Science

Asteroids and comets in near-Earth space

Although Gaspra is not a near-Earth asteroid it is interesting, nonetheless.

Although Gaspra is not a near-Earth asteroid, it is interesting nonetheless. Gaspra was the first asteroid to have a fly-by when, in 1991, the Galileo spacecraft flew by on it’s way to Jupiter. Image credit: NASA

Although our solar system is, for the most part, fairly sparsely populated, it is far from empty space. Along with the major bodies that we’re familiar with, there are countless smaller objects making their way around the sun. The vast majority of these are in harmless orbits that take them nowhere near Earth. There is, however, a large population of objects whose orbits do cross the Earth’s. Many of the big ones have already been discovered. This is good. Many more small ones have not. This is not so good…

At the present time scientists are doing a good job of discovering most of the NEOs larger than 1km in diameter. As of mid-2013, 862 had been discovered. This represents more than 90% of the expected population greater than 1km and is consistent with NASA’s Spaceguard Survey goal. The problem is that even an object 100m or less can cause significant damage should it impact with Earth. So, the goal, generally speaking, is now to find 90% of the population greater than 140m in diameter. How are scientists doing in this regard? Well, take a look at NASA’s NEO stats for some clues.  What you find is that, although nearly 10,000 small (>140m) objects have been found with near-Earth orbits, there are plenty more to be found. This is evidenced by the increasing growth rate of discoveries in the small sizes compared to the decreasing growth rate of discoveries in the >1km sizes. And, what this means for us, is that a lot of good work can still be done with the right instruments.

PHASTTER

Currently, the proposal is to use PHASTTER as, primarily, an NEO search instrument for an initial 5-year period. Due to the incredibly large field of the instrument, however, it is best to think of PHASTTER as a synoptic survey instrument. Not only will it discover asteroids and comets, but the acquired images can also be analysed to search for variable stars, supernovae, and monitor exoplanets.

Since the telescope will be capable of fully robotic and remote operations, not only will we be able to queue observations from around the world, but we’ll also be able to bring real-time astronomy into the classroom.

This 60s image taken with the TFRM Baker-Nunn camera shows the wide field of view possible with such a system.

This 60s image of the Andromeda Galaxy taken with the TFRM Baker-Nunn camera shows the wide field-of-view possible with such a system.

One exciting aspect of this project for us is the concept of providing an open database of observations so that anyone can access them and work on them at home. Generally, the workflow might look something like this… As observations are made, the images will be archived on a processing machine that will automatically perform the necessary corrections, do the required astrometry and try to identify moving objects. As objects are identified they’ll be cross-checked against known objects then submitted as a batch to the Minor Planet Center. The images will then be archived and shortly afterwards be made available for public download. Users will then be free to download the archived images and perform their own analyses.

Bringing the PHASTTER Baker-Nunn into the 21st century through crowdfunding will be an achievement to be proud of. Too often, instruments like this sit, unused with little hope of being funded through more traditional channels. The Baker-Nunn is such a wonderful design that, even 50+ years after its construction, it can still  make significant contributions to the field of astronomy. Together, let’s make it happen!

PHASTT-1

The planned PHASTT-1 telescope is a different instrument when compared to PHASTTER. Although it boasts the same aperture as PHASTTER and will use the same CCD camera, it has a smaller field-of-view. Even so, PHASTT-1 will have a large field of view compared to most telescopes (~5x) of a similar aperture. Competing in the area of asteroid search, however,  is difficult due to a large number of teams doing similar work. Because of this, PHASTT-1 is being designed as not only a targeted search telescope but also as a follow-up and characterization instrument – two key areas where it can make an impact. Follow-up observations are important as they help to refine the orbits of potentially hazardous objects and narrow the uncertainties around how close an asteroid will come to the Earth. Characterization of asteroids is important as it helps with our understanding of the physical properties of asteroids. This understanding is critical if we want to know how to best deal with a ‘rogue’ asteroid that is on an impact course or if we just want to know which asteroids would make for interesting near-Earth exploration targets.

Critical to the characterization aspect of this project is the choice of filter set. Developed in the 1980’s, the ECAS (Eight Colour Asteroid Survey) filters help determine the composition of faint asteroids by matching the filter bands to characteristic absorption features and reflection peaks. With the proposed system, we hope to be able to image asteroids, with filters, at magnitudes fainter than 18.

Planets in other solar systems (exoplanets)

The large FoV of PHASTTER, together with its moderate aperture and its planned robotic nature, will allow for the efficient detection of exoplanets by means of transit measurements with high signal-to-noise ratio in the appropriate magnitude range. The suitability of a refurbished Baker-Nunn camera for exoplanet research was confirmed for the first time by the Australian Baker-Nunn APT during their UNSW Extrasolar Planet Search 2004-2007 (Christiansen et al.,2008). The subsequent catalogue that they produced shows that refurbished BNCs can accomplish millimagnitude photometry to at least V14 mag. TFRM has further improved on the APT photometric precision performance by working down to magnitudes V15.5 (del Ser, 2013).

Irwin et al.(2009) proposed an interesting alternative observational approach which has been executed by the MEarth project. In order to maximize the probability of detection of rocky super-Earths in the Habitable Zone (HZ), MEarth is photometrically monitoring a sample of ~2000 pre-selected M-type stars. MEarth operates 8 telescopes f/9 Ritchey-Chretien with a field of view of 25’x25′ each. Due to this limited FoV (0.17 square degrees), this project can only monitor a single star per telescope at a time. Despite this limitation, MEarth has been able to detect the first super-Earth (GJ1214b) from the ground (Charbonneau et al., 2009) in its first 4 years of operation.

Beginning in December 2011, the TFRM began to survey a pre-selected series of fields, with an input catalog similar to MEarth’s in search of super-Earths around M-type stars. The survey is called TFRM-PSES (TFRM-Preselected Super-Earths Survey). TFRM-PSES monitors a number of M0 to M5-type catalogued targets comprised in several fields with sufficient frequency each night, and in the magnitude range of 9.0-15.5. The number of M targets per field distribution spans from 6 to 16, with a global median value of 8. However, up to 23 out of 60+ fields contain more than 13 M targets: typically 14 and 15, and 16 in one case. The main difference between MEarth and TFRM-PSES is that, while in MEarth each single telescope monitors one star per CCD field, TFRM-PSES captures approximately 8 times as many stars per field thereby increasing survey efficiency. However, the higher number of telescopes in MEarth compensates for the small field size. Although a magnitude limit of 15.5 has been stated for TFRM-PSES, it could be increased  at the expense of the frequency of measurements (which would penalize efficiency when recording possible transits).

These two projects have shown that a refurbished Baker-Nunn camera can be an excellent tool for surveying exoplanets. The expectation, then, is that PHASTTER will be able to undertake the same kind of survey during periods when PHA work is not feasible. PHASTT-1, on the other hand, will be useful for exoplanetary followup observations post-discovery by PHASTTER.

Finally, note that a by-product result of TFRM-PSES survey is the detection of new variables stars. Preliminary results and statistical efficiency of such detection can be found in del Ser (2013).

Bibliography:

Charbonneau, D., Berta, Z.~K., Irwin, J., et al., 2009, Nature, 462, 891

Christiansen, J. L., et al. 2008, MNRAS, 385, 1749

del Ser, D. 2013, Master Thesis of M.Sc. in Astrophysics, Particle Physics and Cosmology (University of Barcelona)

Irwin, J., Charbonneau, D., Nutzman, P., & Falco, E. 2009, IAU Symposium, 253, 37

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