III. Landing and Movement
IV. Sensor Capabilities
A. The Last Observation
B. Targeting Sensors
V. Communications Systems
VI. Defense Systems
A. Force Field and Hull
VII. Additional Commentary
Serving in a role akin to the modern NASA planetary exploration rover or military UAV, Imperial probe droids were dispatched by the "thousands" in the three years after the Battle of Yavin, used to scour the galaxy in the Empire's search for the Rebellion.
As seen at Hoth, a total of eight probe droids were dispatched from the main hangar bay of an Imperial Star Destroyer. An ISD evidently has multiple probe droid launch ports within the hangar, given the different locations of exit. Several probes are launched from a point apparently in the forward center of the bay, with one other launched from the starboard fore. The relative launch velocity is quite small . . . no more than a handful of kilometers per hour. Seconds after the release, the main engine of the probe fires in a full 'afterburner' mode for just a moment, imparting additional velocity. The engine continues to run after this burn.
Near the hangar we see the blue flare of one of the tiny probes engaging in a main engine burn.
Image zoomed, cropped, and contrast-enhanced
At launch the probe droid itself is encased within a capsule of some sort. There is no opportunity for a clear look, but the shell appears to be largely spherical, and a rough comparison to the hangar bay suggests that the capsule is no larger than the cockpit-ball of a TIE fighter. In the images below one can see the very dark main engine bell on the rear of the capsule, along with a variety of bulges and protrusions along the forward area. The purposes of the bulges and protrusions are not known, though it seems likely they are navigational sensor and thruster assemblies.
It has often been claimed that the Imperial probe droids are hyperspace-equipped. However, there is nothing in the canon to indicate this.
1. The novelization makes no suggestion regarding the probe droid's transit to Hoth VI, except for the rough landing it uses.
2. The script directly contradicts the notion, with the Star Destroyer that released the probe said to be at or near "Planet Hoth".
"EXTERIOR: GALAXY -- PLANET HOTH
A Star Destroyer moves through space, releasing Imperial probe
robots from its underside."
3. And finally, we have the film itself, where seeing that the Star Destroyer is very near the planet is almost as easy as one-two-three. First, let's look at Luke escaping Hoth VI from later in the film:
Now, let's see the ISD just before it launches the probes:
Such an arrangement is far too unlikely for this to be mere happenstance. It is also noteworthy that in the image of the probes launching from the hangar bay, there is a diffuse blue-hued glow on the starboard hull of the Star Destroyer. This may be a reflection indicative of the planet being off to the left. It is certainly not the blue moon, given that it is toward the portside aft of the ISD.
And, finally, let's check the view of the planet after Vader's fleet drops out of hyperspace at Hoth:
While only two of the moons are visible in the shot above (probably thanks to the enormous starships in the way), it's quite clear that Hoth VI has at least three moons in orbit, moons which correspond to those seen when the ISD dropped the probe droid which landed on the planet.
Some have expressed a bit of confusion regarding the evidence above, countering that the probe capsule whose course we seem to follow after launch heads down and to the starboard of the ISD and, in the rear view, apparently toward deep space. However, as seen in the next shot, the probe takes a curving course toward the planet's surface. Given the grand sweeping curves that so often figure into orbital mechanics, it is unsurprising that the probe did not start by heading straight in and then suddenly curving.
(It's also worth noting that Curtis Saxton identifies the scene of the probe-launching ISD as being in an "anonymous rimward system" and not Hoth. The basis for his determination seems to be a non-canon image of Hoth VI and its moons which makes the gray moons appear red and the blue moon appear gray. This acanonical image is then used to assert facts regarding the film's imagery. As a result, his analysis misses the canonically-obvious, and his in-depth work on the relative moon sizes and orbits is rendered almost entirely useless.)
In addition to the lack of evidence for hyperspace capability, there is also a bit of counterevidence. As noted earlier, the probe capsule is little larger than the ball of a TIE fighter cockpit. Given that TIEs are not hyperspace-equipped, it follows that vehicles of similar size would be similarly handicapped.
The probe capsule appears to engage in a largely ballistic entry into the Hoth atmosphere, crashing into the surface like a small meteor. The capsule is not directly visible at impact, obscured by a glow suggesting that it is still quite hot. This is actually quite unusual . . . a normal meteor of such a size and re-entry profile is likely to be extremely cold upon reaching the ground. While in certain contexts this glow might indicate some sort of energy shielding, none is apparent during the re-entry itself, at which point we see the glowing plasma right against the probe's outer shell. It is possible that this glow is the result of some sort of retro-rocket being fired just before impact, for reasons illuminated further below.
