Beyond Gaia: A Deep-Space Astrometric Mission To Obtain ~1% Parallax Distances at 10 kiloparsecs
By Khee-Gan Lee
The Gaia space telescope is currently measuring parallax distances for Galactic stars with ~1% precision out to ~1kpc. This will be a revolutionary data set, but order-of-magnitude improvements are possible with a next-generation mission that is already achievable with existing technology.
Every parallax distance measurement to-date has used a 2AU baseline, i.e. the Earth-Sun orbital diameter. By launching an astrometric satellite with similar resolution and sensitivity into a deep-space orbit, however, the parallax accuracy could improved in proportion to the orbital major axis. A telescope on an elliptical orbit with a semi-major axis of 20AU would have a parallax accuracy ~10x better than Gaia, allowing 1% distance measurements out to 10kpc, encompassing the Galactic Center and a significant sample of globular clusters. It could also directly measure distances to bright Cepheid variables in the Large Magellanic Cloud, bypassing the first rung in the cosmological distance ladder and its associated systematics.
Launching a Gaia-like telescope into a ~20AU orbit would be a significant endeavor, but is achievable with current technology using multiple gravity assists, and is even more compelling with anticipated cost reductions from reusable heavy-lift launchers. The primary challenge would be the low data-transfer rates from the outer Solar System, although >10^3 bandwidth increases are planned for the Deep Space Network by ~2030.
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...but a 20 AU orbit has a 90 year period, and you need most of that time to clearly distinguish proper motion from parallax. Wouldn't it just be faster to make a Gaia with 10x the precision?
Joshua Peek · 24 May, 2017
Not really since such a mission would use gravity assists from the inner planets to speed up transit. So you could get the first epoch near perihelion before the gravity assists, which would send the spacecraft to 20AU in about a decade, see, e.g. New Horizons took 10 yrs to get to Pluto. So say 15 yrs max between perihelion and aphelion...I do not envision such a mission taking more than 1 epoch of data!
Khee-Gan Lee · 25 May, 2017