Talk:16 Psyche

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[edit] Dimensions and mass

The mass and size data is difficult to make sense of at the present. This is what the data highlights look like (as far as I have been able to tell). Diameters:

• IRAS diameter: 253 km
• An occultation on 16 May 2004 gave a best fit ellipse of 214x181 km (±7 km), but there were only 5 observations, and the fit was described as "poor". (PDS)
• Speckle interferometry gave 315x262 km (Cellino et al 2002, proceedings, on ADS)
• Lightcurve fitting by the group that usually appears most reliable (IMHO) indicates diameter ratios of about 1.44:1.2:1, but they mention residuals, which is not encouraging. Other pole solutions (PDS) range from 1.38:1.16:1 to 1.9:1.4:1, with a clustering around 1.75:1.32:1.

If you equate the geometric mean of the diameters with the IRAS diameter, you can usually get a decent estimate of the bodie's shape. To get agreement with the smallest occultation diameter, you need then the more elongated pole solutions, but the second diameter ends up around 250 km, which is significantly bigger than the second occultation diameter. You might then assume that the occultation was with the asteroid showing its small face towards us, while the IRAS measurement when it was showing its large face. Taking this into account, you can get something like 192x231x277 km using the 1.44:1.2:1 solution, which is a bearable fit to the occultation. But now the speckle interferometry doesn't agree! Actually, I would be inclined to keep this last attempt, since I seem to recall from somewhere that speckle interferometry is good at giving axis ratios, but often has systematic error in the absolute size. Can anyone confirm this? Deuar 19:27, 16 May 2006 (UTC)

Now mass is interesting as well. There are two estimates:

• In 2000 M=1.7±0.5×10¹⁹ kg was obtained (Vitaeau A&A, 354, 725).
• In 2002 M=6.8±0.6×10¹⁹ kg was obtained (Kuzmanoski A&A, 395, L17).

Now that's some discrepancy! Normally I would be more inclined to believe the second value because it seems to be based on a more significant perturbation of the test asteroid. However, with the dimensions 190x230x280 that look best, this gives a density of 10.6 g/cm³. That's like Uranium or something. The first estimate gives a density of 2.7. A bit small for a presumably metallic body, but maybe it's a rubble pile. So after all that, I'm suggesting we use ~190x230x280 km, 1.7×10¹⁹ kg for now Deuar 19:27, 16 May 2006 (UTC)

[edit] Mass

New source lists mass as either ≈3.22×10¹⁹kg or 3.14×10¹⁹ [1]. I recommend using median value. I will wait 1 week for feed back before updating. Abyssoft 05:08, 4 September 2006 (UTC)

Here is what a search in PDS and ADS for other mass estimates turned up:

Author	Year	Mass (in Solar mass × 10-12)	note	link
Viateau	2000	8.7 ± 2.6		[2]
Kuznetsov	2001	14.9 ± 3.1	repeated by Kochetova
Kochetova	2004	13.4 ± 2.2	reference mentioned in the link above	[3]
Chernetenko et al	2005	6.8 ± 1.4	the webpage mentioned above (same group as Kochetova)	[4]

Well, purported errors are similar, but the actual values are all over the place. A worrying symptom is that the same group that includes Kochetova gave two widely varying values in the space of two years whose errors do not overlap (fairly spectacularly). This makes me suspicious whether they have got their method worked out properly yet. I would suggest just going with the old value by Viateau that is in the article now. We could add the new reference in the infobox without changing the value, though. Deuar 23:18, 4 September 2006 (UTC)

To add to the confusion, this is supposed to be a metallic asteroid from radar observations. The Viateau mass gives it a density of only 2.7 g/cm³ which is a pretty poor effort for metal, while the lates Chernetenko etal gives it even less. Ah the madness. Deuar 23:28, 4 September 2006 (UTC)