We are scientists studying the comet about to engulf Mars on Sunday. Ask away!

Which one reaches the Oort cloud first - Siding Spring or Voyager 1?

That's a great question...we're calculating it now. [MK]

I can't easily run their orbits out later than the year 2020, but it looks like Voyager 1 would win. It's got a big head start (~100 AU) and is moving very fast (~17 km/s). Siding Spring is currently moving faster (~56 km/s), but as it moves away from the Sun it will be slowed down a lot by the Sun's gravity. [MK]

A colleague just pointed out that Voyager 1 is moving so fast that it is unbound from the solar system, while Siding Spring is still bound. Thus, Voyager will win, even without the head start. [MK]

I can visualise what you just said because of KSP.

And somewhere there's a NASA rocket scientist playing it right now ;) [MK]

That there is a mighty fine question. It's the kind of question that'll have professional astronomers dropping everything they are doing at the moment cause they just have to know, right now! Kudos

The classic nerd snipe

Guilty as charged [MK]

Will @MAVEN2Mars be able to detect guanine, adenine, thymine, cytosine in #CometSidingSpring with spectroscopy? How could any of the Maven Mission designers have imagined that a week after arriving at MARS this amazing craft would have the opportunity to analyze a very close "long period comet". Surely this in itself will be a historic achievement. But find evidence consistent with Panspermia will be momentous. I wait with bated-breath. It is my 68th birthday :-)

PS. Happy birthday! [MK]

Not that I am aware of. [MK]

Hey guys, thanks for doing this. Two questions: Has anything remotely similar to this ever happened to Earth? And what are you hoping to discover from studying this?

To the first question: No, that's why this is so exciting! Siding Spring is coming about 1/10 the distance of the closest known comet passing by Earth since human's have been studying the sky. Here's a list of closest comets known to Earth: http://www.minorplanetcenter.net/iau/lists/ClosestComets.html [MK]

From the perspective of MAVEN, this will be a fantastic opportunity to study how dumping a large amount of gas and plasma (ionized gas) into the upper martian atmosphere can change it. This is exactly what MAVEN is designed to do, except normally we'd be looking at the effects from the plasma of the solar wind or solar storms and in this case it'll be cometary plasma and gas. [JE]

Thanks! How do you expect the atmosphere to change? Would the effects of cometary plasma and gas on the Martian atmosphere be applicable to a similar scenario on Earth?

One model shows that we might expect the temperature in the upper atmosphere to rise by as much as 30 degrees Celsius. And we also expect dramatic changes in the atmospheric magnetic fields ("induced magnetosphere") -- changes comparable to a large solar storm. At Earth, we would expect similar effects in the upper atmosphere (though slightly less due to Earth's thicker atmosphere -- even the upper one is thicker). [JE]

What causes the upper atmosphere's temperature to rise? Cometary gas slamming into it, or something else?

Yes, the gas hitting it at 56 km/s. [JE]

To the second part: there is a chance that one of the spacecraft at Mars will be able to image Siding Spring's nucleus. We've only previously ever imaged a comet nucleus when we've sent spacecraft directly to a comet (like Rosetta, Deep Impact, etc.). Those have been missions to what are called "Jupiter Family Comets", comets that have short periods and have been in the inner solar system a long time. Siding Spring is newly arrived from the Oort Cloud and this is probably its first passage through the inner solar system. We have never imaged one of these comets up close before, so the nucleus could be very different than Jupiter Family Comet nuclei. [MK]

If the gas surrounding the comet envelopes mars, could the gas provide the building blocks of a future atmosphere?

Yes but only if a ton more comets "hit" Mars. Even though the effects will be really interesting on the upper atmosphere, that's only because it's so thin up there. The amount of material actually delivered by this one comet is very small compared to the amount of material in the comparatively thick lower atmosphere. That said, you do occasionally see SF stories that deal with terraforming Mars by delivering comets to it. In the stories, however, they usually have the actual cometary nucleus hit the planet and deliver all its water content at once. Boom! [JE]

Thanks. I've never heard the idea of using comets for terraforming. Makes sense

It's also seriously debated how much of Earth's water was delivered in the ancient past by comet impacts. The flux of comets hitting Earth (and other planets) was likely far higher in the first few millions years of the solar system than it is today. [MK]

How likely is it that this could happen to Earth within the next decade, century, or even millennium?

