Clark R. Chapman William J. Merline Southwest Research Inst. Boulder, Colorado

1. Small Bodies in Near-Mercury Space: Satellites, Vulcanoids, Trojans, Inner-Earth Objects Clark R. Chapman William J. Merline Southwest Research Inst. Boulder, Colorado MESSENGER Science Team Meeting Arizona State University, Tempe AZ 23 January 2007 Part 1: Science Context (CRC, 5 min.) Part 2: MESSENGER Obs Logistics (WJM, 10 min.)

2. Background • Biggest news from 1st Mariner 10 encounter: Mercury’s “moon” (actually a background star observed by UVS); MESSENGER continues Mariner 10’s “exploration” • Literature: dozens of papers on satellites, vulcanoids, Mercurian trojans. Motivated… • …by “limits of completeness” (“exploration”) • …by dynamics (“why do Venus/Mercury lack moons?”) • Geology/geophysics relevance: • Origin of Mercury: different models “predict” left-over objects exist (and their composition) or don’t exist • Small bodies crater Mercury (or did…or don’t) in various interesting ways

3. Satellites of Mercury • Numerous predictions that satellites do (or did) exist, or don’t: MESSENGER can look! • Distant “captured” satellites • Closer satellites • Strong dynamical forces affect evolution • Solar tides • Mercury’s large orbital eccentricity • Yarkovsky Effect large due to solar proximity • Early satellites may have tidally decayed, producing elongated craters near equator • None detected in Mariner 10 images • Much more of Mercury to examine, smaller sizes

4. Vulcanoids ? • Zone interior to Mercury’s orbit is dynamically stable (like asteroid belt, Trojans, Kuiper Belt): 0.08 – 0.21 AU? • If planetesimals originally accreted there, it is unclear if they survived mutual collisional comminution • Searches during last 20 years have so far failed to set stringent limits on current population of vulcanoids (none >60 km) • Vulcanoids few km in size could have • Decayed with few Gy timescale • Cratered Mercury after the Late Heavy Bombardment, with little leakage to Earth/Moon zone; would compress Mercury’s geological chronology toward the present (e.g. thrust-faulting might be ongoing)

5. Trojans & Inner Earth Objects • Trojans: Dynamical analysis shows that stability is more difficult for Mercury than Venus: But we can look and see! • Near-Earth Objects are dynamically dispersed throughout inner solar system: Inner Earth Objects. • 3 IEOs are known, but there are strong biases against discovering them from the Earth; looking outwards from near Mercury orbit is much more advantageous • There could be cometary objects of opportunity (Comet McNaught) • IEOs are the projectiles that are currently cratering Mercury • MESSENGER could search for trojans and IEOs simultaneously.

6. Vulcanoids • Current limiting size ~ 60 km • Expected smallest ~ 1 km • Range expected 0.08 – 0.21 AU from Sun • Very hard to see from Earth • MESSENGER puts us ~ 5x closer

7. Vulcanoids with MESSENGER • Spacecraft pointing constraints limit observations to > 33 deg from Sun • Want to observe when spacecraft is as close to Sun as possible --- this happens 12 times --- 4 Mercury encounters and 8 others

10. Vulcanoids with MESSENGER • At 90 deg phase (typical middle of search region) and • At limiting mag of WAC (V=8) and • With one image at max exposure (10s) and • Clear Filter : • EXPECT limiting size = 15 km, improving size limit by 4x

11. Vulcanoids with MESSENGERCompleteness • Coverage of Vulcanoid cloud with one observation set (one side of Sun only) is ~ 6% • Both sides of Sun at all 12 opportunities would cover about 46% of the total cloud volume • In all 12 opportunities, we’d be able to re-sample an object up to 3 times and therefore refine the orbit

12. Solar C/A dates, MESSENGER

13. Vulcanoids with MESSENGERMotion, Timing • FIRST want to make definitive detection by multiple images (rejection of cosmic rays) • Confirmation in subsequent few hours • Orbital determination by follow up over subsequent ~week • This observational strategy would require 84 images during one spacecraft perihelion (one side of Sun)

14. Vulcanoids with MESSENGERMotion, Timing • Expected star motion about: • 410 pix/day • 17 pix/hr • 0.3 pix/min >> no streaking until ~ 3 min • Rough estimate, Vulcanoid motion: • +/- 59 pix/day relative to star field • +/- 2.5 pix/hr • Star motion is predominant

16. Vulcanoids with MESSENGERHighlights • We can improve detection limits by at least 4x, more with additional data • All stars will be identifiable – all are in catalogs due to bright limiting mag of WAC • We can coadd images – motion is “slow” • May be able to fit downlink in existing passes • Should not affect other science observations

17. Satellite Search • Mariner 10 limits: 10 km within 30 Mercury radii • Want to scan entire gravitational sphere of influence, for smallest possible objects, sphere radius is roughly 250,000 km (~ 100 Mercury radii) • In a single field, the sphere diameter is captured by WAC at distance of 2.9 Mkm = 0.02 AU

18. Satellite Search • At this distance, sensitivity in a single image, assuming 50% phase is 2 km. • This distance is about 5-7 days from an encounter • We could get smaller at closer distances, but brings us closer to encounters & requiring a mosaic to cover region

22. IEO Search • Can extrapolate from known NEO population the expected numbers of IEOs observable from Mercury’s orbit • Search can be done at non-busy mission times • Size limits, e.g. • From Merc perihelion, Object halfway to Venus 7 km • From Merc perihelion, 0.025 AU out, ~2 km • Similar for Trojans • Do them together ?