What does Glib Deepak have to do with deep doo-doo?

[cheesehead]

I missed reading about this a few months ago.

Ever wonder what the Deep Impact space probe did after its primary mission (http://www.nasa.gov/mission_pages/de...ion/index.html) to knock out and observe some chunks of comet Tempel 1 ?

If I'd been paying attention, I might've remembered that it (renamed as EPOXI) was to (a) observe exoplanet transits and distantly observe the Earth-Moon system in order to characterize its appearance for comparison to exoplanet observations (http://deepimpact.umd.edu/), and (b) fly by another comet closely (1000 km) (http://epoxi.umd.edu/1mission/index.shtml).

Here's a movie of Moon transiting Earth, from Deep Impact EPOXI's point of view:

http://epoxi.umd.edu/7press/news/20080717.shtml (Note how dark the Moon looks beside Earth: its albedo is low.)

[ewmayer]

Of course, I just had to make this thread sticky.

[cheesehead]

Whoops -- didn't notice in advance:

EPOXI just made a swing by Earth yesterday (http://epoxi.umd.edu/7press/news/20100625.shtml) to get its final gravity assist for visiting comet Hartley 2 on Nov. 4 this year.

Since accomplishing its Deep Impact mission at comet Tempel 1, EPOXI has been observing transits of exoplanaets around other stars and maneuvering to shape its orbit to pass by comet Hartley 2.

From http://epoxi.umd.edu/2science/targets.shtml

DIXI Target

The DIXI component (Deep Impact Extended Investigation) of the EPOXI mission will observe comet 103P/Hartley 2 to compare it with comets observed by other spacecraft missions. Comparisons with data from Tempel 1, taken with the exact same instruments, will be particularly useful for determining which cometary features represent primordial differences and which result from subsequent evolutionary processes. Comets Boethin and Hartley 2 were identified before the launch of the Deep Impact spacecraft as possible targets for an extended mission upon the completion of Deep Impact. Boethin was selected as the primary target since it was a shorter mission and had better encounter conditions. Hartley 2 was designated the backup target in case Boethin was not recovered before the December 2007 Earth flyby.

(A periodic comet is said to be "recovered" when it is re-sighted on its way from aphelion to perihelion.

Most comets are too faint to see with current technology when they are at aphelion. As they re-approach the Sun, warm up and resume emitting particles to form a coma, eventually they become bright enough to see again. Once they do, their positions can be measured and orbits updated.

Comet orbits are less precisely predictable far in advance than asteroid orbits because they are subject to significant non-gravitational forces from the jets of material evaporating from their nuclei when they are close to the Sun.

That isn't much of a problem if all you want to do is look at them from Earth -- a million km one way or the other doesn't make it all that hard to find. But if you're trying to send a space probe close to one, and need to do so at the best angle for photography and other data-taking, you need a more precise orbital prediction. That can't be done until the comet has been "recovered", or re-sighted.

Why not just update the orbit as the comet is pulling away from the Sun? Because the slight differences due to non-gravitational forces take some time to make enough difference in position for us to measure accurately. That's more readily done after half an orbit, several years later, than during the few months the comet stays close to the Sun, where the non-gravitational forces are greatest but have not yet had time to make much difference in distance.)

Boethin was not recovered,

(or, at least, not recovered in time for its orbit to be refined enough to begin steering EPOXI to intercept it)

which simply means that we were unable to definitively locate the comet's current position according to the best orbital information that we had from the last time it was sighted in 1986. The flyby spacecraft has performed the appropriate Trajectory Correction Maneuver (TCM) to put the spacecraft on course for Hartley 2.

• 103P/Hartley 2 is a Jupiter-family comet.
• Discovered in 1986 by Malcolm Hartley
• Period is 6.4 years, next perihelion is 28 October 2010 at 1.059 AU
• Inclination is 13.6°, descending node is 27 October 2010

Hartley 2 is smaller and more active than Tempel 1, which makes the comparisons more interesting. This plus more sunlight (because the encounter occurs slightly closer to the sun) means our signal-to-noise ratio (SNR) on all our measurements will be significantly improved.

[Graff]

Originally Posted by cheesehead

Comet orbits are less precisely predictable far in advance than asteroid orbits because they are subject to significant non-gravitational forces from the jets of material evaporating from their nuclei when they are close to the Sun.

The key point here is that the non-gravitational forces are not constant.
If they were, comets would be as predictable as minor planets.

Originally Posted by chessehead

That isn't much of a problem if all you want to do is look at them from Earth -- a million km one way or the other doesn't make it all that hard to find.

Depends on the field-of-view of the instrument that you're using to
observe the comet. If that 1 million km is tangential to the geocentric
vector to the comet, the difference in sky positions will amount to
atan(1/d), where d is the geocentric distance to the comet in millions
of km. For d = 1, that's 45 degrees; d = 10, ~5.7 degrees; d = 100,
~0.6 deg.