The capsule is destroyed in the impact, with flying debris and shrapnel surrounding the crash site. The capsule's content . . . the insectoid robotic 'rover' itself . . . soon emerges from the crater, using a repulsorlift to move about.
Emergence of the Imperial probe droid . . . note the capsule debris behind and to the right.
Note also the small blaster box on the right between the head and body.
The reasoning behind this landing pattern is unclear. Certainly it implies that the probe droid is a fairly impressively-built item, given that it survives the sudden deceleration that destroys the capsule. However, alternative landing maneuvers could have been employed . . . parachutes to slow the descent, inflatable shells for bouncing landings, antigravs, et cetera. Or, the capsule could eject the antigrav-equipped droid before smacking into the ground. Any of these could've produced a less impactful trip for the droid, but none were used.
Our recent experience with the Genesis capsule also makes for an interesting note of comparison. When the parachute triggers failed, the Genesis probe simply tumbled to the ground at its terminal velocity of about 160k/h (100mph), gouging itself a hole in the dirt. The probe was similar in width to the Imperial probe capsule, striking the denser soil at a greater speed . . . hence the idea of a retro-rocket producing the unusual glow effect above (since otherwise we would have to assume that the Imperial probe was extremely light so as to produce a lesser terminal velocity). As can be seen below, the capsule was severely damaged but largely intact, with many of its delicate sample containers surviving.
Image from Space.com
Since all of the alternative techniques except antigravity are available even to the current NASA space program (albeit with varying degrees of success, as seen above), it is interesting to consider why a far more advanced spacefaring civilization would choose such a violent method for landing a probe. The only rationalization I can think of is that the ballistic re-entry and crashing is intentional for what the novelization describes as a "surveillance machine". The probe capsule would presumably be designed to act like a meteor so that even if it is tracked, there would not be the same cause for suspicion that might occur if the probe were to engage in powered flight maneuvers. And, given that the Imperial probe capsule effectively shatters upon impact (unlike Genesis), it is likely either that the capsule self-destructs at impact or that it is intentionally made of heat-resistant but quite brittle materials.
The metal shell of the emerged probe droid features a head section that includes "dark-lensed blister eyes" , along with "several extended sensors" (per the script) and other limbs dangling below, most of which seem to be simple manipulator arms. The description in the novelization has the probe "equipped with cameras, sensors, and metal appendages, some of which terminated in crablike grasping pincers", along with the blister eyes.
Although Luke Skywalker is said to be carrying a small lifesigns detector "sensitive enough to zero in on even the most minute life readings by detecting body temperature and internal life systems", the probe does not seem to be so well-equipped. In a scene appearing only in the novel, the probe droid discovers a heat signature beneath a large snow mound, but detects none of the "internal life systems" present . . . the conclusion that it is a lifeform comes only due to the heat. After annoying the wampa beneath the snow, the creature is said to produce "a roar that nearly destroyed the probe droid's auditory sensors". On the other hand, the probe droid managed to discover life within hours of arrival, whereas the Rebel base had been established for a month and no one had noticed a thing.
Eventually the probe droid discovers the Rebel base. As seen in the film, it sends back images of the Rebel power generators. It isn't clear if the probe droid analyzed the images or if the analysis was performed on Vader's Star Destroyer which received the images, but in either case the result is the image below, in which the upper portion of the Rebel generators are seen and analyzed as being part of four tokamak-shaped objects placed together. Given the different angle as seen below versus what can be seen after the probe transmits, it seems clear that the probe must have at least partially circled the reactors for different views.
Whereas we were kind enough to find a way to rationalize the primitive landing, there is simply no way to rationalize the probe droid's last scanning efforts. After dispatching a message to the Empire, the probe droid is floating in the direction of the Rebel base. En route, the probe droid stumbles upon a waiting Han and Chewie. Per the script, the droid:
"[...] raises a large antenna from the top of its head and begins to
send out a piercing signal.
The probe droid has spotted Chewbacca who, not thirty feet
away, has popped his head over a snow bank."
As seen in the film and noted in the novelization, Chewie did more than just pop up . . . he actually had to howl to get the probe's attention. The novelization also supports the fact that the probe was only reacting to noticing him pop up and howl. Even this delayed reaction only happened after Han had already popped his head up from behind another snow bank and observed the droid's flight. And of course, in order to even get into such a favorable position Han and Chewie had to have either tracked the surveillance droid without being spotted (which is bad), or else they exposed themselves and retreated to a more favorable position, meaning the droid already knew they were around (which would be worse). This suggests that even in a frigid environment where warm-blooded lifeforms ought to stick out like sore thumbs on any sort of thermal sensors, the probe droid's sensors were only good enough to pick out something wampa-sized.
