Inside Mercury's orbit

By Phil Plait | December 3, 2010 1:58 pm

Regular readers may know me as the beloved online blogger for Discover Magazine, but I also sometimes write longer articles for the print version as well.

Last summer, I wrote a piece on the search for small solar system objects that might, theoretically, circle the Sun inside Mercury’s orbit. Called vulcanoids, they are extremely difficult to observe, which is why it’s still not certain if they exist or not (I wrote a brief post about this back in 2008). Two astronomers (and friends of mine), Dan Durda and Alan Stern, are hot on the trail of the purported possible planetesimals; I talked to them about their chase and the history of the search for these hot little objects.

Until now, the article was only available in the print magazine or to online subscribers, but now my brilliant prose is open to the public. Seriously, this is a pretty cool topic, and one that most people don’t know about. The region between the Sun and Mercury is closer to the Earth than the main asteroid belt, yet we know much less about it. Read the article and find out why.

CATEGORIZED UNDER: About this blog, Astronomy, Cool stuff

Comments (26)

Links to this Post

  1. Astronews Daily (2455537) | December 6, 2010
  1. I remember reading that print article. :) It is amazing how much we are learning about things that are so close to us. Most people get too comfortable in the things that they think they know.

    Reminds me of George Carlin’s list of things you need to know from Brain Droppings.

  2. Chris

    I was wondering what the smallest size SOHO could see. The SOHO LASCO camera routinely see small comets and the background stars. Any decent sized object should have already been seen.

  3. Mapnut

    Are there known objects between the orbits of Earth and Venus? Or between Venus and Mercury? I don’t recall hearing about any such objects.

  4. From the article:

    anything smaller than a few hundred yards across would be long gone from the inner solar system by now… By the middle to late 20th century, these upper and lower bounds for both size and location were well defined.

    Does the ‘few hundred yards’ limit take account of the Yarkovsky effect, which I thought had only been taken seriously in the last 10 years or so?

  5. Pete Jackson

    One could watch bright stars near the sun’s limb for a sudden cutout due to a vulcanoid transit. This could be done from space, near-space, or during total solar eclipses.

    The occultations would be darn fast, a one km vulcanoid traveling at 200 km/s would cause the star to dim out for only 5 milliseconds! The search plane, 10 to 30 million km from the Sun’s surface would cover a staggering 2400 trillion square km and five minutes following a bright star would sample only about 60,000 of these. If we optimistically assume there are 1,000 vulcanoids, we should have one chance in 40 million of observing an occultation in five minutes. Of course, each observer more than 1km apart gives an independent observation, so a thousand amateurs along an eclipse path could narrow the odds…

  6. Grimbold

    The ideal thing would be a geostationary satellite placed so that the Earth blocks out the Sun, to search for the vulcanoids. If you place the satellite so that the Earth is directly between it and the Sun, the Earth’s disk would have an angular radius of about nine degrees. Since the vulcanoids would be about twelve degrees from the Sun, that would make it a very effective eclipse indeed.

    Of course, it would be hard to justify the expense if this is the only purpose of it; who would spend so much for the sole purpose of looking for little asteroids that probably don’t exist. You’d have to think of other scientific goals for the satellite. Or maybe you could go halvesies with a telecommunications company; they are always putting satellites into geosynchronous orbit.

  7. Phil

    Grimbold, any orbit would do. With no atmosphere scattering light even a small disc between the camera and sun would be enough.

  8. kevin

    so we are to believe that we can analyse the atmosphere of a plant a trillion miles away but we can’t figure out if an astroid is closer to the Sun than Mercury? It surely makes me doubt the accuracy of the distant planet stuff. after all, couldn’t we detect said astroid’s buy seeing a bit of extra iron in a spectrum shift?

  9. Grimbold

    @Phil, #8,

    Yes, that’s true, but space telescopes cannot point too close to the Sun for other reasons, such as preventing direct sunlight from illuminating the equipment. Hubble, for instance, can’t even observe Mercury. I figure any satellite like this would suffer similar restrictions, which Earth’s permanent shadow would prevent.

  10. Richard Woods

    @Grimbold #10

    Your generalization about space telescopes is too broad.

    Whether a space telescope can point close to the Sun depends on the design of the telescope! Hubble was not designed to do so, but SOHO obviously was!

    Any satellite designed to search for Vulcanoids while outside Earth’s shadow simply has to incorporate features SOHO and other near-Sun exploring probes have, such as using shields and positioning to prevent direct sunlight on some of the equipment. Doing so with Hubble would’ve interfered with optimizing its design for its primary missions of observing faint objects in the majority of space that isn’t close to the Sun’s direction.

  11. Diane

    On a side note, I’m enjoying a mild optical illusion with the photo here. it looks like it’s moving towards me, and I had to look carefully to make sure that it was, in fact, staying still.

