Comet Flyby Missions for Mars Rovers

On October 19, the Mars rovers—like their orbiting cousins—will become comet flyby missions. Comet C/2013 A1 (Siding Spring) will pass within 140,000 kilometers of Mars. This is less than 20 times the distance from Mars to its smaller moon, Deimos. Those distances provide a sense of scale: Deimos appears only a few pixels across to the rover cameras, so the nucleus of the comet will effectively a point source. The coma of the comet, tens of thousands of kilometers across, will take up a substantial fraction of the sky.

Planetarium software may show you a gloriously bright cometary tail stretching across the sky, but don’t believe that hype. While the comet may reach magnitude -5 or brighter (brighter than anything but the Sun and Moon in our sky, and comparable to the Martian moons), the brightness is spread around the sky and very little appears especially bright.

So, what can the rovers see and do? Up to closest approach (Martian day, or sol, 3817 for Opportunity and 783 for Curiosity), the comet will be a morning object. That means the comet will be viewable in pre-dawn hours from either rover site. After then, it will be an evening object. While inbound, the comet will likely be too faint for rover cameras until the last few sols, during which its apparent brightness increases rapidly. Generally, the increase in brightness will exactly match an increase in angular area that the brightness is spread over. This is not so good for imaging. What we do know is that the nucleus itself will get brighter as it gets closer, as a point source (never as much as one full pixel in the rover cameras). What we do not know, is whether there will be any interesting activity. If there is extra material concentrated near the nucleus, there could be significant brightening of a small area of the sky.

The comet’s closest approach to Mars occurs during a small interval when the Sun is up at both rover sites. The comet will be above the Meridiani plains, but Opportunity will unable to image it due to the bright, dusty sky. However, a few hours before that will be the closest either rover is to the comet while the comet is up and the Sun is not. Opportunity will have a challenge to face if sol 3817 morning observations are done. The rover has needed “deep sleep” to get through the night since very early in the mission: power is disconnected from the system (and especially a faulty heater) in the evening, and is not restored until sunlight on the arrays wakes the rover up. Pre-dawn imaging of the comet will require foregoing deep sleep and using much more energy than usual. In today’s energy environment, that may be possible due to clean solar arrays. But, as the rovers get deeper into dust storm season, we cannot predict whether mid-October will be free of storms (but still quite dusty) or in the early stages of a planet-encircling dust storm or somewhere in between.

Pancam is Opportunity’s best instrument for comet imaging. Pancam has a clear filter that has been used for astronomical images. We have found (with the Spirit rover) that in moderately dusty conditions, Pancam can see M31—the Andromeda Galaxy—but just barely. That means that we can see only the brightest diffuse objects, and short of an outburst of some sort, the comet’s coma would be unlikely to be so bright. On the other hand, we also saw magnitude 6 or 7 stars. Essentially, the rovers can see what many people can see from dark skies—but if you’re picturing any comet you’ve seen from Earth, keep in mind that there will be half as much sunlight at Mars’ orbit, both for reflecting off the coma and for being absorbed to drive activity. We would expect to see the nucleus appear like a medium-bright star, unless the comet is active enough that we see a bright inner coma instead. Based on past experience, the energy available to the rover will not be enough to heat motors, so there will be a single aim for the night. But in that aim, we can watch the comet move against the background stars. We can also look for variations in the inner coma that indicate the presence of jets and the rotation of the comet. The rover’s Navigation cameras have a nice, wide field of view; however, they are not nearly as sensitive as Pancam and thus they may not be used.

The evening following closest approach, Curiosity faces a more favorable situation. Evening temperatures are warmer, and the motors need less heating. The cameras can use multiple aims. The comet will be a bit further away, at 600,000 km range (vs. 400,000 km for Opportunity) once the long twilight glow has subsided. But the Mastcams have color imaging and 34 and 100 mm focal lengths: Mastcam-34 has a similar field of view to Pancam, and slightly better resolution; Mastcam-100 has 3X the resolution over a 5 degree field of view. And, Curiosity has Chemcam.

Chemcam is a laser-induced breakdown spectrometer—while the range will be a hundred million times too high for the laser, the comet’s own reflected and emitted light can be measured. With no worries about ozone or oxygen absorption, ultraviolet emissions can be measured more easily than from Earth. However, there are no guarantees there will be emissions strong enough to be detected at this time. Mastcam can take color images, and can repoint to see the comet in different directions—for instance, a few hours after the first images can be taken, the comet passes near Deimos in the sky. The nucleus or inner coma can be imaged, with potentially more detail in the coma from the 100-mm camera. MAHLI, the Mars Hand Lens Imager, can also image the sky. It requires the robotic arm to aim it, and there are more constraints on the ability to do that. But for landscape imaging including a bright coma, MAHLI offers the widest angular views.

Both rovers can potentially detect the comet for a few sols around closest approach, although Opportunity will likely not get more than one shot at pre-dawn imaging. Rover plans will not be made until the last few days before the event. Any number of things happening on the surface could take priority over comet images or spectra. While there will be great interest in getting the data, the energy needed for night wakeups and operations is a luxury.

A final note: we’ve been quite interested in meteors in the past. Unfortunately for rover imaging (but quite happily for the survival odds of the orbiters), the flux of meteors is unlikely to be high. It will peak within 2 hours after closest approach, when Mars has its closest approach to the tail. This will be in full daylight for Opportunity—it would take a ‘meteor hurricane’ to make a fireball likely. As energy availability means Opportunity will be able to image, at most, one of the comet or the meteor shower, it is unlikely we’ll see meteors. The predicted meteor fluence is so low that a meteor image would be unexpected anyway. Curiosity, like the orbiters, will be shielded from the meteors by Mars. By the time the radiant (direction meteor would appear to come from) rises, the comet will be quite distant. The low expected mass of impacting meteors also means that compositional changes in the lower atmosphere (that Curiosity could measure) are unlikely.