Upon seeing Chewbacca, the probe droid attempted to fire at him. However, the three shots fired all missed, and the aim worsened after each shot. Below we see the layout of the event from just behind Han's snowbank.
The probe droid blunders into the trap. Chewie stands post-howl
in the distance "not thirty feet away" from the droid.
Zoomed and contrast-enhanced view.
Chewie ducks down toward the right.
The probe droid has fired its first shot, missing left.
The other two shots miss even further to the left.
Overlay of Chewie's position and the clean miss to the left
There is no excuse for the atrocious aim coming from a machine not subject to human foibles. Indeed, given that the blaster is built-in, it seems unthinkable that the sensor-laden probe wouldn't have exquisite accuracy. As seen above, however, this is not the case. The droid even turns back toward the left for the latter shots.
Some might be tempted to retort that the droid was somehow seeing Chewbacca attempting to maneuver to the left behind his snow bank. This thought, though an admirable try, would be wrong. Below, we see Chewbacca's original howling position (contrast-enhanced) followed by his position upon re-emerging after the probe droid is destroyed. Note the same pattern of snow in front of him, along with the same cracked-looking area behind him and to the right:
In short, Chewie did not go anywhere. He simply ducked, and this was enough to confuse the probe droid and cause it to fire every which way. Since it had not observed him in the first place, it is little wonder that it became confused when he ducked.
The probe droid featured two small antennae which seem to have been its primary method of phoning its information home. Though a Rebel sensor operator referred to the signal as being weak, it is possible that it was actually just a signal directed toward the intended receiver or relay. Thus, much as a spotlight's output is weak toward the sides of the spotlight, so too would the probe droid's signal seem weak at the Rebel base.
One interesting note is that the probe is not seen to transmit 3-D holographic information, but instead transmits a 2-D image or video feed.
Unlike most sci-fi science probes, probe droids quite nicely reflect the Imperial ethic by being armed.
The novelization includes a scene of the probe droid setting down for the night, presumably either to recharge or to try to evade the frigid nighttime temperatures. The interesting part of the scene is the method the probe uses to hide:
" [...] the robot gently lowered itself, coming to rest on the ground. Like spider legs, several probes separated from the metal hull, dislodging some of the snow that had settled there.
Something began to take shape around the robot, a pulsating glow that gradually covered the machine as if with a transparent dome. Quickly this force field solidified, repelling the blowing snow that brushed over the droid's hull. After a moment the glow faded, and the blowing snow soon formed a perfect dome of white, completely concealing the droid and its protective force field."
Later when the field is shut down and the probe droid is lifting off again, the "droning machine began pulling back its retractable sensor arms". Is the force field related to the separable probes, or is the unit leaving those outside the field because the field renders it sensor-blind? Are the retractable sensor arms the same as the separating spider-leg probes that knocked snow off thetop of the probe? Which of these are represented by the probe droid's visible dangly-bits that hang beneath? It isn't all that clear, unfortunately.
What is of interest, however, is the gradual covering of the machine in a dome . . . the description sounds not unlike the activation of a Gungan shield. It's also possible that the force field was of a design similar to that of the droideka shields during the Clone Wars era, and simply lacking the design flaw wherein the droideka shields would burn out if a solid object were touching the shield constantly.
In any case, one must explain why the force field was not used on any occasion when the probe really needed it. There's no mention of a shield in use against the wampa in the novelization. And, in the final confrontation Han hit the unit more than once, and that was after the unit had time to raise an alarm and fire several shots when it was surprised by Chewbacca's howl. Either the activation sequence is extremely gradual, or the shield is meant only for hiding, or else the droid has to release the other probes to raise the force field. Or, perhaps the most likely answer, the probe droid may not be able to activate its force field at the same time as it is using its repulsors to float. Whatever the case, the force field is of no apparent tactical utility.
Han's multiple hits did little to the droid. The novelization notes that Han's first shot (which in the film produce a rather large fireball) just "nicked" the hull, and confirms Han's statement that a self-destruct was initiated. The novelization even describes the whine that came in advance of the explosion, suggesting that the self-destruct system was similar in design to the device used to blow the door of Cell Block AA-23 in ANH.
