Monday29 March 20046:48pm MST2004-03-30 UTC 0148 back top next  
2004 FH from New Mexico Skies by Robert Hutsebaut

The Asteroid/Comet Connection's
daily news journal about
asteroids, comets, and meteors


Today's issue status: done
yesterdayMarchtomorrowIndex

Cover: At left, Earth-buzzing small asteroid 2004 FH. It was in the constellation Libra when observed on March 18th by Robert Hutsebaut, who was running a Rent-a-scope telescope at New Mexico Skies from Belgium. These frames are from the same session that later saw two satellites fly through the scene during a 300-second exposure, which was the March 18th "cover" image.
      How big is 2004 FH? See an explanation below.

From brightness to size – panel 1/2 Major News for 29 March 2004 back top next  
From brightness to size

By Marco Langbroek

The size estimate of a near-Earth asteroid (NEA) is based on a determination of its absolute brightness (H) and an assumption about its albedo (p or pV). The absolute brightness is the brightness that the object would have if it were at one astronomical unit (AU) from both the Sun and Earth, and fully illuminated as if seen from Earth. H can be calculated directly from the NEA's observed brightness and its calculated distance and phase angle (like Moon phases). The albedo is the reflective property of the NEA's surface. A dark surface has low albedo value, and a bright surface has a high value. In the usual absence of a known albedo, an assumption has to be made, which of course influences the resulting size estimate. Asteroid albedos can vary between about 0.03 (very dark) and 0.22 (bright), depending on the type of asteroid.

The often stated size estimate of "30 meters" for the recent Earth-grazing NEA, 2004 FH, is based on a low albedo estimate of 0.05, which is the mid-value

Chart of 2004 FH size from brightness 
created by Marco Langbroek
2004 FH brightness-to-size range

for C-class asteroids — dark asteroids probably similar in composition to carbonaceous chondrite meteorites. If 2004 FH is an M-class asteroid, however, composed of nickel-iron (similar to iron meteorites), or an S-type asteroid (similar to common stony meteorites), the size estimate goes down to about 18 meters. Thus the 30 meter estimate is near the upper end of the possible size range for 2004 FH. This object is not likely to be larger, but could be smaller. For NEAs, albedo is usually assumed to be 0.15, and the JPL NEO Program uses pV=0.154 in judging impact hazards.

references >>

From brightness to size – panel 2/2 Major News for 29 March 2004 back top next  

<< continued from panel 1

An Excel spread sheet created by the author is available for download to use to obtain diameter estimates from brightness. More on the relationship between albedo, absolute magnitude, and size, including the formula, which is easy to incorporate into a spreadsheet or script, can be found at these links:

Editor's note: When Marco Langbroek wrote March 19th about "2004 FH as meteor event," he mentioned the possible consequences if 2004 FH had entered the Earth's atmosphere instead of merely making the closest-ever observed flyby. In judging whether an incoming small object might self-destruct harmlessly at high altitude or could reach deep enough into the atmosphere to cause blast or even impact damage, size is, of course, one critical factor.
      See above for an animation of 2004 FH.

2004 FU4 brightness-to-size chart by Marco Langbroek

The above chart shows how the brightness-to-size range relationship works with a larger asteroid, in this case 2004 FU4, which is the only object currently under observation that has impact solutions (see below for today's news).

News briefs – panel 1/2 Major News for 29 March 2004 back top next  
P/2004 F3 (NEAT) from 29 March 2004 
 by Robt. Hutsebaut via N.M. Skies
P/2004 F3 (NEAT) this morning by Robert Hutsebaut
News briefs

Comet news: The Minor Planet Center announced two comet discoveries today, C/2004 F2 (LINEAR) and P/2004 F3 (NEAT). MPEC 2004-F81 for the first comet has LINEAR observations going back to March 25th, with confirmation from four other observatories through this morning. The first preliminary calculation has this object now going away from perihelion (1.43 AU last December 26th) on a just-past-perpendicular retrograde path.

For P/2004 F3, which travels a path from Jupiter's orbit into the asteroid Main Belt, MPEC 2004-F82 shows the first NEAT observations as coming from its Haleakala telescope yesterday, and includes positions from the 15th from LINEAR and from the Spacewatch 0.9m telescope on the 21st. Among the 16

participating observatories was New Mexico Skies, where today's cover astronomer, Robert Hutsebaut, used a robotic telescope to make his first comet confirmation (see at left), reporting in his batch to the MPC that the object seemed "neboulous."