We haven't had a comet come anywhere near this close to Earth since we've been seriously studying the sky with telescopes (~400 years). Based on that sample size, I'd say it's likely to be at least centuries. Here's a link to the closest known approaches of comets to Earth: http://www.minorplanetcenter.net/iau/lists/ClosestComets.html [MK]

It's really no more or less likely than it has been for millennia. We have a pretty good handle on most of the big asteroids, and certainly short-period comets. If one were to come close to Earth - and that's a big IF - then I personally think it's more likely it would be a random Oort Cloud comet like Siding Spring (or ISON). But there's nothing in the pipeline we know of, so don't cancel your vacation! [KB]

When do you all expect Siding Spring to become visible to Curiosity or Opportunity's cameras? How bright will it be if it doesn't flare up or fade out in the next few days?

My understanding is that it will be potentially visible to both around the time of close approach. Opportunity will be near sunrise and Curiosity will be near sunset. It's hard to predict how bright it will appear because the comet will be so close that it will be very spread out. We're hopeful that it will be bright enough that the rovers will be able to see it, but won't know until it happens. [MK]

Thanks for doing this AMA guys, I have a couple of questions on this. Firstly, is it known how large this comet actually is? and secondly, a hypothetical one, what would the effects be on Mars if this comet were to score a direct hit instead?

1-10 km is decent guess for the nucleus. That's one of the major questions that we'd like answer with this encounter. The gas cloud is 200,000+ km. Second answer: boom! [JE]

Ok, real answer to second question. Major impact on surface. Significant atmospheric disturbance. I don't know the details. Someone should model it. [JE]

It's estimated that the comet/asteroid that caused the Chicxulub crater was ~10 km wide. So if there were any dinosaurs on Mars they wouldn't be very happy about an impact... [MK]

So basically no more robots up there I guess, I might have a go at working out some of the numbers involved in this but i'm by no means a physicist, cheers for the replies!

Even an impact by a considerably smaller comet would likely kick up enough dust into the atmosphere to be problematic for the solar panels on the rovers. [MK]

If a comet passed this close to earth, what would it be like from the ground seeing it? I'm just trying to get a feel for exactly how close 140 km is to Mars.

I wasn't old enough to appreciate it, but the closest comet in recent memory was IRAS-Iraki-Alcock in 1983 which was something like 30 times further away from Earth than Siding Spring will be from Mars. People who saw it described it as being a diffuse glow about the size of the full moon that moved visibly in just a few minutes relative to the background stars. Because Siding Spring is so much closer to Mars, it will appear to move a whole lot faster, so you would likely see it move very rapidly. [MK]

It would be freakin' awesome! The close approach to Mars (~140,000km) is just one third of the Earth-Moon distance. That's really close... Even a small comet would look pretty impressive at that range! (And still be no threat to us on Earth) [KB]

What led to you guys choosing a career in the field of astronomy?

I kind of stumbled in to it. I always liked math, which led to physics in college, then astronomy. I think it's pretty cool to be able to study things that people can actually go out in the backyard and see with their own eye. [MK]

I always enjoyed space, astronomy, math, physics, etc but had no real aspirations to be an astronomer (early career choices included farmer, vet and meteorologist!). But I semi-randomly chose to do astrophysics for undergrad, and it all kinda snowballed from there. In hindsight there was some luck, but laced with a lot of hard work. I certainly wouldn't change it - I love my job! [KB]

This is fascinating. What would be the most interesting observations you could find? What do you expect to find?

Hmm, I think we all might have different opinions here. Personally, I'm really excited at the prospect of MRO/HiRISE hopefully being able to resolve the shape of the nucleus. We've never seen an Oort Cloud nucleus before! They give no promises, but they're going to try!

Also, I think MAVEN brings some fantastic capability to this situation. It's a spacecraft designed to study a gassy atmosphere sitting in the solar wind... and this weekend it will get to see not one but TWO gassy atmospheres sitting in the solar wind! I really hope they're able to detect some good strong interactions between the comet's coma and Mars' atmosphere.