We've already touched on the probe droid's built-in blaster. This relatively small weapon is fired several times in quick succession at Chewie and Han. The two are both well-protected by their respective snow banks, implying that the blaster is no more effective than a bullet.
We also learn in the novelization that the probe droid is armed with a secondary beam weapon, apparently distinct from the blaster we see in the film given the context of the descriptions. The novelization describes the blaster as "The blasters that had been built into the probe robot", whereas the secondary beam weapon is always called a "laser beam" and described differently:
"A thin blue-white beam of light shot from one of the probe robot's appendages, its intense heat boring into the white mound and scattering gleaming snow flecks in all directions.
The mound began to shiver, then to quake. Whatever existed beneath it was deeply irritated by the robot's probing laser beam."
Whether this was a real laser beam or not is, as always with Star Wars, a bit uncertain. Even Han's blaster bolt was referred to as a "red beam" instead of a bolt. However, the description seems to be one of continuous fire poking into the snow mound, not the "blasting" with removal of chunks of snow that the novelization describes regarding the blaster.
At maximum setting, this secondary weapon is vastly superior to the primary blaster:
"The droid made an internal adjustment to regulate the potency of its laser beam. Less than a moment later the beam was at maximum intensity. The machine aimed the laser at the creature, enveloping it in a great flaming and smoking cloud. Seconds later the few remaining particles of the Wampa were swept away by the icy winds.
The smoke disappeared, leaving behind no physical evidence—save for a large depression in the snow—that an Ice Creature had ever been there."
The droid managed to hit the wampa, doing so with a beam that quickly killed it and soon burned it down altogether. The wampa was totally disintegrated in some unknown number of "seconds". If we assume that this is less than a minute, then we have what may be the most profound example of small arms firepower in the entire Star Wars canon, and by a comfortable margin. This is vastly superior to anything Han's blaster has put out, even in the wanked-out retellings that some fanboys rely on. After all, there's more than just the creation of a smoking corpse a la poor Greedo in ANH . . . he at least was mostly intact, though holed and scorched. Judging by the Greedo shot, the notion that Han could have vaporized Greedo in less than a minute seems quite absurd. He could've pumped shots into Greedo for several minutes and perhaps gotten Greedo down to a chunk-covered skeleton, but the probe droid does far more in this example. This level of firepower is seen in ground combat only on much more massive beam weapons like the ball turrets of clone gunships or the huge SPHA-T multi-story Republic artillery cannons of AoTC.
As with the force field, then, we are forced to try to explain why the probe droid would have such a device but would fail to use it. After all, a weapon capable of reducing a wampa to debris within a minute would make short work of a snow bank and the Wookiee or human behind it. This is especially true given that the beam had already been seen in use during a tactical situation.
The situation is worse than that, though. The novelization suggests that there are multiple built-in blasters on the droid, along with the laser beam on one of the appendages. As can be seen in the zoomed and cropped image below, however, the droid completely swings its torso around in order to fire on Han.
We could attempt to rationalize the novelization's multiple blasters as all being mounted to the same emplacement (a la a ship's multi-gun turret), but there is no evidence for multiple barrels in any of the images, nor a firing rate suggestive of them. In short, the novelization is wrong on the blaster loadout of the probe.
Is it also wrong about the presence of the secondary weapon? Well, not necessarily. Although the droid should've used a second gun if available in order to be able to cover both Han and Chewie simultaneously (not to mention using the blue-white beam in the first place so as to burn through the simple snow the Wookiee was hiding behind), it's rather cavalier to address the reliability of the canon based on what we think a character should've done. However, the disparity of the firepower between the blaster and secondary weapon is simply too great to ignore. A weapon capable of vaporizing a wampa in less than a minute simply does not conform to a universe where such firepower invariably comes from far larger devices.
The solution, happily, is a simple one. The disparity only exists due to our assumption that the "seconds" referred to was 60 or less. This is usually a safe type of assumption, since more than that would've been "minutes". However, since that assumption leads to contradiction, the assumption was simply wrong in this case. Problem solved.
Our exploration results in several interesting implications.
For instance, the Rebel base had been on Hoth for a month, per the novelization. And yet the ISD, which apparently cruised in to within the orbit of Hoth's moons, did not detect the base. Since antigrav range (and hence hyperdrive activation range) is said in ANH to be some six planetary diameters, then the ISD must have cruised out to that range before departing. Similarly, the base did not detect the ISD. While it's possible that the two were simply on opposite sides of the planet and were lucky enough not to detect one another because of that, it seems an inefficient way to perform a search. One would think they'd have given the planet a once-over.
