Talk:16 Psyche

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Dimensions and mass[edit]

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)

The reason for the seamingly to low density is that it is for bulk density, which is up to 3 times lower than material density. The two values given in the article reflect this distinction, which (as I think) should be pointed out to avoid further confusion.Renerpho (talk) 06:02, 16 November 2015 (UTC)
It may make sense to add a 'Bulk density' row to the Infobox for use with minor planets. Praemonitus (talk) 16:29, 17 November 2015 (UTC)


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


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)
The reason for the seamingly to low density is that it is for bulk density, which is up to 3 times lower than material density. The two values given in the article reflect this distinction, which (as I think) should be pointed out to avoid further confusion.Renerpho (talk) 06:01, 16 November 2015 (UTC)

Dead link[edit]

During several automated bot runs the following external link was found to be unavailable. Please check if the link is in fact down and fix or remove it in that case!

--JeffGBot (talk) 19:14, 1 June 2011 (UTC)

Hydrostatic equilibrium[edit]

It seems the proposed mission to Psyche for the Discovery program is assuming Psyche is in hydrostatic equilibrium - should that be of note here? DN-boards1 (talk) 22:46, 28 September 2015 (UTC)

Also, this:

Page 104 comments that the larger asteroids are likely in HE, should we note that?

And you keep throwing around "rock collapses into a sphere only at a larger size than ice does". Can someone give me a source here for that assertion? DN-boards1 (talk) 22:52, 28 September 2015 (UTC)

How about you find a source instead that states that 16 Psyche is in hydrostatic equilibrium? A standard Wikipedia procedure. Cheers, BatteryIncluded (talk) 23:23, 28 September 2015 (UTC)
There is also a HUGE difference between "roughly" spherical (with partial differentiation) and true hydrostatic equilibrium. Page 104 also generically suggests you need a diameter of 290km while Physche has a shortest axis of about 150km. The bottom left of page 104 also admits, "our knowledge of the composition of the internior of an asteroid, as oppsoed to its surface, is non-existent." I do not even think the paper mentions hydrostatic equilibrium. Our own article goes onto to say, "Psyche thus appears to be an exposed metallic core from a larger differentiated parent body." And as such it formed from disruptive high speed collisions more than lower speed accretion (astrophysics). -- Kheider (talk) 00:23, 29 September 2015 (UTC)
Isn't Psyche like 290km? Also, let's look at this logically.

-The cores of fusors and planemos are round. -Psyche was the core of a planemo.

Therefore, we can deduce that Psyche, if it has not been horribly disrupted, will at least be in some form of HE. Haumea is in HE, and is far less round than most objects. DN-boards1 (talk) 01:34, 29 September 2015 (UTC)

Psyche has a mean diameter of around 200km. Psyche may have been part of a dwarf planet when it was much larger, but Psyche *IS* horribly disrupted and odd-shaped. That is because Jupiter's massive gravity "disruptively scattered" the asteroid belt before a major planet could form between Mars and Jupiter. 9 Metis is probably another disrupted protoplanet. -- Kheider (talk) 02:04, 29 September 2015 (UTC)
Haumea is a different beast altogether as it formed in the Kuiper belt well beyond the frost line. The frost line is why Jupiter is the most massive planet. Planetesimal Jupiter started as a large clump of frozen volatiles that had more surrounding accretion disc material to collect than further out Saturn where the disc would be thinner. The rocky planets had to accrete from dust and rock as volatiles would not be in the form of ice when closer to the Sun than the frost line. -- Kheider (talk) 16:19, 29 September 2015 (UTC)
If the asteroid belt was totally disrupted, how is Ceres a DP? DN-boards1 (talk) 21:40, 2 October 2015 (UTC)
Because of Jupiter, Ceres did not accrete to become a major planet and it is the only surviving dwarf planet in the asteroid belt. Most of the original mass of the asteroid belt is now gone. -- Kheider (talk) 22:32, 2 October 2015 (UTC)
Let me show you the lightcurve model for Pallas.
2 Pallas Lightcurve Model.png

There. That is quite clearly in HE, a prolate spheroid. So at the very least Pallas is a possible DP. We can throw out all the others, but the evidence points to Pallas being a candidate. But how did Ceres survive while no other asteroid DPs did? DN-boards1 (talk) 22:44, 2 October 2015 (UTC)

You will need a reliable and recent source to claim Pallas is still an accepted dp candidate. Looking at Vesta and Ceres, I do not see how anyone can still expect less massive Pallas to make the dp cut. Resembling a sphere does not make an asteroid a dp. And why are we having this talk on the wrong page? I will be reverting you a second time. -- Kheider (talk) 23:03, 2 October 2015 (UTC)
Dumb luck might be the only reason Ceres survived. The early asteroid belt probably had 5 dwarf planets or more. Some of them may have been ejcted from the asteroid belt and impacted the major planets. Had a large enough asteroid hit Ceres, Ceres would not have survived. -- Kheider (talk) 23:03, 2 October 2015 (UTC)

DN-boards1, there seems to be some confusion as to what we do here at Wikipedia. We don't "deduce" anything. Nothing. We only summarize results from reliable sources, and we cite those reliable sources. Otherwise, we don't write it. Please read this: WP:OR. Isambard Kingdom (talk) 06:18, 29 September 2015 (UTC)

Surface gravity, escape velocity[edit]

The previous figures were too low for the given mass and diameters. I used 240 km with the given mass (2.27E19 kg), which is still too low at the ends, as when a sphere is squeezed to a rod with the same length the mass is closer to the end and pulls more down and less sideways. Not sure if the gravity at the waist is higher or lower at the ends. If a sphere were compressed to a line the surface gravity at the middle would be 0 and would rise with altitude initially, then drop. OTOH a sphere compressed to a line might be a linear black hole ... Darsie42 (talk) 22:23, 4 January 2017 (UTC)