The thing to remember here is that we're doing something completely unique, so we really don't know for sure what we'll find out. This is what makes it so exciting! [KB]

If there is indeed life/water on Mars, will this comet destroy it all?

No, all the Martians are safe for now! :) Seriously though, Siding Spring will pass a very harmless 86,000 miles (138,000 kilometers) from Mars. The comet's gas coma will envelop the planet but the dust will almost certainly miss. So even if Mars were a thriving biosphere, there would be no significant risk from the comet. [KB]

Hello and thanks for doing this AMA!

On a scale of 1 to Neil deGrasse Tyson, how sexy will this cosmic phenomenon be?

Also, will photos of this event be available to the public?

Phil Plait

Edit: fixed link for reasons[KB]

My bad. I just quickly googled "bad astronomy" and that was the top hit. This is why I don't tweet. [MK]

I gave a longer answer to when photos will be available below, but the short answer is that it varies from telescope to telescope. [MK]

Can you describe what it was like to realize that we would be getting this amazing encounter less than a month after MAVEN gets to Mars?

I imagine it was at least a little scary at first.

How much extra work has planning for this weekend required?

It was awesome when I first heard about this -- especially as you say having it happen just after our planned arrival. A once in the centuries event and it happens just after our spacecraft arrives. Awesome! That said, the spacecraft engineers quickly pointed out the potential danger from the high-speed dust. Now that we know that the dust will miss (just barely!) Mars it's a bit easier to relax. We will nonetheless take some basic preparations for MAVEN. We'll time the orbit of MAVEN so that's behind Mars when the dust tail comes by. We'll turn the solar arrays edge on to the dust tail. And lastly, we'll turn off the high voltage instruments while the dust tail is going by. Besides that extra work the spacecraft engineers have done, we scientists are excited about taking our observations and preparing our models to compare the data to. [JE]

Thanks (been reading the CIOC blog and watching all the conferences) - but planning the science observations on a comet vs a planet must have caused some headaches?

Especially since there is only one shot to get it right.

I'm not involved with any of the missions directly, but based on the number of telecons and emails I have been involved in, the missions people must have been working like crazy to get these observations planned. Observing a comet is completely different than anything their instruments were designed to do, so it's quite a challenge to get the observations just perfectly. [MK]

Do you guys have a ton of awesome ho def cameras ready to take a picture of this?

There are numerous observatories (like Hubble) and the ones at Mars prepared to take observations of both the comet and Mars. However, the nucleus will still be comparatively far away from Mars such that even HiRise onboard the Mars Reconnaissance Orbiter will only get a few pixels of the nucleus. The gas cloud will be all around and encompassing the planet so during closest approach that'll be hard to image. So, short answer, is there should be some great images and great science but it won't be super high resolution like the images coming from Rosetta and it's target comet 67P. [JE]

Just for comparison, Rosetta is currently only 16 km from comet 67P and was designed to image a comet. MRO will be ~140,000 km away and was designed to look at Mars, which is way brighter! [MK]

Shit. I have no clue what's happening to Mars. I read your summery. I just don't have the education. All I want to know is this: will it be cool looking?

We sure hope so! This artist's rendition would be amazing, no? [MK]

Will there be any meteorites detectable on Mars when the Comet goes by?

A lot of people have done modeling of the comet's dust and it seems very unlikely that the dust will get to Mars. Theoretically the cameras on the rovers might be able to see a meteor, so if one does occur, it might be imaged. That would be pretty awesome! [MK]

Hi! Thanks for your work :)

I'm wondering if you have any advice on selecting a college for studying astronomy. Is it best to attend school in a region with little light pollution? Do you think attending a college in a highly polluted city would be less rewarding research-wise (Atlanta, for example)?

