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Direct determination of NEO distance by parallax

Presentation delivered at MACE 2006 by Monty Robson and Peter Birtwhistle

[Comments by M. Robson]

Hello, Iím Monty Robson from 932, the McCarthy Observatory in Connecticut in the United States. Thank you for having us at this fine conference.
I am here with Peter Birtwhistle to speak about our ongoing project to range Near Earth Asteroids. We hope it will be as exciting to you as it is to us.

Here are the two observatories.

The McCarthy Observatory is located on a high school campus in Connecticut and it is primarily an educational facility. It is located about half way between Boston and New York City.

On the right is J95, the Great Shefford Observatory in south central England, not too far from Stonehenge. This picture shows me, Monty Robson (left) and Peter Birtwhistle (right) in front of J95 in November 2005. Ever since Great Shefford began operations it has been a leader in confirming newly discovered NEOs.

In 2002 and 2003 it was my pleasure to mentor a wonderful young student during the research portion of her science fair project. Miss Lisa Glukhovsky is shown here with Dr. Brian Marsden, Director of the Minor Planet Center. They were both invited to speak at a meeting of the Rhode Island Astronomical Society, Skyscrapers.

Lisaís project was entitled A Rapid, Accurate Method of Determining the Distance to Near-Earth Asteroids. Her project won many awards including winning one of the worldís largest science fairs, the Intel International Science and Engineering Fair.

Lisa used simultaneous CCD imaging between observatories separated by a long baseline distance to determine the objectís parallax and then derived its distance. Peter and I are using the same basic technique, but are using improved equipment and software and have improved procedures.

Here are some of the projectís statistics.

24 simultaneous or nearly simultaneous observation pairs of nine different newly discovered NEOs have been reported to the MPC.

3 NEOCP objects, objects on the Confirmation Page of the MPC, were reported.

The mean range difference between our projectís derived range and the range published in the Horizons Ephemeris service from the Jet Propulsion Laboratory was 0.41%. Our mean astrometric residual was 0.35 arc second for the project to date. Our mean timing error was 0.36s. 932 uses a GPS-based time and J95 uses a corrected Internet-based time.

This is a chronological summary of our projectís observations.

The nearest object observed was at a range of 0.047 AU and the most distant object was at a range of 0.313 AU.

I want to point out that there is little or no correlation between an objectís range and the percentage of range difference between our derived range and JPLís published range. In other words, no matter how far away an asteroid is our project range is within 0.5% of JPLís range.

There is, of course, a correlation between the range difference and the range to the NEO. The range difference between our derivation and JPLís range increases with increasing target range.

This slide shows the spreadsheet we use to determine range to the target. Peter also uses other software, including FindOrb.

Data is only entered into the yellow boxes. Answers are given in either the blue boxes or the lavender box.

The date and decimal day time are entered first. Then the two observing stationís elevation, east longitude, and latitude are entered to calculate the baseline length (a chord line distance through the Earth). Then the targetís astrometric coordinates from the MPC report are entered. This gives the parallax in degrees and is converted into range. Ranges shown are: R1, range from target to station 1; R2, range from target to station 2; R is range from target to the geocenter.

I would like to introduce my collaborator and friend, Mr. Peter Birtwhistle. Most of you know him and he doesnít really need an introduction; I think he is the best there is at finding and confirming newly discovered near-earth objects. All of us at 932 have learned a lot from working with Peter, we are a more capable observatory from the experience. PeterÖ

[Comments by Mr. P. Birtwhistle]

Good afternoon.

This page is showing simultaneous observations of a NEO. The spreadsheet that Monty showed relies on the fact that the observations are simultaneous or nearly so, preferably to the second. So we spent a bit of time finding the way to do this, in practice itís not straightforward.

For the first little while we were emailing to coordinate our observations. Trying to arrange observing sessions, with the number of communications required, using email was almost impossible. So a few weeks into the project I suggested using instant messaging over the Internet as the way to communicate. It made things much easier.

Next is a series of our images.

This is the first target we went for. At the time I was still using my 12-inch telescope, Montyís is a 16-inch. The pixel scale is different, but we tried to show the same field of view.

This is an example of a rather large parallax of 117 arc seconds.

Here is 2004 VD17 showing a rather small parallax of 23 arc seconds since it was a fair distance away.

This animation toggles between the 932 image and the J95 image.

This is a set of five simultaneous images of a relatively bright NEO with a moderate parallax of just under 70 arc seconds.

This was one that was on the Confirmation Page when we were going for it.

This was a fainter object so we had to stack images using Astrometrica (software) to see it. It is much easier to work with targets that can be seen on individual images.

Here is the MPEC announcing the confirmation of 2005 CC37. Our timing was slightly off, the fifth digit of a decimal day is about 0.8 seconds.

This is a time frame of the history of 2005 CC37 observations.

20 minutes after appearing on the NEOCP, J95 sends the MPC a confirming report. This is 2 hours 24 minutes after discovery.

When it is dark enough at 932, we try to simultaneously image the asteroid. The first joint observation takes place 3 hours 42 minutes after discovery.

4 Ĺ hours after discovery we had a range to the target Confirmation Page object. This range stayed within JPLís 3 sigma range uncertainty for 21 days. In other words, it took three weeks and almost 90 observations of 2005 CC37 before its orbit was known well enough for our early range determination to fall out of JPLís range uncertainty.

This is the spreadsheet for one of our 2005 CC37 observations.

This is a graph showing the way 2005 CC37ís range uncertainty decreased as more observations were reported.

Blue diamonds represent JPLís range. The range uncertainty is shown as error bars above and below the blue diamonds.

The magenta square on the left represents our project range, 4 Ĺ hours after discovery.

This graphic shows, at first, a large uncertainty area for an asteroid observed by only one station.

If simultaneous observations are conducted from two (or more) observatories the uncertainty area is greatly reduced.

This slide and the following two slides illustrate the power of the technique of simultaneous imaging to quickly refine the orbit of a newly discovered object.

What I show here are the residuals for both J95 and 932 from FindOrb. The FindOrb orbit was calculated using only J95 observations, then the 932 observations were evaluated. Notice the very high residuals from 932. It is obvious that this orbit solution is not representing Montyís observations very well.

Putting 932ís observations into the Findorb orbit solution you get the accuracy of the parallax. Here are the residuals for the same observations when FindOrb includes all of them for orbit determination. J95ís residuals change slightly, but 932ís residuals are drastically reduced. Now the orbit represents all observations very well.

Using my 8 observations, taken over a time span of 1 hour 45 minutes, for orbit determination, FindOrb returns a range to 2004 XJ of 0.0410 AU. But when 932ís observations are included in the solution FindOrb gives a range of 0.0472 AU for the same time span. This is very close to JPLís range, 0.0471 AU, at the time of our observations, when using all 75 observations collected over a 13 day arc. This demonstrates the power of observing new objects at the same time.

There are limitations to this project. Weather at both stations must be acceptable. We are 5 hours apart in longitude and 10 degrees different in latitude. Targets have to be visible from both stations. In the summer months it is difficult to image together since morning twilight sets in at J95 as it is getting dark at 932. In many situations it may be preferable to use stations orientated on a north-south baseline, rather than an east-west baseline as in our case. Maybe an observatory in South America would be a good match for North American stations, likewise a southern Africa site may be well suited for those in Europe.

As for requirements for participants, accurate observatory time and Internet access while observing seem to be most important.

[Comments by Robson]

We would be happy to answer any questions you may have about this project.

We would welcome the participation of others in the project.

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