  12. Wayne on the plains

    Grimbold et al,
    A more serious problem to this proposal is the fact that geostationary orbits aren’t permanently shadowed. They maintain the same orientation to the Earth’s surface, not the Earth-Sun system. You’d want a satellite at the L2 Lagrange point for that, but those orbits are too far away from the Earth to completely eclipse the Sun…

  13. Navneeth

    Regarding the sub-orbital space flight, will three minutes be enough to spot a potential vulcanoid moving against background stars?

  14. Gary Ansorge

    I predict that IF we discover Vulcanoids, they will be rich in heavy elements.

    Gold, Platinum, etc. I can imagine a 400 meter wide solid chunk of platinum. What would THAT do to the world market for platinum? Of course, unlike gold, it IS a very useful industrial metal.

    Gary 7

  15. Grimbold

    Yes, but can you haul it back to Earth for less money than you’d make selling it? That’s the trick. Your hypothetical platinum clinker, if it’s a sphere of radius 200m made of pure platinum, would weigh about 7.2×10^14kg. This is about the mass of two billion International Space Station (which is the heaviest thing we’ve hauled into space) and about 6.6×10^11 times more than MESSENGER (which weighs a ton and is the heaviest thing we’ve sent to Mercury).

  16. Messier Tidy Upper

    Nice article BA. Liked it. :-)

    @3. Mapnut :

    Are there known objects between the orbits of Earth and Venus? Or between Venus and Mercury? I don’t recall hearing about any such objects.

    Add to the list linked by #4.Chris (if they’re not incl. already) – Sun-grazing comets.

    Plus at least one semi-famous asteroid named Icarus :

    That apparently glows red-hot at perihelion. Arthur C. Clarke has suggested it could be used as an observing post.

    I wonder if sungrazer comets could help in the Vulcanoids search too?

    Of course, any future explorers there might have to ask that Spanish womans’ permission :

    for straying close to “her” solar territory – NOT! 😉

  17. Gary Ansorge

    16. Grimbold

    There is a lot of difference between boosting a payload into earth orbit or boosting the same load up from the sun, using solar power 16 times more intense than that available at earth orbit(roughly 20,000 Watts/m^2) to power a vasimir engine. In space, free fall and vacuum allow continual low level acceleration to move mountains anywhere you want. It just takes a while. At earth orbit, dropping that payload to the surface is easy.

    Not all at once, of course,,,

    Gary 7

  18. amphiox

    I would imagine that the principle cost outlay is boosting the equipment off earth and to the object in question. After that, moving the payload back to earth and getting it to the surface for processing will only be a minor additional cost.

  19. Michael Burkley

    Hmmm….this sounds like a pretty HOT idea rather than a cool one! But whatever it’s temperature it reminds us that there’s always more to learn!


  20. Nigel Depledge

    Gary Ansorge (15) said:

    . . . the world market for platinum? Of course, unlike gold, it IS a very useful industrial metal.

    Gold is very useful in electrical circuitry. Although it is a slightly less good conductor than silver, it does not corrode in air, so a gold-plated silver electrical contact gives you the best of both worlds.

  21. Tom

    @12. Diane: I’m glad I’m not the only one that saw that, I was beginning to think I was having a medical problem!

  22. Nigel Depledge

    So . . . is the study of vulcanoids vulcanology?

    (In the same way that the study of meteors is meteorology, right?)


  23. Messier Tidy Upper

    @ ^ Nigel Depledge : Somehow I think ‘vulcanology’ is taken! 😉

    In this context, it may be worth noting that :

    … the cosmic origin of meteors was now firmly established. Still, they remain an atmospheric phenomenon, and retain their name “meteor” from the Greek word for “atmospheric”.

    Source : Wikipedia (under meteors -history)

    Also the term for someone who studies meteors is …

    … an astronomer! 😉

    @15. Gary Ansorge & 22. Nigel Depledge :

    A few quotes for y’all on the usefulness of gold :

    “Gold is an outstanding heat insulator, many spacecraft have been gold-plated, not as a symbol of extravagance but because a thin layer of gold is the most efficient way to keep the spacecraft from overheating.”
    – Page 172, Ben Bova, ‘The Story of Light’, Sourcebooks Inc., 2001.


    “When human explorers head out toward the stars, they will bring gold and diamonds with them – not only as personal adornments, but as practical and efficient materials for their starships as well.”
    – Page 320, Ben Bova, ‘The Story of Light’, Sourcebooks Inc., 2001.

    Additionally, remember that gold comes in very handy for defending against cyberman too! 😉

  24. Messier Tidy Upper

    For example check out this :

    Ace videoclip. :-)

    See :

    for the source of the quote on the originof the term ‘meteor’ cited in #25.


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