Much thanks, Lalita

Hi Lalita, I'm glad to hear that you're thinking about a career in astronomy! For undergrad classes, you can probably go to school anywhere. I went to grad school at the University of Maryland and we have an active observatory in the suburbs of Washington DC. The sky brightness affects what you can do, but there are still plenty of ways to get practice in being an astronomer. [MK]

Personally, I wouldn't let light-pollution dictate your choice of school. Yes it's a factor to consider, but it's more important you choose a school you're happy with, and that has a good program in general. A telescope operates just the same in a city as it does in the desert - you just can't see quite as much. Far more crucial IMHO is to be happy and enthusiastic in what you do (and seek out internships during breaks). Good luck! :) [KB]

When I was in the navy I used to sit and watch the stars at night when we were deployed. I used to think that astronomers would be so jealous of the sky I saw. Literally 1000 miles of nothing in every direction. Such a beautiful sky.

Not many places on land that are that remote anymore that aren't in the middle of the Sahara or the Himalayas.

Yes, I would be very jealous for that sky! [MK]

How did you guys get jobs in your field? What previous positions did you have that led to it?

Luck and persistence. Lots of resumes sent out to countless places as I desperately looked for a job doing something vaguely science-related, anywhere I could find one. I randomly got an email one day from someone saying they'd seen my resume and were interested. (TBH, I had no recollection of even applying to this group!) One thing led to another and long-story-short, in 2003 I ended up working in the solar physics group at the Naval Research Lab in DC. And there I've happily stayed. I never directly sought out the specific job I have - I just made sensible decisions along the way that stacked the deck in my favor. Luck got me in the door, doing good work kept me there. [KB]

I had a pretty traditional route. I majored in physics as an undergrad, got a Ph.D. in astronomy, then got a postdoc position at Lowell Observatory. My nominal position was for 2-3 years, but I've been successful at getting funding and really like where I am so I stayed (although I have been promoted to "research scientist"). [MK]

My previous jobs were as a waiter and working at a call center. Not sure either helped me land this job! [MK]

Is the large CME that's just become visible in LASCO imagery likely to have any effect on the comet, as it looks to be headed in that direction? Are there any instruments that would be able to make such observations?

Agreed, the recent CME does appear to be headed in Mars' general direction but it's hard to tell for sure from the 2-d view provided by SOHO. Either way, CME's tend not to have a major effect on comets. We have witnessed ion (gas) tails being ripped off by CMEs in our STEREO images, but that's still not "harmful" to the comet.

We have nothing that will directly image any interaction between a CME and the comet, but MAVEN is already detecting CMEs, so hopefully it could pick up something if one interacts with the comet this weekend. [KB]

Any chance the dust cloud will give us the fantastic four?

Only if the sequel is better than the first movie. [JE]

Do you know if there are proprietary data periods for these observations?

It completely varies from observatory to observatory. The observations will be fairly challenging and in many cases (both at Mars and on Earth), will push the limits of what the telescopes are capable of doing. Thus, they require quite a bit of calibrating and processing before they can be used for scientific purposes. For the spacecraft from Mars there are added complications since the bandwidth for transmitting data from Mars back to Earth is very limited, so it can takes days or even weeks before all the data have arrived at Earth. Furthermore, some of the spacecraft, like MAVEN and the Indian MOM mission, have just arrived at Mars, so they still have to finish all of their post-flight checks and calibrations, so the period could be even longer. On the positive side, I'm sure there will be lots of amateurs on Earth who get great images that they are posting as soon as they get them! [MK]

This was a truly great AMA. Thank you. What can we do to support your efforts?

Thanks! I don't want to get preachy, but a positive word about why science is important to your senator or congressman the next time science funding is on the chopping blocks can't hurt.

And I'll give a shameless plug to stop by Lowell Observatory the next time you're in northern Arizona! [MK]

Are there any plans to monitor Deimos or Phobos during the movement through the dust cloud? Do either of these two bodies have a "shell" of material around them already and would bombardment of dust from the comet show these shells?

It looks very likely that both Phobos and Deimos will be just missed by the dust tail. The gas cloud will engulf the moons but the gas is not likely to cause any dust to get kicked up from the moons themselves. So the bottom line is that we don't expect any particular rings or shell from the moons. Nonetheless, numerous observatories (like Hubble) will be looking at the encounter so Phobos and Deimos will be generally observed so it's possible we'll see something unusual. Yay, science! [JE]

Based on what you know how, what do you expect the local and palnet-wide effects to be? Will anything capture high-res images? Will rovers on the planet just get "fogged in"?