This also seems somewhat odd given the later events . . . the Rebels detect the hyperspace exit of Vader's fleet, and clearly exit well outside the Hoth lunar orbits. The Rebels then "reroute all power to the energy shield" which the Imperials detect. Before those two events neither apparently detected the other.
This implies that a single Star Destroyer might not have tripped the hyperspace exit alarms, whereas Vader's entire fleet did. It's possible that the probe-launching ISD exited hyperspace far from Hoth VI, but that would seem to require that the ISD and Hoth base ended up in line-of-sight of one another at some point, which is an alternative one would hope to avoid. Presumably, then, the probe-launching ISD dropped out of hyperspace at a range of one to six planetary diameters, cruising in to within the lunar orbits and then cruising back out, inadvertently staying on the opposite side of the planet from the Hoth base the entire time. Meanwhile, Vader's fleet was unlucky enough to drop out of hyperspace within sight of the base.
As an alternative view, hyperspace exit could be something that can be detected through the bulk of the planet, perhaps via a sudden spike in neutrinos. If so, then the probe-launching ISD would presumably have been just far enough away at hyperspace exit to fail to trip the alarm, whereas the huge spike that one might expect from the Executor and other Star Destroyers exiting hyperspace did trip the alarms. This would elegantly serve to explain why Vader considered the Admiral clumsy and stupid for coming out of hyperspace too close to the planet . . . had they come out further away, they might've had the surprise factor the Admiral claimed to want.
In any case, the Admiral's report that Hoth is supposedly not inhabited would imply that the ISD was sent to a system where they expected no one would be found. This suggests a methodical search of many possible worlds, which would suggest that a significant portion of the fleet was so employed in order to make the effort timely and worthwhile. However, as we learn in the opening crawl and from Admiral Ozzel, only "thousands" of probe droids were engaged in the search. This seems a small number given how many planets we would expect them to search. Further, even in the uninhabited Hoth system eight probe droids were launched. If each system got a similar number of droids, then for 9999 droids the number of systems searched would max out at 1250. Even if we assumed 999,999 probe droids, the number of systems searched tops out at 125,000.
Whatever the case, the fact that probe droids are used at all has certain implications for the Imperial fleet. Dispatching probe droids to search 125,000 systems (maximum) implies that the Empire did not have 125,000 ships available that could enter orbit and scan the surface of planets, which would correspond to other fleet size estimates. Even if we assume that they did have the ships, though, we still have to answer why they didn't simply use them. Stealth is an unlikely reason, given that to drop off the probe droids an ISD had to come within close range of the planet in the first place. It's possible that the Empire feared that the Rebels could detect and overwhelm a single Star Destroyer or other vessel before Vader's task force could arrive, but given the far easier dispatch of a probe droid this thought seems unlikely, too. And so the original idea that the use of probe droids seems to be a limiting value for the Imperial fleet seems justified.
Finally, the extraordinarily poor equipping of probe droids is quite damning to the idea that Imperial sensor technology is of significant value. While this and other examples of poor Imperial detection abilities have been commented on previously (1,2), it is worth reiterating in the modern era of extreme Star Wars tech-wanking by desperate fanboys.
Though capable of covering more ground (especially rougher ground) than a walking man, probe droids are about as effective as a walking man for planetary scanning. Slow and equipped with a very poor suite of sensors, the fact that a probe droid on Hoth managed to stumble across the Rebel base can only be considered the wildest stroke of luck. While armed and hence more capable of self-defense than the spaceborne Federation science probes, the ridiculously-bad aim plus the fact that the probe droid is skimming the surface of the planet with its bad sensors (thereby putting itself in harm's way) nullifies the advantage. Finally, the apparent lack of hyperspace capability seen in the probe droid's capsule (as compared with warp-capable Federation probes capable of crossing light-years to scan other planets ("The Defector"[TNG3] et al.)) means that while a ship can go off and do other things while the painfully-slow coverage of a planet takes place, the ship still has to go there (which largely defeats the purpose of an unmanned probe anyway).
It would be helpful if the number of probe droids in use by the Empire during its three-year hunt for the Rebels had been vastly larger . . . at least then we could have dismissed the many problems of the probe droids as being due to the comprises of budget that come when you have to build a zillion of something rapidly. However, given the technology and industrial might of the Empire it seems astounding that their mere thousands of probe droids would've been so poor in design.