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Discovery story of Apollo object 2006 WH1

 
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Stefan
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Čest korisnik
Joined: 15 Jun 2005
Posts: 33
Location: Zagreb
PostPosted: Wed Nov 22, 2006 0:13    Post subject: Discovery story of Apollo object 2006 WH1 Reply with quote

... written by Reiner M. Stoss, and posted to MPML:

Let me share the discovery story with you, which will also answer some questions that were asked here on MPML. After that I want to add some comments on the discovery rules and also to provide some links to things mentioned in the discovery circumstances. Let's go with the discovery
of 2006 WH1 first...

La Sagra Observatory (J75) is a rather new observatory, located in Andalusia in Southern Spain, near the village Puebla de Don Fadrique. It is a joint venture between the resort Collados de la Sagra, the Observatorio Astronomico de Mallorca (620) and the Instituto de Astrofisica de Andalucia in Granada.

In September we started to use two 36-cm f/2 telescopes for asteroid and comet searching. While the large 1 square degree FOVs of these telescopes might suggest that we go for large sky coverage, this is not the case actually. Each telescope remains on the same field for about 40min and we prefer to go as deep as possible.

Imaging along the ecliptic we usually find several dozens of unidentified MBAs each night, all of them Rmag 19.5 and fainter. Anything brighter is usually known already and IDed with MPC's MPChecker. We do not report ONS but try to get a second night.

Data acquisition "on the mountain" is done by Nicolas F. Morales and Jose Luis Ortiz (both IAA). The images get transferred via a satellite link to IAA in Granada from where they get distributed to the folks in charge of the astrometric data reduction. That's currently two high-school students affiliated to Visnjan Observatory (120) in Croatia - Aleksandar Cikota and Stefan Cikota, with some help by myself.

Both students were on a national science competition (Vip Eureka) during the whole weekend, so I reduced friday night's images myself on saturday afternoon. Along dozens of new mag19+ objects, there was this mag18 mover. The field was located only 10 east of opposition and all those movers moved at very similar speed and P.A., including the mag18 object, which we labeled OLSB28. OLSB28 moved at 0.6"/min in P.A. 228, nothing unusual at all.

On sunday evening, after Aleks and Stefan returned from the science competition, they started to reduce saturday night's data of the same fields, to get those needed second nights. They reported me their findings and one of those was again a unusually bright mag18 object.

I ran both nights of the mag18 object through a linear regression, first separately, all being okay. And then together, nothing being okay.

The second night didn't fit at all and rather both nights didn't fit together at all. My first guess was that they did some measuring error, so I grabbed the original images, measured myself and got the same results. A measuring error was completely impossible with such a high SNR object.

I decided to measure now all frames individually as the object was bright enough and didn't need any stacking, so I got about 9 data points per night instead of just 3. With them the linear regression looked like this:


First night (Nov. 17):

OLSB28

-0.25 +0.06
+0.20 +0.28
+0.16 -0.06
+0.17 -0.16
+0.14 -0.19
-0.09 +0.15
-0.62 -0.03
-0.04 +0.09
+0.49 +0.08

Second night (Nov. 18):

OLSB28
+0.11 -0.07
+0.01 +0.11
-0.16 -0.12
-0.10 -0.01
+0.08 +0.18
+0.10 -0.05
-0.21 +0.01
+0.17 -0.04

Both nights together:

OLSB28
+2.15 -0.21
+2.05 +0.07
+1.49 -0.21
+0.95 -0.25
+0.39 -0.22
-0.46 -0.20
-0.93 +0.24
-2.00 +0.16
-1.97 +0.29
-2.00 +0.35
+3.34 +0.60
+2.50 +0.63
+0.78 +0.07
+0.11 +0.03
-0.45 +0.06
-1.16 -0.32
-2.22 -0.42
-2.58 -0.63


Clearly both nights have different "slopes" and the linear regression tries to fit both with an average "slope" resulting in the above ugly fit in the RA residuals.

>From my experience so far, this is unusual. Usually I can fit two consecutive nights of an MBA in opposition with a linear fit to check the astrometry and that they belong together.

But OLSB28 clearly showed "curvature" after just one day. My first thought was something from Andrea Milani's talk at MACE2006 in Vienna, where he presented his proposal for a new discovery definition. As far as I remember, one of the essentials on which his definition is based, is the "curvature". You need to measure the curvature to be able to tell the orbital type.

Now I did a real orbital calculation and got an Apollo type orbit, with a=1.97, e=0.60, i=2.8. I reported both nights to MPC on sunday to monday night, at 00:17 UT. A few minutes later it was added to the NEOCP. At the same time I requested targeted follow-up from Nicolas at La Sagra. While the third night was exposed there and uploaded to the server, MPC had already removed the object from NEOCP and MPECd it in MPEC 2006-W28.

We got a nice Apollo discovery that gets as bright as mag16 shortly before Christmas Eve when it crosses the Earth's orbit, going from the dark night side into the bright light of the day side.

Obviously MPC had found prediscovery observations within their ONS files, from Oct. 23, Nov. 10, 11 and 15.

To be honest, when I found the object on our Nov. 17 images, I was sure that this bright rock must have been imaged before already. This is often the case with the two-nighters we report. But usually MPC has just an ONS or several of them but separated too far to be linked together. So we report linked two-nighter and get credit usually.

