THE SMART-1 IMPACT ON THE MOON
By Francis G. Graham
US Amateur Observations Coordinator, SMART –1 Impact
On the evening of
September 2, observers in the eastern half of
The following information pertains to the SMART-1 Impact on the lunar surface, from various reports by Bernard Foing of the European Space Agency and colleagues. The information and impact model comes from them. I have added some ideas of my own, but none in conflict with the models.
The SMART-1 impact on the moon might be viewable by amateur astronomers with large instruments. What observations might be performed will be discussed also.
Time of Impact
At present, the SMART-1 probe is scheduled to
impact on the night side of the Earth-facing hemisphere of the Moon on the
evening of 2-3 September, 2006. The most
probable time of impact is 2:00 UT September 3, which is 10 PM, September 2,
Eastern Daylight Saving Time. At that
time, the Moon will be at R.A. 18 h 29m 13s, Dec. -28o 27’ 32”, true
equatorial geocentric coordinates, at a distance of 377,338 km. . Thus the Moon
will be near its most southern point in Sagittarius. For observers in
It is possible the impact will occur on the previous orbit, in which case it would be 5 hours before the time stated; or 5 hours after the stated time, on the following orbit. The reason is that the probe is coming into the Moon at a very shallow angle of one degree! It could also impact an unknown high topographical high up to 2 hours before the impact, or 2 hours after, on each possible impact orbit.
If a planned orbit adjustment scheduled for June 23 to July 8 is not successful, then the probe will impact on August 27. However there is every reason to believe the burn will go as planned, and subsequent adjustments July 26 and August 30 will too, and this will select the orbit of impact even more, to the one which terminates at 2:00 UT September 3.
Visibility on the Earth
At the time of
impact, the Earth facing the Moon will extend from
Location of Impact
At the time of impact, SMART 1 will be orbiting the moon in an orbit with apolune, or farthest point from the Moon, at 4420 km. or so, and
Fig. 1 Impact Site: Lacus Excellentiae
perilune, or closest point, just intersecting or immediately above the lunar surface. The orbit will be inclined 90.6 degrees, which means it will orbit essentially north-south.
The location of the perilune on 2-3 September will be 36S, 44.2 W on the lunar globe, which is in Lacus Excellentiae (“The Excellent Lake”). The probe is expected to impact near the perilune point into a topographical elevated feature. The most probable impact point is 34S, 44.13 W in Lacus Excellentiae north of the crater Clausius. However, if the probe impacts on the previous orbital approach to perilune, which is possible, then the impact will most probably be at 36.5 S, 41.4 W. If it impacts on the following orbital approach to perilune the orbit will be most probably at 37S, + 2 o , 47 W. The first picture is a Lunar Orbiter 4 picture of Lacus Excellentiae; the second map shows the topographic features, derived from Clementine probe data, near the final impact point, the elevation topographical lines showing elevations in order of 1 meter. The straight, nearly vertical lines are the SMART 1 orbits, the line in the middle shows the most probable impact orbit and the dot the perilune. The lines on the right of the center line represent the previous two orbits, the lines on the left the following two orbits. Immediately north the periline on the center line is an elevated ridge. It is this ridge the SMART-1 probe is predicted to strike, as it travels from north to south on its orbit. The ridge itself rises in a 2-8 degree slope, the probe will come to it about 1 degree downward and southward from the horizontal.
Fig. 2 Decay of the Perilune
The lack of highly detailed information on the lunar topography 400 km north or south of Lacus Excellentiae makes an exact prediction of impact impossible; we can only give the most probable. It is possible the probe might just barely clear the ridge on the center orbit and impact on the following orbit.
Fig, 3 Perilune Meets Surface
Orienting the SMART-1 solar panels vertically with respect to the lunar surface may assure impact at the desired location, rather than miss the topographical feature by a few meters and go on to the next orbit. The efficacy of this would depend on how much torque the impacting panels can give the remainder of the spacecraft, and the scale of the uncertainties of the closest approach to the topographical feature. If the scale is thousands of meters of uncertainty rather than scores of meters, the solar panel orientation would of course not matter.
Fig. 4 Impact Map of Final Orbits
Visible Circumstances of Impact
The impact of the SMART-1 probe will be a low velocity (2 km/sec = 4,400 MPH) crash, spread out over a large linear swath. The impact energy of the 285 kg craft will be thus about 600 megajoules. If half of this kinetic energy goes into the explosion, the impact will produce a 7.4 magnitude flash. But the impact velocity is lower, so it is likely the percentage of kinetic energy being transformed into heat is far less, on the order of 1% to 0.1%, then the flash may be magnitude 16. The duration of the impact flash is 20 milliseconds.
The flash will also be caused by 3 kilograms of fuel ullage, mainly hydrazine, N2H 4, which may also raise a significant portion of the 200 kg. of aluminum above the 600 Celsius point, but not to vaporization.
The impact will likely make a crater 5- 10 m in size, shaped like Schiller i.e., elongated. 10-80 cubic meters of excavation will occur, of which 80% will be an unheated, cold ejecta plume. The dominant size will be about 15 microns per dust grain. The area of the ejecta could be as high as 25 square kilometers, which would produce a fuzzy appearance for telescopes that can image in reflected Earthshine. This would require a telescope of 2 meters aperture or larger.
The dust will be ejected with a wise distribution of velocities, the most probable normal (vertical) component will be 130 m/s. However, a small fraction will be greater than 280 m/s. If that is the case, the particle has enough velocity to ascend upward into the sunlight and be visible. The best modeling gives the magnitude of visibility of the plume as
V= 11.5 - 2.5 log (f/1%)
So if only 1% of the plume exceeds 280 m/s, the magnitude of the sunlit-visible plume would be 11.5, that is, it would appear as a magnitude 11.5 centrally condensed “comet” above the night surface of the Moon.
