km 

25 deg. 

350-600 m 

24/P024_01,02) 

Olympus Mons (12-26 deg. N, 126-138 deg. W) 

176-310 km 

0 deg. 

~ 1km 

Table 1 Images Posted on the MSSS Web Site as of 1 November

MSSS states that 132 images were acquired from September 11 to October 28. Of these 58 are low resolution, and 74 high resolution. They also state that 28 high resolution and 24 low resolution images were taken after aerobraking was suspended, from October 16 through October 28. On average they seem to be taking about three images per orbit which seems consistent with statements concerning the limited opportunities to take pictures during aerobraking [Ref. 3]. However, the existence of image ID P006_5 suggests that during orbit 6, five or possibly more images were acquired.

The pointing accuracy of the MOC has been a subject of great controversy especially with respect to taking pictures of the Face and other objects in Cydonia. The USGS Mars Mosaicked Digital Image Model (MDIM) is used by MSSS to determine the coordinates of features on the ground. According to Michael Malin, the estimated accuracy of the latitude longitude grid defined by the MDIM is 5-10 km [Ref. 2], i.e., a given point may actually be up to 5-10 km from where it is thought to be, assuming the position of the camera is precisely known. However, Malin goes on to state that "There will be a substantial uncertainty in the predicted inertial position of the spacecraft (and hence, the camera)" and gives a value of 7.4 km or more in the crosstrack direction at 40 deg. latitude [Ref. 2]. Thus togther we are told to expect positional uncertainties in the 12.4 to 17.4 km range.

The MDIM and a subset of Viking Orbiter images that have been registered to the MDIM [Ref. 5] are used by MSSS for target planning. An example output from the planning software showing the Cydonia targets is reproduced in Figure 1.

 

Figure 1. Portion of screen from planning software showing Cydonia targets from MOC target database [Ref. 4]. This portion shows the Viking Orbiter context image. Inset is the image reduced to the same scale as that of the Viking Orbiter context image in Figure 2 (see below) for comparison. (Original image courtesy Malin Space Science Systems, Inc.)

Figure 2 shows a reduced version of P0005_3 and its corresponding Viking Orbiter context image. Because of the narrow field of view of the high resolution camera, context images provide a visual reference for interpretation, showing the relative location and footprint of the MOC image within a larger area. The correspondence between the MOC image footprint shown in the context image (which is assumed to have been used for targeting as well) and the actual MOC image suggests that the positional errors are far less than 12.4 - 17.4 km. P005_03 is about 12 km by 12 km in size. If its corresponding context image was in fact used for targeting, it seems as if the targeting accuracy is far better than we have been led to believe. Comparing this to Figure 1, one would conclude that, given the opportunity, MGS/MOC could probably acquire a high resolution image of the Face or City from the current orbit.

 

Figure 2. Reduced versions of P0005_3 (left) and its corresponding Viking Orbiter context image (right). (Images courtesy Malin Space Science Systems, Inc.)

 

Could Cydonia Have Been Imaged by Now?

So, could Cydonia have been imaged during the initial aerobraking phase that began on September 15 (orbit 3) lasting to the suspension of aerobraking on October 6 (orbit 15)?

Aerobraking occurs at periapsis when the spacecraft dips into and is slowed down by the atmosphere. According to MSSS, following periapsis a roll-out maneuver from the aerobraking to array-spin normal orientation occurs. It is only at this point in the orbit, shortly after periapsis, that the camera can be used. Since images can be acquired just after periapsis, imaging opportunities would have been severely limited. It is thus unlikely that MGS had the opportunity to image Cydonia during this initial aerobraking phase because Cydonia was probably too far to the north.

However it is possible that during the aerobraking hiatus, there might have been greater flexibility in changing the attitude of the spacecraft and more opportunities to aim the camera at Cydonia.

The orbital period during the hiatus was about 35 hours. The orbit thus precessed about 154 degrees in longitude to the east each revolution. If the first orbit passed over the equator at some longitude, say zero degrees, the longitudes for that and the following seven orbits would be

0, 154, 308, 102, 256, 50, 204, and 358.

Putting these into ascending order,

0, 50, 102, 154, 204, 256, 308, and 358

we see that during the aerobraking hiatus, MGS passed within about 50 degrees of longitude of any point on Mars. Since the operation of the camera was not limited to the roll out maneuver, it could have in principle imaged just about any point on the surface to within the accuracy of the targeting software.

Given the ability to move the camera up to 35 degrees off nadir, evident in the four acquisitions reported thus far, one would have to conclude that the MGS was being tasked by MSSS to point the MOC toward a predetermined set of targets of interest. That the Face and objects in Cydonia could have been one of these targets is a possibility.

Future Imaging Prospects

JPL plans to resume aerobraking on 7 November. It will occur at a more gradual pace, taking perhaps as long as 8-12 months to complete according to MGS Project Manager Glenn Cunningham. Extending aerobraking means that if a sun synchronous orbit is ultimately achieved, it will be one that passes over the equator earlier in the day. This is an exciting prospect as it would permit the Face and other anomalies to be imaged in the morning with the sun coming from the east instead of from the west as in the Viking Orbiter photographs. Cunningham also states that worst case they could achieve a 16 hour elliptical orbit.

Although the mapping orbit provides the highest resolution (1.4 meter/pixel), the field of view is so small (3 km crosstrack) that given positional uncertainties of 5 km or more, MSSS cannot guarantee that it will be able to successfully image an isolated target like the Face. However targeting Cydonia from an elliptical orbit - either now during aerobraking, or later from a 16 hour orbit has the advantage of a larger field of view and thus a significantly better chance of imaging the Cydonia targets.

Because of the difference between MGS's orbital period and the length of a Martian day, the longitude at which the orbit passes over the equator will shift from orbit to orbit. Over time MGS will likely fly close to most points on the surface. This combined with the ability to steer the camera should provide opportunities to image Cydonia at lower and, from the standpoint of targeting uncertainties, more forgiving resolutions, like those seen thus far from orbits 5 and 6.

MGS may be the last chance for a long time to verify the possible discovery of ancient ruins on Mars. It is hoped that MSSS and JPL will take advantage of any and all opportunities to image these enigmatic objects as soon as possible.


(Updated November 5, 1997)

© 2019 Mark J. Carlotto