IMP-8 Confirmation

Tracking a very fast moving NEO candidate - NK0154 by Reiner Stoss

On the late evening of February 24th a new Near-Earth Object (NEO) candidate designated NK0154 was added to the Minor Planet Center's (MPC) NEO Confirmation Page (NEOCP). The object was quite faint and moving very fast, so it seemed to be a tough object to confirm and with rapidly increasing ephemeris uncertainty a quick confirmation was crucial to not lose this NEO candidate again. The confirmation succeeded and after several hours of follow-up the orbit turned out to be rather unusual. The object was clearly in a geocentric orbit and the question was: Is it an asteroid or a piece of space junk?

It was a clear night over central Europe, as it has been for several weeks already. A very unusual period of good weather for this region especially during winter. Together with my team mate Matthias Busch I was at Starkenburg Observatory, Heppenheim and we prepared our equipment for a long night of observing. Since January 1998 the main targets at Starkenburg Observatory (station code 611) are the newly discovered NEO candidates posted on the MPC's NEO Confirmation Page. These objects are in urgent need of confirmation and follow-up observations. If enough tracking is achieved and a first reliable orbit can be determined, the MPC issues a Minor Planet Electronic Circular (MPEC) where the discovery is published, in case the object turns out to be a NEO, which is mostly the case.

On this evening the NEOCP showed six NEO candidates, discovered between February 22.5 and 23.4 UT. Four of them by NEAT, one by LINEAR and one by LONEOS. These professional asteroid surveys scan the night sky for potentially hazardous asteroids and whenever they discover something moving unusually which could thus be a NEO, it gets onto NEOCP and a community of professional and amateur observatories around the world try to confirm and track them as long as they are visible.

Two of the six NEO candidates on this night were still one-nighters and not yet confirmed. One of them was already two and a half days old and the other one just one and a half days old but quite faint at predicted V 19.9 mag. So we decided to first go for the other four. These were at magnitudes between V=18 and 20 mag and moving at motions between 0.5"/min and 12"/min. We used the first few hours of the night to hunt them and reported the astrometry to the MPC when there was suddenly a new object added to NEOCP. The temporary designation of this was NK0154 and it was discovered only a few minutes before at February 24.8 UT, which was a clear indication that it must have been discovered by a station in Europe. Retrieving ephemerides we noticed that it moved very fast across the sky and was quite faint at predicted V 18.4 mag, with quickly increasing uncertainty. A very quick confirmation was needed and so we decided to cancel the other candidate that we had already pointed the telescope at and switched to this new fast-mover.

As the object moved at slightly over 1"/s, we had to keep exposure time short to not get trails, which would be almost impossible to be measured accurately.

With our 0.45-m f/4.4 reflector and the AP7 CCD camera we have 2.51"/pixel and the seeing was not too good that night so we decided to take 5s exposures, which should give maximum SNR before the object would start to trail. But could we see a V 18-19mag object on 5s exposures? Definitely not with just 0.45-m aperture. Track and stack was needed, a technique where one takes several images and then stacks them on the asteroid's motion. Thus the SNR improves and if the asteroid was barely visible on the single frames, after stacking it would be better visible and measures would be much more accurate. After stacking the first series of 8 images we found it. It was quite good on prediction but already a bit off. We assumed that this faint and fast object would not get followed-up by many stations so we kept tracking it to get as many positions as possible and enable the folks at MPC to improve the ephemerides so far that observers in the US would be able to find it and continue with the tracking when in Europe the night would be already over. Or in the worst case, if no station in the US was able to detect such faint objects would have clear weather, to allow a recovery from Europe one night later. Our first position was done at 22:52 UT and thus only about 2.5 hours after it was discovered. After tracking it for more than 3 hours we quit. This should be enough to find it again later that day from the US or on the next night from Europe.

On the evening of February 25 it was still clear over Starkenburg Observatory and a few more observers had arrived there, ready to go again after NK0154, as long as it would be possible. We had been in contact with Kyle Smalley from MPC by email and learned that M. Glaze and R. Trentman from Powell Observatory in Kansas were able to pick it up only 4 hours after we quit at Starkenburg and tracked it for more than 3 hours. This meant that the ephemerides provided on the NEOCP should be accurate enough to bag it again that night from Europe. At the same time we talked about the possibility that NK0154 would be on a geocentric orbit and thus circling around the earth.

But to get a definite answer to that, more follow-up was needed. We got it around 22:11 UT and thus almost 13 hours after Powell quit tracking it the night before and this night we kept tracking it for almost 6 hours until 4:00 UT when dawn began and we had to quit. Now it was clear. NK0154 was on a nearly circular 12 day orbit around the Earth, half way to the moon. And it would be observable every 12 days for 3-4 days. But what was it?

I was already in contact with Bill Gray, author of the astronomy software "Guide" and Tony Beresford, who both are experts in identifying satellites that get detected by asteroid hunters. And Tony did a superb job. He found out that NK0154 was the old IMP-8 spacecraft, which was launched back in 1973 and was still operational. IMP-8 is the last of ten Interplanetary Monitoring Platforms (IMPs), launched to measure the magnetic fields, plasmas, and energetic charged particles of the Earth's magnetotail and magnetosheath and of the near-Earth solar wind. This satellite was a drum-shaped spacecraft, 135.6 cm across and 157.4 cm high. Its initial orbit was more elliptical than intended, with apogee and perigee distances of about 45 and 25 earth radii.

Its eccentricity decreased after launch. Its orbital inclination varied between 0 deg and about 55 deg with a periodicity of several years. Its current orbit is nearly circular at 35 Earth Radii and with a period of 12 days. This data provided on the IMP-8 Project website fitted perfectly to the orbit we derived from our astrometric observations. So there was no doubt that what was
discovered as a potential Near-Earth Asteroid (NEA) was indeed this old spacecraft.

