Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system, according to NASA.
Yes, you can invest in both an IRA and a 401k (or a Roth 401k and a Roth IRA) at the same time, or any combination of those accounts. And in some cases, based on the fees in your 401k, an IRA might actually be a better place to invest your money.
1 The fact is, for tax year 2007, an individual, even if they contributed to a 401(k) in 2007, can contribute up to $4,000 to an IRA. And they have until April 15, 2008 to make a contribution. For 2008, an individual can contribute up to the annual contribution rates for both a 401(k) ($15,500) and an IRA ($5,000). Those age 50 or older can also add an extra $1,000 to an IRA and $5,000 to a 401(k).