LIGHTCURVE ANALYSIS OF TWO NEAR-EARTH ASTEROIDS: 2010 VB1 AND 2014 JO25
Brian D Warner
Email:briain@MinorPlanetObserver.com
Abstract
CCD photometric observations were made of the near-Earth asteroids (NEAs) 2010 VB1 in 2017 June and 2014 JO25 in 2017 April. The lightcurves for both asteroids showed significant day-to-day evolution due to changing viewing aspects. For 2010 VB1, the average synodic period was 0.18919 ± 0.0002 h while the amplitude decreased in near step with the phase angle, going from 0.99 mag at 54° to 0.61 mag at 27°. For 2014 JO25, the average synodic period was 4.60 ± 0.04 h. Its amplitude ranged from 0.39 to 0.14 mag.
The somewhat close flybys of the near-Earth asteroids 2010 VB1 and 2014 JO25 in 2017 provided a good opportunity to study their lightcurve evolution as the viewing aspects (phase angle and phase angle bisector) changed by significant amounts.
Table I gives the telescope-camera combinations used for the two campaigns conducted at CS3-PDS. All observations were unfiltered since a clear filter can result in a 0.1-0.3 magnitude loss. The exposure duration varied depending on the asteroid’s brightness and sky motion. If necessary, an elliptical aperture with the long axis parallel to the asteroid’s path was used.
Table I.
List of CS3-PDS telescope/CCD camera combinations.
| Desig | Telescope | Camera |
|---|---|---|
| Squirt | 0.30-m f/6.3 Schmidt-Cass | ML-1001E |
| Borealis | 0.35-m f/9.1 Schmidt-Cass | FLI-1001E |
Measurements were made usingMPO Canopus. The Comp Star Selector utility inMPO Canopus found up to five comparison stars of near solar-color for differential photometry. Catalog magnitudes were usually taken from the CMC-15 (http://svo2.cab.inta-csic.es/vocats/cmc15/) or APASS (Henden et al., 2009) catalogs. Period analysis is also done withMPO Canopus, which implements the FALC algorithm developed by Harris (Harris et al., 1989).
In the plots below, the “Reduced Magnitude” is Johnson V as indicated in the Y-axis title. These are values that have been converted from sky magnitudes to unity distance by applying −5*log (rΔ) to the measured sky magnitudes with r and Δ being, respectively, the Sun-asteroid and Earth-asteroid distances in AU. The magnitudes were normalized to the given phase angle,e.g., alpha(6.5°), usingG = 0.15, unless otherwise stated. The X-axis is the rotational phase, ranging from −0.05 to +1.05.
If the plot includes an amplitude,e.g., “Amp: 0.65”, this is the amplitude of the Fourier model curve andnot necessarily the adopted amplitude for the lightcurve. The value is provided as a matter of convenience.
2010 VB1.
This NEA made its closest approach in 2017 June. The exposures on June 19 and 20 were only 20 seconds. This was because 1) of the asteroid’s rapid sky motion and 2) given its estimated size of only 70 meters, the possibility that its rotation period could be on the order of only a few minutes.Pravec et al. (2000) showed that exposures can be no longer than about 0.187x the period. Otherwise, “rotational smearing” occurs, meaning that details of the lightcurve are lost since the single observation covers too much of a rotation. Because of the rapid sky motion, up to five “sessions” were required, i.e., five different sets of comparison stars were used.
The plot above shows the raw data from just one session on June 19. At first glance, it appears to be useless data because of large scatter. However, because of the possible super-fast rotation, a period search was run from 0.001 to 1.500 h in steps of 0.001 h using all the data from that night. The result was a dense, well-defined bimodal lightcurve with a period of 0.18908 ± 0.00004 h and amplitude of about 0.98 mag.
As the asteroid retreated from the Earth’s neighborhood and the sky motion decreased, exposures were gradually lengthened to keep the signal-to-noise as high as possible. From June 22-25, 40 second exposures were used while for June 26 exposures were extended to 120 seconds. In all 1687 data points were acquired over the eight consecutive nights of observations.
The lightcurves below go from June 19-26 and show how the shape and amplitude evolved as the phase angle decreased from about 54° to 28°. The biggest change is the decreasing depth of the second minimum at about 0.75 rotation phase.
The final plot in the set shows the amplitude versus phase angle. As expected, the amplitude decreased along with the phase angle (Zappala et al., 1990). From the trend line alone, the amplitude would be about 0.28 mag at 0° phase angle. That should be taken with a healthy dose of skepticism since the shadowing effects (or lack of them) at near 0° phase angle could easily change things.
2014 JO25.
This NEA made its closest approach in mid-April 2017 when it was the target of radar and optical observations. The first observations at PDS were made on April 20. Here again, due to rapid sky motion, exposures were very short, only 5 seconds. Fortunately, the asteroid was V ~ 10.5 and so a reasonable SNR was obtained. Exposures on April 21 were 10 seconds and increased to 30 seconds for April 22-24. Unlike for 2010 VB1, the estimated size of 2014 JO25 (D ~ 860 m) was well above the limit where a superfast rotator might be expected.
The initial analysis of the April 20 data showed three likely periods: about 3, 4.5, and 6 hours, the second and third periods being 1.5x and 2x the shortest period. A bimodal lightcurve is typically the correct solution when the amplitude is about 0.2 mag and greater, but – in truth –Harris et al. (2014) allow for a non-bimodal solution up to an amplitude of about 0.37 mag. Since this was a known to be a target for the radar team, an email was sent to them to see if their observations supported the shortest period. They did not (Patrick Taylor, private communications), but instead favored a period of about 4.5 hours.