It's highly unlikely that the rovers will get "fogged in." The amount of material that will get to Mars is a lot compared to what gets there normally from the solar wind, but the amount will be miniscule compared to the lower Martian atmosphere. I believe that the rovers will try to observe the comet, but it'll be a really challenging image so no guarantees that it will work. [MK]

Effects will be mostly confined to the upper martian atmosphere. Even though it's thin, it's still thick enough to absorb all of the incoming gas and plasma and most, if not all, of the incoming dust. However, in the upper atmosphere, the effects could be comparatively large at least temporarily. One model shows that we might expect the temperature there to rise by as much as 30 degrees Celsius. And we also expect dramatic changes in the atmospheric magnetic fields ("induced magnetosphere") -- changes comparable to a large solar storm. [JE]

The best images from the Mars spacecraft will probably come from HiRISE on MRO. Even that will comparatively low resolution because the nucleus will still be far away compared to Mars which is what HiRise is designed to study. But it's likely we'll get our first multi-pixel image of a Oort Cloud comet (i.e. one that comes from the far reaches of the Solar System). [JE]

Do certain comets and/or asteroids have well known density characteristics? Would a Oort comet be more likely to end in an "above ground" explosion ( Tunguska Event) as opposed to making it all the way down to the surface and making an impact?

In general, asteroids are believed to be considerably more dense than comets. There's a really cool relationship between asteroid size and asteroid spin rate that very clearly indicates the limits of internal structure for asteroids. We don't have a similar plot for comets because their nuclei are very hard to image since they are generally shrouded in gas and dust and we just don't have enough measurements yet. Most of what we know definitively about comet densities is based on spacecraft flyby, and there has never been a flyby of an Oort Cloud comet.

I'm not an expert in what causes air bursts vs. impacts at the surface so I'm really not sure if there's a reason to think that one type of comet would be more likely to make an impact. [MK]

I gotta say this is pretty amazing and thank you so much for doing this AMA!

Now to the questions: So if you do get a good shot at the nucleus what could this tell you? What could the information about its size and color and density tell us about Oort Cloud Comets or the Outter solar system?

What exactly are the possible lasting/short term effects of the magnetosphere/outer atmosphere changes to Mars from the comet if any?

Any possible side effects of the gravity of the comet itself or is it going too fast for it to actually have an effect on Mars or its moons?

Again thank you!

I'll take a quick stab at these but MK and JE will probably add more.

If we see the nucleus, it will be the first time we've ever seen an Oort Cloud comet nucleus. Are they brighter than JFCs? Or darker? Smoother? Lumpier? We have no idea at all - we've never seen one! Intuitively you'd think the nucleus will be different from that of a comet that has spent centuries or millennia bathed in solar radiation, but we simply don't know (and hope we find out!)

I'm really shaky on Mars atmosphere stuff, but intuition tells me that effects from the comet will be relatively short lived and the atmosphere will soon return to a more normal state. Yes, for a short time the atmosphere will be locally enriched with incoming high-velocity gas, but Mars has a reasonably dense atmosphere so I'd bet equilibrium wouldn't take too long to be restored. Jared will hopefully chime in here though.

I keep kicking myself because so many people have asked me what effect - if any - the comet will feel from Mars' gravity, and I still haven't looked at the numbers. Again, my gut is telling me that even at 138,000km from Mars, the Sun's gravity will be by far the most dominant force at play and the comet probably won't even know Mars is there. [KB]

Would this cloud be able to help Mars create an atmosphere?

No. The amount of gas that will get to mars from the comet is trivial compared to the atmosphere Mars already has. [MK]

Would you consider reopening this AMA for a couple of hours after the COMET Siding Spring pass-by? Will MAVEN Mission have a USTREAM event for the pass-by? What time is it from/to on Oct 19?

