More on SMART-1 impacting the Moon

By Phil Plait | March 15, 2006 8:30 pm

A little while back, I blogged about plans to let the SMART-1 probe impact the Moon. My friend Emily Lakdawalla interviewed Bernard Foing, the Project Scientist for SMART-1, about their plans, and she clears up some of the questions I wasn’t able to answer. The impact will occur on or about September 2-3. The uncertainty is due to — get this — the fact that the angle of impact is incredibly small, about a degree, so the probe will be skimming the surface of the Moon for the last few hours. If a hill rears up, smack! Since we don’t know the topography of Moon very accurately, they’re not sure exactly when or where it will hit. That’s amazing, and makes me realize that even though the Moon is the closest of the objects in the sky, there is still a lot more to learn and to know about it.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (23)

  1. Hope it doesn’t hit any LM descent stages or parked rovers. ;)

  2. Troy

    That sounds like a wild, wild ride.

    SMART-1 was supposed to be able to photograph Apollo hardware on the moon does anyone know if any images of Apollo hardware were returned? Maybe Fox will issue a retraction?

  3. Chip

    Interesting article by Emily Lakdawalla. She states that SMART-1 will impact “on the near side of the Moon, in the southern hemisphere, in highlands, in the dark, about 80 arc seconds from the first quarter terminator.”

    Also, 1 kilogram of hydrazine on a spacecraft in impact could produce a flash visible from Earth, “at least in infrared wavelengths (around 2 microns.)” SMART-1 will have 3 kilograms! The article also explains the wide window of time before impact.

    An ambitious amateur astronomer might be able to set up several video cameras mounted to tracking telescopes. The cameras could be running in an overlapping sequence over the impact time window in the hope of capturing a flash.

    One thing I may have missed in the article; will SMART-1 be functioning up to the moment of impact?

  4. Troy: I remember seeing a news story or two describing the imaging of Apollo hardware, but I wouldn’t get your hopes up. According to ESA:

    “Despite the fact that SMART-1 will cover virtually the whole Moon with observations by several instruments, unfortunately we will not be able to image the Apollo landing sites with enough resolution to detect the presence of any remains from the Apollo missions.

    A resolution of about 1 metre would be necessary for that. We have a camera with a resolution of 0.005176°, which translates into a resolution of about 90 metres from an altitude of around 1000 kilometres. In order to image the Apollo remains with our camera, we would then need to fly at an altitude of about 11 kilometres. This is due to the fact that the optics of our camera were designed for the specific scientific needs of this mission. We could have easily designed an optical system with a narrower field of view and capable of detecting the Apollo remains, but alas this was not a mission goal.”

    Not much imagery from SMART-1 has been released yet; while it’s possible they may have detected traces of the Apollo sites, I don’t think there’s much chance of seeing any satisfying images of the remnants. Besides, since the hoax believers dismiss the existing Apollo images as fakes, I doubt they’d be very receptive to anything new.

    Back to the impact though — mmm, possibility of visible ejecta. That’d be a neat sight!

  5. P. Edward Murray

    Impact?

    Almost sounds like they are trying to do a soft landing?

  6. Scott

    “One mind-boggling fact he mentioned was that on the orbit before the crash, they could well be sailing along only 400 meters above the lunar surface. Yikes.”

    Kind of reminds me of Arthur C Clark’s Maelstrom II. The main diff is that, fortunately, this will be sans human!

  7. Scott, that was my very first thought as well!

    Anyway, the impact site expected is if there is no large hill or anything in the way.

  8. Reminds me of Arthur C. Clark’s Maelstrom II…. arrgg, never mind…

  9. Emily Lakdawalla mentions Chuck Wood’s question to Foing. Wood also blogs about this ( http://www.lpod.org/?p=152 ) at his Lunar Photo of the Day site ( http://www.lpod.org ). If you are a Moon junkie, be sure to check this site.

  10. Mark Martin

    Very cool. If it approaches the surface at such a small angle of attack, it’ll undoubtedly flake apart as it initially grazes the soil. It’s meaningless then to speak of an impact “point”; it’ll necessarily be a long, stretched out impact trail.

  11. Kim

    I haven’t read the book, but my first thought was: How scary that would be to see from the ground.

  12. pumpkinpie

    The BA wrote: “Since we don’t know the topography of Moon very accurately, they’re not sure exactly when or where it will hit. That’s amazing, and makes me realize that even though the Moon is the closest of the objects in the sky, there is still a lot more to learn and to know about it.”

    That’s why I’m excited about the Lunar Reconnaissance Orbiter, launching in 2008. It should provide us with much better topography data. I’m attending a talk by a LRO mission systems engineer tonight! :^)

  13. Sid

    I say we should put the entire current U.S. administration on the moon, quickly. There are no larger objects that can get in the way of innovation and learning oportunities than them! SMART-1 couldn’t miss ‘em! (And I wouldn’t, either.)

