Sensex opened over 173 points up today on buying in heavyweight stocks, led by Reliance Industries. The Sensex, which gained 142.87 points in the previous trading session, added another 173 points to touch 17,238 in the first five minutes of trading.The broad-based National Stock Exchange Nifty gained 45.55 points to 5,165 at the same time. The good start in the market was amplified by an upsurge in the Asian stock market.