Predicting Pluto's moons and moondust
Posted by Emily Lakdawalla
19-02-2014 13:39 CST
A reader comment on an old blog post about the discovery of Pluto's moons made me wonder about the timing of their discovery. Giant Charon was discovered in 1978, but the much smaller moons required the Hubble Space Telescope to be seen. Nix and Hydra were found in images from May 2005, and Kerberos and Styx in 2011 and 2012. Given the fact that Hubble has been taking great space photos since 1994, why did it take so long for Pluto's moons to be discovered, especially with serious interest in a Pluto mission in the form of Pluto Kuiper Express (New Horizons' predecessor) before 2000?
I asked Mark Showalter (leader of the team that discovered Kerberos and Styx) this question and he wrote me a lengthy response. The Hubble Space Telescope is, to put it mildly, oversubscribed. For an astronomer to get time on Hubble, they have to write a proposal justifying the time. The competition for time is stiff. Mark told me that the original proposal by Hal Weaver and coworkers was rejected by Hubble. Mark said, "They got the time on the telescope later only because one of HST's other instruments failed, so Space Telescope Science Institute had time to fill and they resurrected a few of the proposals that had fallen just below the cutoff. Hal's group got their telescope time after all, and the rest is history."
It's an irony that's central to pretty much all of science that you usually have to predict the possibility of something new before you can discover it. For the most part, science is advanced in very small steps by people who master the stuff that we know and use that mastery to predict something we didn't know before, then go looking for evidence. In that way scientific discovery is an act of creativity, of imagination. Unfortunately, it can be hard to tell ahead of time whose ideas are likely to lead to breakthroughs, and whose will just turn out to be fantasies not grounded in reality. When you're trying to win time on the world's greatest telescope, the bar is set very high, and it can be very hard to win time for a search for something that you aren't sure exists.
Mark went on: "Later, my own proposal was to search for dust rings, such as we often see in the orbits of small moons. We had a very clever idea about how to push the detection limit much deeper than Hal's group had been able to achieve. That proposal was also rejected by the review panel. It was only when the New Horizons management contacted STScI and raised the importance of our proposal for the mission that we were granted Director's Discretionary time. We didn't find any rings, of course, but because we were taking longer exposures than had been attempted before, Kerberos (about 5% as bright as Nix and Hydra) became visible. After that, STScI worked closely with us on the follow-up observations in 2012, and that is when we discovered Styx."
Small moons can cause a problem for spacecraft: dust. Micrometeoroid impacts onto tiny moons with near-zero gravity send dust into orbit around a planet, making a tenuous ring in the orbit of the moon. You can see several such rings in this Cassini photo.
Pluto's much smaller than Saturn, of course, but the physics works at Pluto's scale as well as Saturn's; it's reasonable to predict that Pluto's small moons have dust rings too. Like Saturn's, they would be incredibly difficult to spot from Earth, because they're so tenuous. Cassini's photo (above) took advantage of a special opportunity of flying into Saturn's shadow to look in the direction of the Sun and see the fine ring particles forward-scatter light to the cameras. We won't get to take the same kinds of photos of Pluto's possible dust ring system until New Horizons has flown beyond Pluto.
To take those pictures, New Horizons will have to survive the Pluto flyby. And dust could jeopardize that, as I've discussed before. Mark pointed out that the New Horizons team has been preparing to deal with this potential problem all along. "They planned the Pluto encounter accordingly. New Horizons will pass Pluto roughly at the orbital distance of Charon, but on the opposite side of the planet. The laws of dynamics indicate that there are no stable orbits in this region--if you put something into an orbit similar to that of Charon, it will quickly crash into Pluto or Charon or get tossed out of the system. This remains true to the best of our knowledge, so we still have good reasons to believe the spacecraft will be safe.
"Nevertheless, small satellites around the other planets can be very effective dust generators, so we now have to take the possibility of dust rings more seriously. These rings arise when meteoroids impact the surfaces of small moons and the cloud of ejecta escapes. Nix and Hydra have enough self-gravity to hold onto a most of their impact ejecta, but smaller Kerberos and Styx do not. Predicting the appearance of a ring produced by the small, outer moons is tough, but we think most of that dust would remain well outside the orbit of Charon. New Horizons is flying through the bulls-eye of the system, so these dust rings (if they exist at all) are probably not a danger to the spacecraft. Nevertheless, this question is still under investigation."
So New Horizons is most likely safe from the things we know about. But it's entirely possible that there are still more moons lurking in the Pluto system that could be generating dust that would be more of a problem. Some moons could even orbit in the region between Charon and Pluto. New Horizons' last defense against this potential hazard is its own vigilance. Mark said: "During the months leading up to the July 2015 flyby, we will be using the New Horizons camera to search for any moons or rings that Hubble might have overlooked. We have designed a couple of alternative 'SHBOT' (Safe Haven By Other Trajectory) flyby options, which we can invoke as late as 10 days before the encounter. However, we will lose a lot of important Pluto science if we have to switch to one of the SHBOTs at the last minute. I remain optimistic that we won't need to use a SHBOT, but I suspect we will have a few tense months leading up to next year's flyby."
2015 is going to be quite an exciting year, with every single image that New Horizons gets of Pluto being the best image we've ever seen of Pluto. I think of it as the Year of the Dwarf Planet, because not only will we be seeing Pluto up close for the first time, we'll also see Dawn approach and orbit Ceres for the first time. And don't forget about Charon -- Pluto's large companion makes it three big round worlds that will finally turn from points of light into places with geography and geology next year. The fact that New Horizons may yet have to take evasive maneuvers to avoid Pluto's dusty defense force just adds an extra level of tension.