    Maybe the immovable object stricken by the unyielding force would clear the way for the Earth to get on a better track.

  14. Irishman

    P. Edward Murray Said:
    >Impact?
    >Almost sounds like they are trying to do a soft landing?

    Soft landing? Not really angle, more to do with velocity. Rough back of the envelope:
    Diameter moon = 3476 km
    Time orbit = 5 hrs

    V = D/T = 3476/5 ~= 695 km/hr ~= 435 mi/hr

    Doesn’t sound slow to me.

    The problem with the angle I would think is that there’s no atmosphere (of any importance) to slow the SMART-1 down, so the only way to change velocity is expend fuel. Their fuel expenditure will put it on a (probably elliptical) slightly degrading orbit, but essentially it will be close to the orbital path around the Moon. Very hard to make a larger circle hit a smaller circle if the centers are in the same spot. Thus the low impact angle.

    The counterintuitive thing is that if they want a soft landing, they would do like Apollo and come in at a much steeper angle, but much slower. Using more fuel. That they don’t have.

  15. It appears that someone is working on an independent movie version of “Maelstrom II”, although it doesn’t seem to be going anywhere.

    http://www.maelstrom2themovie.com/

  16. During the Leonids, ESA had a nice research program to monitor the flashes from the meteoritic impacts on the Moon. This seems like a good opportunity to resurrect the experiments. http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=38059&fbodylongid=1800

  17. Irishman said:
    “Soft landing? Not really angle, more to do with velocity. Rough back of the envelope:
    Diameter moon = 3476 km
    Time orbit = 5 hrs

    V = D/T = 3476/5 ~= 695 km/hr ~= 435 mi/hr

    Doesn’t sound slow to me.”

    It’s faster than that. The orbit is the circumference of the moon, not the diameter. You’ve got to multiply your numbers by Pi, i.e. ~2,200 Km/hr or ~1,400 mi/hr.

    – Jack

  18. Buzz Parsec

    IIRC, a low circular orbit of the moon takes about 2 hours, not 5, which works
    out to about 3400 mi/hr. I forget most of my freshman physics, but
    an independent approach – the moon is about 1/80 the Earth’s mass, but
    1/4 the diameter and a low earth orbit is about 18000 mi/hr, which
    works out to 1/80* (1/4)^2 * 18000 = 3600 mi/hr.

    According to the ESA Smart-1 web page at http://sci.esa.int/science-e/www/
    object/index.cfm?fobjectid=36801, the initial orbital period was about 5
    hours, but this was an elliptical 471 x 2880 km orbit. The 2 hour period
    applies to a low circular orbit, such as that used by the Apollo missions.

    It sounds like they will be lowering the perilune (low point of the orbit) until
    it just grazes the surface. This is how they can know the approximate impact
    location. An object in a 0 x 2880 km orbit would have to be travelling
    considerably faster than a circular orbit just above the surface, so the 3400
    mi/hr figure is a minimum.

  19. Buzz Parsec

    P.S. Smart-1 would be travelling considerably faster *at the low point of
    the orbit* than an object in a circular orbit at the same height. At the
    high point in its orbit, it would be travelling slower than an object in a
    circular orbit at the same distance as the high point. As a rough
    approximation, an object in a elliptical orbit would be moving at same
    average speed as an object in a circular orbit at the average of the high
    and low points of the orbit, faster at the low point, and slower at the high
    point. The lower the perilune, the smaller the average distance, and the
    faster it has to go. (Remember, the closer to the Moon, the stronger its
    gravity, so an object has to move faster to overcome the force.)

    I hope this makes sense without having to resort to calculus, conservation
    of energy, etc. to explain it all. :-)

  20. You probably already know about this but just in case you don’t, here’s the Planetary Society’s blog. You can follow this topic from there.

    http://www.planetary.org/blog/

  21. Irishman

    Ack! Thanks for the cleanup, Jack. Good additional info, Buzz.

  22. Mark Martin

    This also brings to mind a passage from Jim Irwin’s book, “To Rule The Night”, in which he tells of orbiting the Moon at low altitude:

    “Now we are in a 9- by 45-mile orbit- that is, 9 miles above the average surface of the moon at ourlowest point. There are many mountains higher than that. You look out on the horizon and see these peaks and you are just skimming along. Now you really know you are moving fast. You are traveling about 5,000 feet per second, that’s Mach 5 or 3,000 miles per hour. Your orbit is defined; you can’t dodge anything. You don’t have any control over the vehicle, and if you did you probably couldn’t react fast enough. You just assume that Houston knows where the mountains are and how high they are. But you see the high mountains on the horizon and you move toward them very fast. You wonder if you are going to clear them.”

  23. LoneSnark

    Point: If they are going to crash the SMART-1 into the lunar surface, then why not make its orbit over one or more of the lunar landing sites? If an altitude of 11 kilometers will let it see something, imagine how good the pictures will be at 800 metres, right before impact :-)

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