Speaking for Jared and Karl, I think we'd be happy to do another AMA post-flyby. As we've talked about elsewhere in this thread the data won't be coming in in real time and it will likely be days or weeks before it's all back on Earth due to the bandwidth limitations of the deep space network. Thus, I'm not aware of any plans for a real time event on October 19. It's a good bet that there will be a press release as soon as there are results to report (and hopefully some nice pictures as well)! Pay attention to http://cometcampaign.org/ and http://mars.nasa.gov/comets/sidingspring/ for updates. [MK]

Hey, thanks for doing an AMA and alerting me to the upcoming event! Just a quick question:

Do you or any actual rocket scientists/astrophysicists/spacecraft engineers/Actual NASA scientists play Kerbal Space Program? It would make my day if actual scientists were playing the dumbed-down version of their job in their free time...

Given how many of the scientists I know play games, I'm sure there must be some actual rocket scientists playing Kerbal Space Program. [MK]

How long have you known about this event? Is this something that scientists have been anticipating and preparing for for a long time?

The comet was discovered in January 2013 (hence "C/2013 A1"). We knew pretty quick that it was going to pass close to Mars, and for a time I believe there were some orbit solutions that had them getting very closely acquainted! But as is always the case, more observations mean more accurate orbit solutions and it was probably only Feb or March 2013 when we knew the comet would miss Mars, albeit by a fairly short distance.

We have been planning for a while, but 20-months is a long time for Oort Cloud comets (which can, and do, behave exactly as they choose) so the bulk of the planning waited until this year when we had a better handle on its behavior. (Plus, last year we had that whole Comet ISON thing keeping us busy...) It was early this year that we did the dust modeling analysis to determine the potential debris impact risk to the Mars fleet (it's minimal risk, btw) [KB]

Jared and I actually talked about this the day the orbit was identified and we noticed how close it came to Mars. We've known each other since undergrad and finally found a project that we could work on together! [MK]

Have there ever been any attempts or is it even possible with current technologies to use a comet's gravity to sling shot a satellite into deep space?

Technically you could get a bit of a boost from one, but it's not worth the risk. Comets are (relatively) extremely low mass, and hence not gravitationally strong. You'd need a huge comet, and you'd need to fly insanely close to it, at quite a high velocity... you'd almost certainly get severely damaged or destroyed by all the dust, ice, etc that surrounds the nucleus. An asteroid would be a much safer bet, but still nowhere near as handy as the Moon, Mercury, etc, which we use fairly frequently for this. [KB]

What's also crazy with comets is that their shapes can deviate a lot from a simple spheroid and since they are too small to have differentiated (e.g. the heavy stuff hasn't necessarily sunk to the middle) so that they could potentially have their center of gravity be outside the nucleus! Another potential problem is that they may be a collection of smaller bodies that aggregated together, with lots of empty spaces in between. Imagine trying to get a gravitational assist from something where you alternate between feeling wildly varying gravitational pulls every few meters. I'm pretty sure that no engineer would ever agree to work out a trajectory for you. [MK]

Okay, maybe Wash would. [MK]

Are there any plans and have images streamed from curiosity live on Sunday?

Not that I'm aware of. Due to bandwidth limitations it's unlikely that there will be anything to stream live. [MK]

Will the comet interfere with communications between Earth and the Martian satellites or rovers? Are these devices set up to automatically perform these operations at this point, so that they can do it even if communications are lost? Can they store their observations long enough to get them sent back after communications are reestablished?

I'm not involved in any of the Mars missions, but my understanding is that they do not anticipate having any communications issues. They will all be operating "automatically," but that is not different than normal operations. Because of the time involved with sending commands to Mars and then getting a response from the spacecraft, everything operates in pre-programmed spurts. The teams for each spacecraft have undoubtedly worked out whatever maneuvers and data collection are planned, checked and re-checked the sequences, and sent (or have planned to send) the instructions to the spacecraft. Due to bandwidth limitations of sending information from Mars to Earth, they are already accustomed to storing data until it can be downlinked, so this won't be affected either.

tl;dr Communications shouldn't be affected, but it'll take a while for all the data to arrive. [MK]

Is there any place where I can see the orbital path of sliding spring, preferable animated? I've been looking, but just can seem to find one.

Also, if sliding Springs orbit were different and it was actually on a collision course with Mars, what would be the consequences of such a collision on Mars. Would the rovers on Mars be doomed?