This is what one can hope to see of the SMART-1 impact.
Observers with spectroscopes can hope to see evidence of the artificial nature of the impact. Aluminum vapor lines will be absent due to the low velocity of the impact, but other spectroscopic features should be visible.
3 N2H 4 -à 3 NH 3 + 3 H + 3 N
will be facilitated by the thermal energy of impact and produce excited monatomic hydrogen. Thus Balmer lines of emission will be visible in the spectrum of the flash, and the Paschen and Brackett lines in the near-infrared. Again, a large aperture will be required since the total magnitude of the impact can’t be brighter than 7.4. In addition, the craft contains about 260 grams of xenon gas, which may produce visible lines, such as the 5419 Angstrom line. There is also 14 meters of carbon fiber arrays in the solar panels, but, it is uncertain what effect they will have.
The spectroscopy slit should be oriented north-south along the final impact trajectory.
The Moon will have the same phase as the Impact Day on the evening of July 6-7, and on August 3-4 or 4-5, and so practice sessions can be developed on those days. Astronomers must remember that phenomena observations are one-shot deals; not since 300 kg. Hiten hit the Moon at 2.7 km/sec. with 1 kg. of hydrazine has there been a similar event. No events like this are scheduled further. The wise observer has several possible sites, and cloud-cover predictions will help the observer select the telescope site. Large aperture telescopes that are barely portable are favored.
The spacecraft itself will be magnitude 19, and will be in the sunlight up to 4 minutes before impact. Observing the final orbit of the spacecraft, and the time it becomes no longer visible, will be invaluable for understanding the final impact. If the solar panels are aligned vertically the spacecraft will be dimmer. In any case, a 3-meter or larger telescope is required to see the magnitude 19 spacecraft.
Telescopes for visual observation should be at least 1 meter in diameter, comfortably more, and have a field of view of about 1 arc-minute centered on the impact area.
In the infrared the flash should persist longer, but a large telescope is required. 0.5 arc-second resolution is needed, so adaptive optics must be used. Very large professional telescopes will be used for infrared imaging.
Expect the Unexpected
The flash may also reveal some things about the lunar area upon which it occurs. We have every reason to expect the low-volatiles of the lunar soil is going to produce no significant emission and this will hold true for Lacus Excellentiae as well. But there are many possibilities each with low probability in impact situations. One recalls the surprises that the impact of Schoemaker-Levy 9 on Jupiter in 1994 produced. Even the Hiten impact produced at least one surprise: a flash signature on the K-band due to Br gamma emission.
There are few scientific paradigms for dealing with the unexpected, but there is a philosophical guide. The philosophical extreme position, called “The Great Chain of Being” in the 18th century influenced the encyclopedists; it states that since the Universe is so large, anything that is possible actually exists somewhere (the principle of plenitude). This philosophical principle also asserts that the Universe is continuous, that is, every phenomenon shares with other phenomena some attributes. Finally, this philosophical principle suggests these phenomena have gradation, in some hierarchical order based on magnitude of some attribute.
A more moderate application of this principle is some help as a guide. How does this principle reflect on the unexpected? If many things are possible at a given time and place, though each has a very low probability, one is bound to happen; this is an inversion of the principle of plenitude. It is not always possible for modelers to list exhaustively all the possible events of vanishingly low probability, however, although Fritz Zwicky’s morphological method (See Morphological Methods in Astronomy, by F. Zwicky) might serve as a guide to listing them exhaustively. The difficulty is, to have instruments and experiments designed for each of the low probability events one would have to array extraordinary time and resources. The best chance is to have the widest bandwidth and lowest signal-to-noise. The two are not always mutually possible.
It would be useful to see the area in sunlight, after the impact. This will occur two Earth days later. As the Sun rises higher and higher over Lacus Excellentiae, observers can see the ejecta blanket and note how its photometric properties change for an impact of an object of this type. It would be also interesting to see if any of the artificial elements of the spacecraft, e.g., the carbon fibers, affect the photometric properties (carbon is rare on the Moon; there is less carbon on the lunar surface that would be expected from the impact of carbonaceous chondrites). The first period of post-impact sunlit observations extends from late September 4 to September 17. The actual impact crater itself, 5- 10 meters in diameter, will be much too small to image from Earth, and must await the next lunar reconnaissance mission.
The mineralogy of the area can be examined pre-and post-impact. Dr. Foing has prepared a table of areas of the Moon for which mineralogical characteristics can be compared, using wide-band reflectance spectroscopy.
Table 1 Mineralogy Sites
Tycho LC10 -43.40 348.90 East Large crater; gabbroic plutons
Alphonsus ALP1 -13.70 356.00
DMD, dark halo crater
Apollo 16 AP16 -9.00 15.50 Landing site, calibration check
Apollo 14 AP14 -3.70 342.50 Landing site, calibration check
Luna 16 LC34 -0.40 56.18
Luna 16 landing site
Apollo 11 PC4 0.67 23.47 Landing site
Reiner GammaREI4 4.95 298.70 swirl
Luna 24 LU24 12.50 62.25 Landing site area, calibration check
Apollo 17 PC9 20.19 30.77 Landing site
Aristarchus ARI2 23.23 313.47 Aristrachus crater, Crustal mat.
Apollo 15 AP15 26.10 356.30 Landing site, calibration check
Gruithuisen Delta1 35.90 320.45
Delta dome, early volcanism
Finally, please let me know of your plans, and your results. My contact address follows.
Francis Graham, Dept. Physics
© Copyright 2006, Francis G. Graham, All Rights Reserved.