Usually in such cases where NEA candidates turn out to be artificial satellites, they get removed from the NEOCP with a quite unspectacular comment: "[temporary designation] was not a minor planet" and nobody cares about them anymore. But this time it was different. The MPC wrote that they had IMP-8 in their database of "space junk" which they check every NEA candidate against before it gets on the NEOCP. But IMP-8 would have been abandoned in December 2002 and since then no orbit or ephemerides would be available, so this was the reason why it was be confused with a NEA candidate and got onto NEOCP. The whole story became even more interesting now. Was this spacecraft really abandoned? If so, wouldn't it be good to keep track of it and determine its orbit to be able to distinguish it in the future from NEA candidates? In addition, if we could predict its path in the future, we could use it in the NEO tracking community for practicing how to track fast moving objects. And beside the fact that it is fast moving it is also quite faint and thus a perfect target for training. Because, as the NEO surveys scan the sky month after month, most of the bright asteroids are meanwhile discovered and the NEOs they find nowadays are quite faint and this trend will increase further.

IMP-8 would therefore be a nice test object for trying out new equipment or improved setups and techniques.

We decided that it would be good to recover it around March 10. But we were not sure if the determined orbit which was based on observations from February 24th and 25th only would be accurate enough to find it again after so many days. A few more observations on February 26th or early on 27th would be important to get a better orbit, before it would get too close to the sun in the morning sky and travelling through crowded fields in the milky way where it would be impossible to detect it among so many background stars. But even though MPC kept it on NEOCP, nobody went after it. Either all potential stations were clouded out or they had other targets scheduled. No more observations were made after the ones from Starkenburg Observatory on February 26th in the pre-dawn sky.

Now it was Bill Gray's turn. With his orbit determination software FindOrb, which is freely available on his website, he can do a so called Monte Carlo simulation to estimate the uncertainty area for a certain epoch. He found out that the uncertainty for around March 10 was about half a degree and mostly along NK0154's track. So the observing strategy was clear. We had to point the telescope ahead of prediction and start to image the field in a long series of short single exposures to avoid trailing beginning a bit before NK0154 would enter the field and keeping imaging until it should have left the field already for some time. Thus we would have covered the entire uncertainty area by just ambushing it at one location along its path. If we would fail to recover it early in March, we would have to wait another twelve days to get another chance and by then the uncertainty would be already much bigger, so it would get tougher. To minimize the danger of losing it due to bad weather during the recovery window, we tried to find other observers who would participate if their weather would
allow it.

The recovery team consisted of five stations, all managed by amateur astronomers and experienced NEO observers. Station "333 Desert Eagle Observatory" in Arizona, run by Bill Yeung. "611 Starkenburg Observatory" in Germany. "620 Observatorio Astronomico de Mallorca" where Jaime Nomen and Salvador Sanchez were ready to go. "649 Powell Observatory" in Kansas and "J95 Great Shefford", the very active NEO tracking station in the UK run by Peter Birtwhistle.

Bill Yeung was the first to try it. He had clear skies early on March 9th and took a series of images. But due to a slow internet connection he was not able to transfer the huge amount of data for reduction to us, so he served as a back-assurance. In case nobody else would find it, he would find a way to get the data to Europe. And with his large FOV it should be on Bill's images.

Later that day the sky cleared over Europe too, quite surprisingly and so 611, 620 and J95 were ready to try it. Mallorca was the first to search for it.

Over several hours many series of images were taken with the 30cm robotic internet telescope located there at the Observatorio Astronomico de Mallorca.

I was at 611 Starkenburg, still clouded out but working together with Jaime, who was actually in Barcelona during the week, over internet with the Mallorca telescope. While the robotic scope took the series of images, we downloaded them and began to analyze them, but then it cleared up over Starkenburg and I quit looking at the incoming Mallorca images and began observing with the 45cm of our observatory. I took a series of images, stacked 8 of them together to form one image and there it was. Very obvious and it had the same appearance as in February. I was sure that I got it even without measuring it and trying to fit the observations with the Orbfitter. I quickly announced the recovery to the other stations and to Bill Gray together with the first couple of measures, so Bill could improve the orbit and provide much more accurate ephemerides to the other observers. With this improved orbit it was possible to locate it on the Mallorca images too, which were taken a few hours earlier.

Now with the recovery done, this object won't get lost again, assuming that this old spacecraft has no more propellant left for orbital manoeuvres. It is not yet clear if this is so. Nevertheless, even if the IMP-8 team will do manoeuvres, it should be possible to keep track of it and I'm sure that we can work together with them on this.

Gareth Williams at the Minor Planet Center has set up a new page called "The Earth-Orbiting Space Junk Tracking Page", where observers can get ephemerides for these man-made satellites which could be confused with natural objects. Either if someone wants to do targeted observations for astrometry or in case one of these objects gets again picked up by a NEO survey when scanning the sky for potentially hazardous asteroids.

The survey which "discovered" IMP-8 alias NK0154 on February 24th is called KLENOT (KLEt' Observatory Near Earth and Other unusual objects observations Team and Telescope) and is located near the town of Ceske Budejovice in the Czech Republic. They found it by doing follow-up astrometry on comet C/2001 B2 (NEAT), when a fast moving object, later designated NK0154, crossed the field of view and left a short trail during the exposure.

KLENOT uses a 1-m telescope to search the sky for new NEOs, but also to do follow-up on known ones.

Reiner M. Stoss
Starkenburg Observatory, 10th March 2003

(reproduced with kind permission of Reiner Stoss. (c) Copyright Reiner Stoss 2003
Article originally published in The Astronomer magazine 2003 March, Vol 39,295)