FMO Project news:  MPEC 2004-F78 issued early today announces 2004 FC18, the object mentioned in news yesterday as discovered by online volunteer Michael C. Begam of Arizona in the course of the FMO Project conducted with the Spacewatch 0.9m telescope. The MPEC indicates that Arianna Gleason was running the telescope and also gives credit to Spacewatch associate Miwa Block. This is the FMOP's second confirmed discovery (see Major News Index). From its brightness (H=24.5), 2004 FC18 is estimated to be on the order of 40 meters/yards wide.

more news briefs >>

News briefs – panel 2/2 Major News for 29 March 2004 back top next  

<< continued from panel 1

Morrison reports:  NASA astronomer/astrobiologist and IAU NEO official David Morrison has some new information on his site at NASA/Ames. One item is about "Congressional Bills Supporting NEO Surveys," dated March 26th, including a bill to fund NEO surveying down to hundred-meter diameters.

The other item, dated March 23rd, tells about the 2004 FH flyby. Information from the latter was expanded in Morrison's NEO News E-mail newsletter, which was posted by SpaceRef.com March 26th (references to "2004 AG" and "2004 AH" are actually 2004 FH). He says, "It is very difficult to answer the seemingly simple question of what would have been the consequence if 2004 AG had hit," and goes on to explain at length, referring consistently to 2004 FH being estimated at about 30 meters wide.

Marco Langbroek told A/CC readers about "2004 FH as meteor event" on March 19th, and today shows above why 2004 FH is more likely to be somewhere around or under 20 meters wide, and also tells how you can study size estimates for yourself.

Bits & pieces:  Ball Aerospace has a news release today telling that "The Flyby and Impactor spacecrafts for the Deep Impact mission will be joined in their final flight configuration to undergo thermal vacuum, vibration and acoustic testing" before going out to crash the impactor into comet 9P/Tempel 1.

The National Review has a piece today, "The Politics of Armageddon," that attempts to foresee the difficult decisions a U.S. President would face if told a northern hemisphere impact might be imminent.

Speaking of chief executives and minor objects, the Globe and Mail yesterday, and other Canadian news outlets, tell that the Prime Minister's pilot filed an official "UFO Procedures" report about "seeing a 'very bright light falling' through the air, with smoke trailing" while over Suffield, Alberta Sunday night. And "People aboard at least two other planes also saw the plummeting object, which was travelling 'at a very fast rate of speed' from a high altitude." This may be the big fireball event witnessed across several provinces last Sunday night, or one of several others reported at about the same time from Canada and the U.S.

Risk monitoring - panel 1/1 Major News for 29 March 2004 back top next  
Risk monitoring 29 March

The Daily Orbit Update MPEC issued early today carries observation of 2004 FU4 from yesterday morning from the Spacewatch 1.8m telescope in Arizona and LINEAR in New Mexico. The Minor Planet Center Last Observation page is showing that Spacewatch also observed 2004 FU4 this morning.

Revised update:  During the lunch hour in Pasadena, and just before 10pm in Pisa, NEODyS and JPL updated their 2004 FU4 risk assessments. NEODyS has cut its impact solution count by three and has very slightly lowered its overall risk ratings for this object. JPL, with a longer time horizon, has cut down from 104 impact solutions to just six, but has slightly raised its already high ratings for the 2010 solution, and has now also raised a second solution, in 2085, to Torino Scale 1 (a routine alert that an object "merits special monitoring"). These assessments are still highly preliminary, based on a very short observing arc of barely nine days. The night-and-day cycle of observation and analysis for some previous objects has taken anywhere from a

Summary Risk Table - sources checked at 2358 UTC, 29 Mar

Object

Assessment

Years

VI
PS
cum
PS
max
T
S
Arc 
days
 2004 FU4 NEODyS 3/292010-207924-0.43-0.4419.066
JPL 3/292010-20956-0.25-0.2619.066
 2004 FHJPL 3/242098-20981-7.25-7.2502.771
NEODyS 3/20R E M O V E D
VI = count of "virtual impactors" (impact solutions)
See A/CC's Consolidated Risk Tables for more and maybe
  newer details, and check the monitors' links for latest info.
Note that only objects recently in view are shown here.

An "impact solution" (aka "virtual impactor" or "VI") is not a prediction but rather a possibility that hasn't been eliminated yet. To learn more, see "Understanding Risk Pages" by Jon Giorgini of JPL, and other links related to this subject.


few days to a few weeks to obtain sufficient data for the risk monitors to remove all impact possibilities, and 2004 FU4 will be in view for several months.

See above for a chart showing the size range possible for 2004 FU4 based on its brightness and likely surface reflectivity.

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