In this case, MPC had four prediscovery ONS which were linked only after we have reported our OLSB28. I *don't* blame them. I failed too when I tried blinking them automatically, i.e. quick and dirty.

Only my doubt that there are two undiscovered bright objects so close together made me go back to the images and check and re-check everything, so that I noticed the "curvature" in the data.

MPC clearly doesn't have the means to investigate for so long time each and every ONS.

Now talking about how objects which go to the NEOCP get selected, I don't know. Either the discoverers report them as NEA candidates or the MPC picks them out from a large batch of routine observations. Only MPC can tell us more or the folks involved with the big surveys.

As far as I know they get selected by the motion they show. In different parts of the sky different combinations of speed and P.A. are necessary to reach a certain threshold for being "not MBA". I remember a diagram by Jedicke (?) that showed the confidence region of motion for MBAs in opposition. Anything outside this "speed/P.A." range was probably a NEO
and got flagged.

I guess the big surveys have such diagrams for the entire sky, either derived from a set of their real detections or by some numerical modelling with virtual asteroids. This would be a good topic and I hope someone will get it started on this list.

Re. the definition of discovery, without wanting to re-start the discussion that we had here on MPML about Andrea Milani's proposal, I just want to add a remark.

The publication of this Apollo type PHA was the result of someone making two nights of the object, linking them and reporting them to MPC. With just one night it was not possible to see the nature of the object, as the many prediscovery ONS show us.

Now thinking about discovery credit, my conclusion is that someone reporting just an unlinked ONS should not get discovery credit. With the exception of cases where the object moves so unusual that it gets flagged and added to the NEOCP.

If nobody reports two close nights and MPC finally has many ONS which they manage to link together, of course someone should get discovery credit in these cases too. It could be someone of those stations that reported those ONS. Or it could be someone who did the linking. Or a share of both.

Clearly Andrea tries to put the discovery on a scientific fundament. He would like to postpone the completion of the discovery to the point where the orbital type of the object is well known. This is one extreme.

The other extreme is to give someone exclusive discovery credit who has only provided an ONS. A detection that doesn't allow any conclusion about the orbit of the object and thus to some extent to its nature.

Again, ONS where the curvature is visible within one night are an exception and do merit and get credit already today (NEOCP objects), but the other two extremes do not get credit currently, neither the many follow-up observers and orbital calculators in Andrea's proposal nor the provider of just an ONS in the other extreme proposal.

And I think this is justified, because extremes are always doomed to fail (no pun intended on non-scientific issues/views ;-)

While I like Andrea's scientific proposal/definition, if really put into practice exactly like this, it will fail because it is impracticable.

The other extreme, of giving credit to the submitter of an ONS already, is *very* practicable and would make MPC's life more easy, but it is somewhat dumb. You would get credit for the discovery of something that you don't even know what it is.

The current MPC two-night rule is somewhere in between. It is not so complicated and easy practicable, but it avoids having to share credit among dozens of individuals and institutions like in Andrea's proposal.

In my view, the discovery game is not science. It is fun. It is the hunt for scalps. From private communication with several folks of this list, I know that they share this view. It is nice to have a NEO, a comet, a Centaur, a TNO etc. etc. in your scalp collection after a long time of "warpath" before you bury the hatchet and start to smoke the calumet, hehe.

So don't try to put the definition of discovery on a *completely* scientific fundament either. It will just wash-out everything. We need the room for those scalp-collectors who do invest a lot of effort and resources particularly *because* there are scalps to be captured. Without them, we wouldn't be as advanced as we are today.

Please consider the case of 2006 WH1 as a vote for MPC's "two-night" rule. Any future amendment of the MPC discovery rules should follow these principles:

Keep it simple and leave enough room for the thrill of discovery!

At the end of this message which got way too long, so I beg your pardon, I want to provide a few links to several terms mentioned in the discovery story.

La Sagra Observatory:
http://www.astrosagra.org/

Resort Collados de la Sagra:
http://www.colladosdelasagra.com/
(with their great cuisine and wines, they are the
very pleasurable starting point on a visit to the
nearby La Sagra Observatory)

Observatorio Astronomico de Mallorca:
http://www.oam.es/

Instituto de Astrofisica de Andalucia:
http://www.iaa.es/

Visnjan Observatory and Science Educational Center,
and Vip Eureka:
http://www.astro.hr/
http://www.sci.hr/
http://www.vipnet.hr/cw/show?idc=8926859&idnews=312067

MACE2006, Vienna:
http://www.minorplanets.org/

MPEC 2006-W28:
http://cfa-www.harvard.edu/mpec/K06/K06W28.html

Cheers,
Reiner
OAM Observatories
2006 WH1_forum.jpg




Last edited by Stefan on Sat Dec 02, 2006 18:31; edited 1 time in total
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Stefan
Čest korisnik
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Joined: 15 Jun 2005
Posts: 33
Location: Zagreb
PostPosted: Sat Dec 02, 2006 18:21    Post subject: Reply with quote

Now we have also the first LC from 2006 WH1, made by Brian D. Warner, from Palmer Divide Observatory.
LC_2006wh1_small.jpg


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