Gareth (who has computed quite a number of cometary orbits with NG
parameters in his time).

[cheesehead]

"Tally-Ho! Deep Impact Spacecraft Eyes Comet Target"

http://www.jpl.nasa.gov/asteroidwatc...m?release=2727

On Sunday, Sept. 5, NASA's Deep Impact spacecraft beamed down the first of more than 64,000 images it's expected to take of Comet Hartley 2. The spacecraft, now on an extended mission known as EPOXI, has an appointment with the comet on Nov. 4, 2010.

It will use all three of the spacecraft's instruments (two telescopes with digital color cameras and an infrared spectrometer) to scrutinize Hartley 2 for more than two months.

Also:

"UMD-Led Team Begins Imaging Comet Hartley 2 with Deep Impact Spacecraft"

http://newsdesk.umd.edu/global/relea...ArticleID=2224

[ewmayer]

And we had not one but two "bullet burn"-style near-misses yesterday, both by small (~10meter-ish diameter) objects passing within the distance of the moon's orbit from Earth.

Both of these objects (RX30 and RF12) were well-characterized in advance (first spotted Sunday, orbits determined with sufficient accuracy to be sure of a miss shortly thereafter) by the NEO folks at the MPC. Gareth, were you involved in these particular orbital determinations and if so, about how long did it take to do the followup observations needed to determine the orbits for these?

Just for kicks, I sorted the MPC list of 2010 close approaches by closest-approach distance...RF12 is the closest-approaching object of that list this year, and RX30 is #5. Even though the distances are individually not that unusual, it's pretty unusual to have 2 such close ones in a 24-hour period.

[Graff]

Originally Posted by ewmayer

And we had not one but two "bullet burn"-style near-misses yesterday, both by small (~10meter-ish diameter) objects passing within the distance of the moon's orbit from Earth.

Both of these objects (RX30 and RF12) were well-characterized in advance (first spotted Sunday, orbits determined with sufficient accuracy to be sure of a miss shortly thereafter) by the NEO folks at the MPC. Gareth, were you involved in these particular orbital determinations and if so, about how long did it take to do the followup observations needed to determine the orbits for these?

Just for kicks, I sorted the MPC list of 2010 close approaches by closest-approach distance...RF12 is the closest-approaching object of that list this year, and RX30 is #5. Even though the distances are individually not that unusual, it's pretty unusual to have 2 such close ones in a 24-hour period.

Yes, I did the initial impact-analysis for 2010 RF12 that showed this object
couldn't come closer than about 10 Earth radii. And I did the initial
Minor Planet Electronic Circular announcing the discovery and the
designation, as well as two follow-up circulars later in the day with
additional observations and updated orbits so that the uncertainties the
next night would be as small as possible. Since 2010 RX30 was never
considered to be an earth-impactor, I didn't do anything on that object.
It was a hectic few hours, complicated by the fact that I was also dealing
with a hardware problem with one of our computers.

Gareth

[Uncwilly]

Originally Posted by Graff

Yes, I did the initial impact-analysis for 2010 RF12 that showed this object couldn't come closer than about 10 Earth radii.

Any prediscovery images found yet? Has any one looked into automating the prediscovery search (or is it already happening in some fashion)? I know that often some of the larger objects are found in the Palomar plates.

[Graff]

Originally Posted by Uncwilly

Any prediscovery images found yet? Has any one looked into automating the prediscovery search (or is it already happening in some fashion)? I know that often some of the larger objects are found in the Palomar plates.

Very unlikely. With a diameter under 10 m, it is improbable that there
are any images of this object in the archives of photographic plates
and CCD images. (2010 RF12 reached only mag ~ 13 moving at > 1000"/min
and photographic plates would be hard pressed to record such a
fast-moving faint image.) Prediscovery images are usually only found
for objects with diameters above ~ 0.5 km. There are a number of
individuals who run semi-automated searches for previously unmeasured
images in archives and we run automated searches for reported
measures of previously-unidentified objects.

Gareth

[ewmayer]

Get Fuzzy Bucky Katt has recently expressed skepticism of the K-T impactor-as-dinosaur-killer hypothesis ... here is today's cartoon:

[ewmayer]

Spot the copy-editing silliness in the following meteoric headline - No, not the one I quip with at end of this post, the other one:

Rare daylight meteor seen, heard over Nevada, California: LOS ANGELES (Reuters) - A rare daytime meteor was seen and heard streaking over northern Nevada and parts of California on Sunday, just after the peak of an annual meteor shower.

It was amusing listening to the meteorologists on the local news stations talking about the event ... insert cartoon-style thought bubble above their heads, reading "Finally I get to do what I was hired to do! Screw this 'weather analysis & forecast' crap..."

Also, where in heck is Nevada, CA? I was unable to find a town of that name on my atlas. Is that near Tahoe?