Thanks for the AMA guys.

You bet there is! Check out the Solar System Scope Interactive Orbit! And even better, check out the simulated view from the surface of Mars!

Consequence of a collision? Hmm. I think "not good" would be one way to describe it. I'm guessing - and this is genuinely just my personal guess - that the impact would throw a huge amount of dust up in to the Martian atmosphere, and the rovers would no longer receive any solar power. I have no idea what it would do to orbiting satellites. I suppose that would depend in part on the incoming direction and subsequent dust trail. I also don't know to what altitude the Martian atmosphere would be affected (i.e. if it would reach the orbiter altitudes). But the bottom line is that it's something the Mars fleet operators would be tremendously eager to avoid happening. [KB]

Reports are that the comet is at mag 15. I have an 8" dobsonian telescope. Will it be worthwhile to observe this encounter from my backyard?

It depends on where you live and how skilled you are with the telescope. From our point of view on Earth, it's pretty close to the Sun so it's not up very long in a dark sky. The comet is still pretty far south so if you are in the continental U.S. it won't get very high in the sky. And because it's diffuse, it'll be a lot fainter than a star with a similar apparent magnitude. On the plus side, it is up just after sunset so you don't have to get up early, and if you can find Mars then you'll already be in the right ball park. All in all, I'd say give it a shot! [MK]

Will India's MOM capture pictures?

It sounds like it, although I haven't heard directly from anyone on that team about what they are planning. [MK]

[deleted]

I don't want to sound corny, but we're getting enough good questions that I don't feel the need to plant anything. [MK]

Good to know. I didn't want to imply you needed planted questions, but often with experts, the really interesting questions may never get asked.

And this is far and away my favorite type of AMA. Celebrities can sometimes be slightly interesting, but give me people who know stuff I don't know any day!

Thanks! We've already been asked it elsewhere in the AMA, but personally I am most interested in getting an image of an Oort Cloud comet nucleus. The only comet nuclei we've ever imaged have been "Jupiter Family Comets" (JFCs). JFCs have been close to the Sun a lot, whereas a "new" Oort Cloud comet like Siding Spring is entering the inner solar system for the first time. It's entirely possible that its nucleus properties are different. In particular, we don't know how well they reflect light (astronomer jargon = albedo). We know JFCs are VERY dark, darker than coal. If new Oort Cloud comets are significantly different, their nucleus sizes could be much different than we assume (either bigger or smaller). [MK]

Amazing. So a "virgin" comet, as it were?

<grin>

Some hypothesize these long period comets in fact are slingshot from an adjacent star rather than disturbed (somehow) from the Oort Cloud "shell". Their spectroscopy is key to understanding what they carry into our own solar system. Have your spectroscopy team interfaced to ALMA spectroscopy people (Anthony Remijan and co). It would be a very sad missed opportunity if your spectroscopy experts failed to search for the main complex molecules of RNA/DNA. I am sure they will be doing this. Here is my wish list to look for :

Guanine : C5H5N5O Adenine : C5H5N5 Thymine : C5H6N2O2 Cytosine : C4H5N3O

Good summary of what we have already found in interstellar space. http://en.wikipedia.org/wiki/List_of_interstellar_and_circumstellar_molecules But what is Comet Siding Spring bringing?

Here's a summary of planned observations of which I'm aware. I don't see ALMA on there, but they could have allocated time and just not told us about it (that table is compile by people voluntarily reporting their planned observations). There are also might be some perfectly reasonable technical reasons that they aren't doing it. [MK]

I know you probably are not excited when someone mentions the face on Mars or things like that which people see, and then ask you if you believe its alien. Well I have observed the Moon's "x" and "v" that appears on the terminator. And recently discovered a Large "X" on the moon.I was wondering. How does one (me as an Amateur Astronomer) when I find something new like this "x" on the moon or a feature on Mars, How do I report it and to who?

In general, a truly major discovery like that would require rigorous peer review before other observers would believe it. So, probably your best bet is to find some scientifically minded friends and colleagues and try to show them what you've observed. In a similar way, you could contact your local astronomy club and see what they have to say.