The longer period was adopted, resulting the in second lightcurve for April 20, and for period searches on subsequent days.
The lightcurve for April 21 remains a mystery. Measuring the images required using several sets of comp stars. To help assure good session-to-session zero point alignment, the last 5-10 images of a given session were remeasured for the following session. This should have led to 5-10 data overlapping data points in the raw plot. Slight adjustments (usually <0.05 mag) were required to get the best overlapping fit between the two sessions. This process was repeated for each subsequent session. Despite this, the lightcurve was dramatically different in shape and amplitude from the night before. The same algorithm to correct for changing phase angle and viewing aspect was used throughout, so it would be strange that this lightcurve would be so different. The possible, maybe even probable, cause is the shape of the asteroid, which was found by radar to be a double-lobed (highly bifurcated) body, along with a different viewing aspect.
There are some excellent radar image animations available of this asteroid, for example,
As the asteroid receded on April 22-24, the phase angle and viewing aspects did not change by large amounts. Given this and an assumed spin axis alignment that meant a similar silhouette was seen each night, the lightcurve returned to a low amplitude shape that was then bimodal when keeping the period near 4.5 h. Despite only small changes in viewing aspect and phase angles, there are still signs of slight changes in the lightcurve.
Table II.
Observing circumstances. Pts is the number of data points used in the analysis. The phase angle (α) and phase angle bisector longitude (L) and latitude (B) are the average values for the given date or date range.
| Number | Name | 2017 mm/dd | Pts | Phase | LPAB | BPAB | Period (h) | P.E. | Amp | A.E. |
|---|---|---|---|---|---|---|---|---|---|---|
| 2010 VB1 | 06/19 | 481 | 53.4 | 245 | 17 | 0.18908 | 0.00004 | 0.98 | 0.05 | |
| 06/20 | 250 | 46.9 | 249 | 15 | 0.18912 | 0.00005 | 0.81 | 0.03 | ||
| 06/21 | 357 | 41.5 | 251 | 13 | 0.18920 | 0.00005 | 0.83 | 0.03 | ||
| 06/22 | 136 | 37.2 | 254 | 11 | 0.1891 | 0.0001 | 0.67 | 0.03 | ||
| 06/23 | 217 | 33.8 | 256 | 9 | 0.18895 | 0.00005 | 0.63 | 0.03 | ||
| 06/24 | 47 | 31.1 | 257 | 8 | 0.1895 | 0.0004 | 0.70 | 0.05 | ||
| 06/25 | 148 | 29.0 | 259 | 7 | 0.18915 | 0.00008 | 0.62 | 0.03 | ||
| 06/26 | 51 | 27.3 | 261 | 6 | 0.1894 | 0.0002 | 0.61 | 0.03 | ||
| 2014 JO25 | 04/20 | 328 | 38.3, 30.6 | 197 | 12 | 4.64 | 0.01 | 0.21 | 0.01 | |
| 04/21 | 265 | 24.7, 24.2 | 198 | 3 | 4.53 | 0.02 | 0.39 | 0.03 | ||
| 04/33–04/34 | 473 | 23.9, 25.1 | 201 | −6 | 4.561 | 0.007 | 0.14 | 0.01 | ||
Acknowledgements
Funding for PDS observations, analysis, and publication was provided by NASA grant NNX13AP56G. Work on the asteroid lightcurve database (LCDB) was also funded in part by National Science Foundation grant AST-1507535. This research was made possible in part based on data from CMC15 Data Access Service at CAB (INTA-CSIC) (http://svo2.cab.inta-csic.es/vocats/cmc15/) and the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund.
References
- Harris AW, Young JW, Scaltriti F, Zappala V (1984). “Lightcurves and phase relations of the asteroids 82 Alkmene and 444 Gyptis.” Icarus57, 251–258. [Google Scholar]
- Harris AW, Young JW, Bowell E, Martin LJ, Millis RL, Poutanen M, Scaltriti F, Zappala V, Schober HJ, Debehogne H, Zeigler KW (1989). “Photoelectric Observations of Asteroids 3, 24, 60, 261, and 863.” Icarus77, 171–186. [Google Scholar]
- Harris AW, Pravec P, Galad A, Skiff BA, Warner BD, Vilagi J, Gajdos S, Carbognani A, Hornoch K, Kusnirak P, Cooney WR, Gross J, Terrell D, Higgins D, Bowell E, Koehn BW (2014). “On the maximum amplitude of harmonics on an asteroid lightcurve.” Icarus235, 55–59. [Google Scholar]
- Henden AA, Terrell D, Levine SE, Templeton M, Smith TC, Welch DL (2009).http://www.aavso.org/apass
- Pravec P, Hergenrother C, Whiteley R, Sarounova L, Kusnirak P (2000). “Fast Rotating Asteroids 1999 TY2, 1999 SF10, and 1998 WB2.” Icarus147, 477–486. [Google Scholar]
- Warner BD, Harris AW, Pravec P (2009). “The Asteroid Lightcurve Database.” Icarus202, 134–146. Updated 2016 Dec.http://www.minorplanet.info/lightcurvedatabase.html [Google Scholar]
- Zappala V, Cellini A, Barucci AM, Fulchignoni M, Lupishko DE (1990). “An analysis of the amplitude-phase relationship among asteroids.” Astron. Astrophys231, 548–560. [Google Scholar]